Is it really better to not half-press to focus with the Fujifilm X-Pro 1?

Question

Fujifilm's new camera uses contrast-detect autofocus, and the current Internet hand-wringing is over whether this is a horrible critical flaw or not. As often happens in such discussion, a bit of street wisdom has arisen: it's better to just push the shutter the whole way. For example, this blog comment:

AF speed on the XPro isn’t that slow, the problem is that most people use it as they would an SLR/DSLR for focusing, i.e half press the shutter and wait for AF confirmation then press the shutter all the way, in most cases with the XPro and the X100 you don’t need to do this.

And there's at least one thread on DPReview, but as characteristic for such discussions, there's a lot more smoke than fire.

The X-Pro 1 manual says nothing about this, and in fact says:

S (single AF): Focus locks while the shutter button is pressed halfway. Choose for stationary subjects.

In my experience, if the subject is stationary, the half-press method works as it always does, and if it's moving the full-press method isn't as good as using AF-C (continuous autofocus). I've used plenty of point-and-shoot cameras as well, which of course use contrast-based AF, and the half-press focus lock has always served me well.

But then, a lot of people are saying this is a big deal. Is there any reasonable basis, or is it just wishful thinking?

Asked by mattdm

Answer

No, it is not. There is nothing magical about not waiting at the halfway point.

What you read is silly, as if waiting would make the focus take longer or something. It does not work that way. You can press all the way as fast as you want and you'll get a shot in focus or not. The longer you wait at the half-press (in AF-S) mode or before fully pressing (in AF-C mode), the higher the probability the shot will be focused. Waiting too long might make you miss action but it won't make the camera miss focus.

Note, having an X-Pro1 one in hand and, for all I tried, I cannot see how not waiting at the half-press can improve focus.

Answered by Itai

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What autofocus zone selection mode should I use for action/event shooting?

Question

I have a Canon 7D, and typically shoot everything I do in its single-point autofocus zone select mode, because I usually want fine control over exactly what the camera is focusing on and I often find that its two automatic focus zone select modes don't focus on what I want in focus. For most of the photography I do, this is fine -- I have enough time that I can compose the shot the way I want, use the joystick on the back to select the right focus point, and take the shot at my leisure.

However, recently I've been doing more event/action photography, and I've been running into issues with this mode of operation. Too often, I will see a person start to do something interesting, but my autofocus point is not in the right place -- so by the time I get the camera up, adjust the settings, and move the autofocus point, the opportunity to take the shot has passed. My question is, what autofocus zone selection mode and workflow should I use for this type of photography? I could use the 5-zone partial-auto mode (which gives me a choice of top, middle, bottom, left, or right, and then the camera chooses the points within my selected region), but I find that the time needed to switch in between that mode and the single-point mode is similarly prohibitive, and I have to hope that the camera chooses the right thing to focus on.

Asked by David

Answer

I generally select the focus point that gets me closest to the majority of shots. For an example, in soccer, I'll select the point either above the midpoint or two above in the vertical orientation, and I'll select the midpoint for horizontal orientation.

I would not be comfortable letting the camera pick the point.

Then, if I want the subject off center, I'll either compose loose enough so that I can crop in post, or I'll change the focus point accordingly. In sports like tennis, I'll put the point to the right or left of center depending on which player I'm shooting so that the player's back is closer to the edge of the frame and the ball is in front.

It's more of a matter of learning to anticipate where the action will be. The more you shoot a given subject, the more you'll learn where to put the point for the majority of the shots, and when action happens you didn't anticipate, shoot loosely enough to crop after the fact.

Answered by Eric

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Why does Nikon warn about banding at high ISOs when using autofocus on Nikon D4 or D800?

Question

Nikon says that banding may occur at high ISO sensitivities with autofocus on the D4 or D800:

D800

Noise in the form of horizontal lines may appear in pictures taken with AF-S Zoom Nikkor 24-85 mm f/3.5-4.5G (IF) lenses at ISO sensitivities over 6400; use manual focus or focus lock.

D4

Noise in the form of lines may appear during autofocus at high ISO sensitivities. Use manual focus or focus lock. Lines may also appear at high ISO sensitivities when aperture is adjusted during movie recording or live view photography.

What would explain this behavior?

Asked by DragonLord

Answer

This problem is caused by electromagnetic interference generated by the SWM in many Nikon lenses.

There are cases where Canon lenses with USMs have caused banding. See this dpreview forum thread for an instance of sensor banding caused by a USM lens mounted on a Canon DSLR.

Answered by DragonLord

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How can a teleconverter allow manual focus lenses to autofocus?

Question

I've found that some discontinued teleconverters, such as the Pentax F 1.7x AF Adapter and Nikon TC16A, enable autofocus with manual focus lenses. How is this possible? Does this system have any significant limitations?

Asked by DragonLord

Answer

With the Pentax option, you're basically getting "close" with the manual lens using manual focus and then using the AF of the camera to move the lens elements in the teleconverter itself to adjust for more precise focus. Bear in mind that the TC is a lens itself...

At any rate, it's not perfect answer for making a manual lens do AF, but for the right sort of shooting, it's a very useful feature. I can't say for the Nikon option, but I would imagine that it's similar.

Answered by John Cavan

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How do I keep my Nikon D5100 from attempting to autofocus with every shutter press?

Question

I have a Nikon D5100 and the auto focus works fairly well most of the time. But there are times when I have a subject in focus and want to fire off multiple shots fairly quickly. I do not need to focus every time on the subject yet auto focus will attempt to re-focus the picture every time that I press the shutter down. Only way I have found around this is to switch to manual focus. Is there a way to keep the camera in auto focus mode but have it not auto focus every shot.

An example: I was shooting the moon (no pun intended => ) the other night and the auto focus had the moon in focus without an issue. Every time I pressed the shutter the camera would attempt to focus. I was attempting to fire the shutter as quick as possible to show to motion of the moon rising and having it focus every time causes too much delay.

Asked by Lynda

Answer

You can configure the back button on your camera that is AF/AE lock to lock your focus. You'll focus, press the button, and the focus will lock until pressed again. You want to go to the menu and select "buttons' and then set the button to "AF Lock" I believe.

Answered by rfusca

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Nikon D5100 Autofocus

Question

I have a Nikon D5100 and the auto focus works fairly well most of the time. But there are times when I have a subject in focus and want to fire off multiple shots fairly quickly. I do not need to focus every time on the subject yet auto focus will attempt to re-focus the picture every time that I press the shutter down. Only way I have found around this is to switch to manual focus. Is there a way to keep the camera in auto focus mode but have it not auto focus every shot.

An example: I was shooting the moon (no pun intended => ) the other night and the auto focus had the moon in focus without an issue. Every time I pressed the shutter the camera would attempt to focus. I was attempting to fire the shutter as quick as possible to show to motion of the moon rising and having it focus every time causes too much delay.

Asked by Lynda

Answer

You can configure the back button on your camera that is AF/AE lock to lock your focus. You'll focus, press the button, and the focus will lock until pressed again. You want to go to the menu and select "buttons' and then set the button to "AF Lock" I believe.

Answered by rfusca

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What is the advantage of the huge number of AF points?

Question

If the camera has only 1 central AF point, it is difficult to focus on an off-center subject ( without recomposing after autofocus ) so having 3 AF points seems really useful. Similarly I understand the advantage of having maybe 5 or event 9 AF points.

Nowdays flagship DSLRs have huge number of AF points. For instance Nikon D800 has 51, Canon 5D Mark III has 61. What is the advantage of this? Using 51 AF points really increases the chance that the camera finds out my intention and focuses on my real subject instead of something else next to it?

Asked by asalamon74

Answer

Having many autofocus points opens up the camera's capabilities because it can function in different ways depending upon settings and camera features:

• You can select a single AF point (get precise with exactly which point you want to use, without recomposing at all).
• You can select a group of AF points (a group of AF points means you don't need to be as precise in your selection/aim, and that if the subject moves within the group it'll still maintain focus).
• You can select the best AF point for the job (not all AF points are equal; some are more/less sensitive, and some are horizontal sensitive while others are vertical).
• You can let the camera choose which AF points to use (it'll try to get the most area in-focus at once).
• You can track subjects as they move across the frame (as the subject exits one AF point or group of AF points and enters another, each point will keep the subject in focus).

Throw in intelligent metering systems and the camera can do scene recognition:

• Is this a portrait? (Focus on the face.)
• Is this a group photo? (Try to focus on the most faces.)
• Is this a landscape? (Try to get everything in focus.)
• Is this a sports scene? (Try to use focus tracking of the running person.)

And if you include flash in the mix, those AF points are also used to try to calculate flash exposure based on subject distance.

Answered by Dan Wolfgang

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Why does Canon and Nikon limit or disable autofocus beyond certain f-numbers?

Question

One thing that I've noticed with Canon DSLR camera is they will disable or limit autofocus when the lens is below certain aperture values, which are specified in my answer to What are the aperture limits for autofocus on Canon DSLR cameras? Note that Nikon cameras are also typically limited to f/5.6 except for models featuring the latest revision of the Multi-CAM 3500 AF system as in the D4 and D800.

As far as I am aware, Olympus and Pentax do not have these limitations, even if autofocus may be slow or unreliable beyond f/5.6. (Sony/Minolta requires at least f/6.3, with one exception for a 500mm f/8 autofocus mirror lens, and the Alpha 850 and 900 have a center AF point with high precision at f/2.8.) For example, on Pentax, 9 of the 11 points are cross-type and remain so at all apertures. The Pentax AF system has been reported to work at f/8 and beyond, though extremely slowly. Olympus Four Thirds cameras will continue to attempt to focus below f/5.6 as well. Why do Canon and Nikon choose to limit AF functionality instead of attempting to focus with reduced performance beyond the above cutoffs?

Asked by DragonLord

Answer

The auto focus systems are not capable of using the highest precision cross type focusing points if you do not use a large aperture lens. They aren't artificially limiting the precision, they are simply working within the constraints of the maximum aperture.

Canon puts these limits in to ensure reliable AF. If you don't believe that theory, add a piece of tape between your teleconverter and lens, on the top three left pins. This will trick your body into thinking that you don't have a teleconverter attached, and your lens is simply a 300/4.0 or whatever you have.

Furthermore, only the Canon 1-series cameras are capable of AF with a f/8 lens, albeit at the center point only. Usually this setup comes from combining a long lens with a extender.

This really comes down to a issue of user experience. Sure, they could include ISO 204800 or 409600, but the quality would not make anyone happy. They are trying to keep the majority of users satisfied with what the camera can do, and most consumers are satisfied if AF doesn't hunt endlessly.

Answered by dpollitt

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When is it really necessary to use AF assist light?

Question

I find the AF assist light to be distracting at times when I'm trying to take a candid shot without disturbing the human subjects or taking pictures of pets (they hate getting the AF light in their eyes and turn away).

I find that if I turn off the AF assist light, the picture usually still turns out ok.

What is the AF assist light for and when is it really necessary?

Asked by nmc

Answer

AF assist light does not affect picture qualiyy per se.
What it does is assist the camera to focus by increasing light level.
Once focus lock has been achieved the assist light has done its job - its light does NOT appear in the image.

Experience will tell you when the light will improve focusing.
Often the camera may attempt to use the light when you can focus adequately without it by using a little extra effort. As light levels drop contrast levels fall and focusing becomes more difficult. By focusing on an area of visually distinctive contrast change you can focus in such situations and then recompose. I almost always waste all the nice focusing sensors that modern SLRs provide and use centre 'spot" focus plus recomposition.

A point will be reached as light falls where focusing slows dramatically or becomes very unreliable. At that stage you are well past where the AF assist light would have been of some use.

When shooting in good light levels a camera may 'decide' to use AF assist - often due to a darket area somewhere in the scene. So, even in the brightest day you can not be certain that the AF assiste will not be auto-triggered. In "street photography' or when photographing crowds, fairs, shows etc it is often not useful for people to be disturbed and alerted by the visually bright AF light. I've spent quite a lot of time wandering around china with a camera in recent years. I find that eg Chinese policemen are not keen to be photographed at the best of times. Alerting them to your artistic attentions with a bright red lights seems unwise [tm].

I do my best to try to remember to turn AF assist off when I am in walk around mode :-).

Answered by Russell McMahon

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When a camera auto-focuses, is the current focus distance reported from the lens to the camera?

Question

The question says it all -- does the camera "know" how far the subject is from the lens, i.e. what is the current exact focus distance set at the lens?

Asked by RiMMER

Answer

Many (possibly most) modern SLR lens systems return focus setting data to the camera.

Potentially the precision of data returned could be high - something better than 1% of range would be possible and meaningful with modern systems.

However, it appears that most if not all systems use a simple gray-coded* system with perhaps 16 steps. Number of steps varies with manufacturer and even with lens model.

A Gray code is a (usually but not essentially) binary code in which only one bit changes at each position change. This means that if the position oscillates to and fro at the boundary that erroneous codes cannot be sent. In a normal binary code, where 2 or more bits may change at once, if one changing bit alters before another due to mechanical tolerances or contact bounce then completely erroneous codes may be sent in error.

Std binary code At the 3 to 4 boundary 3 bits change at once. If any of these were early or late in occurring many illegal codes could result

0 0000
1 0001
2 0010
3 0011
4 0100

Out of my head Gray-code I have purposefully NOT followed what may be a most logical sequence.
What is important is that at each step only 1 position changes state - erroneouis codes can not be caused by bounce or tolerance errors:

0000
0001
0011
0010
0110
1110
1100

---

A few brand specific samples - more anon:

Claim here that the Canon EF 50mm f/1.4 USM only uses a 2 bit = 4 position code. - interesting discussion with some relevance.

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Minolta / Sony A mount system lenses that I have inspected use a simple grey-coded mechanical distance encoder with 4 bit / 16 position precision.

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Nikon: Here is a nice Nikon discussion with specifc 16 stage gray coding information for one lens, plus some circuitry and a lot of associated material.

TC16A teleconveter modification - wity much related material - excellent.

Gray codes for an example lens:

Here a4 digit code is returned wit 1 = open circuit and 0 = contact shorted to ground. 0000 70mm
0001 75mm (Approx)
0011 81mm (Approx)
0010 85mm (Approx)
0110 94mm (Approx)
0111 101mm (Approx)
0101 109mm (Approx)
0100 117mm (Approx)
1100 126mm (Approx)
1101 135mm (Approx)
1111 145mm
1110 155mm
1010 169mm (Approx)
1011 181mm (Approx)
1001 195mm (Approx)
1000 210mm

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Adding gray-coded sensors to manual Nikon lenses to allow operation with more modern bodies.
He discusses adding coding for fcal length and focus position.
The Electric Accordion - his aim is mainly "fooling" purposefully crippled low end bodies but the principles apply.

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Example rotary gray code pattern. This is to gray code a knob or dial but lens focus rotation uses the same style of pattern. There are numerous different possible gray codes.

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Excellent gray-code discussion with specific reference to len focusing.
Position encoders and the Gray code
From the above:

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Related:

DIY Nikon lens protocol converter - upgrade old or incompatible lenses.](http://photo.net/nikon-camera-forum/00956N)

Canon gray-code patent February 2012 !!!](http://www.freepatentsonline.com/y2012/0032068.html) and PDF here - may or may not be lens related.

Answered by Russell McMahon

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How to verify that Nikon SB-28DX AF-assist is working?

Question

I have a Nikon SB-28DX which I am using with an F5. I understand from the instructions how to turn the AF-assist illuminator off. But, how do I verify that it's working? Does it still work with the flash in standby? If no, how do I ensure that the unit will perform AF-assist only, and not fill flash?

In case it makes a difference, I'm activating AF using the AF-on button (that is, custom function 4 is set to 1).

Asked by James Youngman

Answer

Try it in a very dark room. If you can focus, and there's no flash, it's working.

I haven't used an F5 (only much more primitive SLRs), but I assume it will let you shoot with no film loaded so you're not wasting anything.

You've edited the question to emphasize the part about ensuring the unit will perform AF-assist only, which is pretty different from the question of verifying if it is working. From the manual (note the DX and non-DX are identical for film cameras), this flash does not appear to have a mode that does what you want — activate the AF assist light but not actually flash. Some flash makers call this "Spot Beam mode"; Nikon, for flash units that have it, calls it "AF-Assist Illuminator only" (AF-ILL ONLY) or, somewhat confusingly, "Firing canceled". I don't have an SB-28DX, but since the manual doesn't mention this as an option, it's probably not. You can still verify this with the test mentioned above.

Answered by mattdm

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What is the practical difference between phase-detect and contrast-based autofocus?

Question

What are the main differences in field use between phase detect AF and contrast based AF?

I understand the technical differences, but I am not sure what the implications from a photographer standpoint are. Is there a noticeable speed difference? Does one drain the battery faster?

Asked by subsub

Answer

To put it into manual focus terms, contrast-detect autofocus is like trying to focus an image on a plain ground-glass screen, while phase-detect is like using a split prism focus aid or a rangefinder. In the one scheme, you are looking for a local maximum on a gentle curve, while in the other, you're just looking for things to line up. It's a lot easier to decide when things are lined up than when things are maximally contrasty.

Now, electronics can make the absolute maximum contrast determination faster than we can, since they can be sensitive enough to go back the moment the contrast curve begins to fall, but that's still not quite as easy as comparing two images to see if they line up. And, since a phase-detect system knows which image is which, it should always know in which direction it needs to focus to make the correction. It's always a guess with contrast detection -- you focus in one direction, and if it gets worse instead of better, you reverse direction.

That said, some of the cameras they're making these days have incredible electronics, so there might not be an appreciable difference to the average photographer. Either way, driving the lens (and the backlight for the monitor, if you're using one) is going to be the main source of power drain. Yes, reading the entire imaging sensor is going to "cost" more than reading a few (or a single) specialized autofocus sensors, but is it a difference you're going to notice? Probably not. The real drain with contrast-detect AF is usually that you can't use an optical viewfinder, so the monitor (or EVF) is active the whole time, not that the focus system itself is running.

Answered by Stan Rogers

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What is an auto-focus motor?

Question

I recently bought a Canon 550D, and I read it has an auto-focus motor, which not all cameras possess. I'd like to know what it is, and what I can do to keep it running smoothly; AFAIK the manual doesn't come with instructions on how to improve the life of the focus motor.

Asked by the.midget

Answer

An auto-focus motor is just what it sounds like: a small electric or piezoelectric ("ultrasonic") motor which moves the lens elements to facilitate autofocus.

In some camera systems, this motor is in the camera body, and the lens moved by a physical coupling. However, you actually read some misinformation. Canon EOS cameras do not have a motor in the body, and instead, all lenses have a built-in focus motor.

With Pentax, all auto-focus camera bodies (to date) have a motor, and some lenses have their own. (In that case, the lens motor is used.) That used to be the case with Nikon too, but in the last few years (starting with the D40), the entry-level Nikons have dropped this. That's a big deal, because it means Nikon AF lenses without a motor don't autofocus. As Nikon refreshes the lens lineup, this will be less of an issue (except if you have or want to obtain older lenses and have them autofocus) — lenses like the AF-S DX NIKKOR 35mm f/1.8G didn't exist when the Nikon D40 came out, and that's part of why the Internet was all in furor about this.

So, you often see "no focus motor" as a disadvantage for entry-level Nikon cameras. This leads to the incorrect but somewhat widespread assumption that the natural opposite (for cameras for which this is not an issue) is "has focus motor". That's technically incorrect, but the practical point is that for your camera, the lack of a focus motor is a non-issue.

And also, since there isn't one, there's nothing to worry about for maintenance. Focus motor failure isn't one of the big, highly-reported problems of cameras which do have them, and I don't think there's any particular recommended maintenance.

Answered by mattdm

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Where can I find advice on using an advanced AF system for wildlife and bird photography?

Question

I recently purchased a Canon 7D, as my previous 450D had become a severely limiting factor in my endeavors to wildlife and bird photography (particularly bird photography, which I LOVE, but for which I have a bare handful of keepers out a few thousand shots.) The AF system of the 450D had a single cross-type point and a total of nine, while the 7D has a much more advanced 19 point system, all of which are cross-type. I've taken the 7D out a few times since I first purchased it, and while I have definitely had better luck photographing wildlife, I am still having trouble photographing birds on the move.

I have read through the 7D manual, however I am still not exactly sure where to start on learning how to use an advanced AF system in the real world for moving subjects. I could really use some insight from photographers who regularly use advanced AF systems for tracking subjects and keeping them in focus. I would also like to know how to take multiple shots of a subject on the move and keep them in focus between shots. While I can often nail one shot in a motion sequence, I regularly seem to fail to get proceeding or trailing shots after the one keeper (and that keeper is often not the most interesting one of the bunch.) The 7D supports a variety of AF modes as well, including AF Expansion and Zone AF, where the camera can supposedly utilize extra points around the selected point in case the subject moves away from the selected points area of sensitivity, or the selected point lands on a low-contrast part of the subject and a neighboring point lands on an area of high contrast (so focus can always be achieved, even if the selected point isn't detecting anything to focus on.) I have also not had much luck using either of these modes, and I think its primarily because I simply don't know how.

From a lens standpoint, I have used the following lenses:

• EF 100-400mm f/4.5-5.6 L
• EF 100mm f/2.8 Macro
• EF 50mm f/1.4

My most frequently used lens is the 100-400, usually at the 400 end, so aperture is only f/5.6. I am not sure if that might be a problem or not (as far as I understand, the 7D supports AF at f/5.6, although I am not sure how many points might be considered "high precision" at that aperture.)

Asked by jrista

Answer

There are several custom function options available to the 7D which can be configured to assist with tracking moving objects:

• C.FnIII -1 AI Servo Tracking Sensitivity You want this set to "Slow" - this will stop the AF system trying to refocus on anything that briefly passes between you and the subject you are tracking - handy with birds where branches etc might flash by as you track

• C.FnIII -2 AI Servo 1st/2nd Image Priority This should be set to 0 so that the camera will give priority to focusing on the subject over taking shots - basically, if it thinks it hasn't got a lock, it won't shoot while it obtains one

• C.FnIII -3 AI Servo AF Tracking Method Set this to 1 - this further adds to the system ignoring obstacles by allowing the system to use AF points around the main focus point when something gets in the way of what you were tracking

Use single point AF with the AF Expansion custom function (C.FnIII -6) switched on - this gives you accuracy on the initial pick up but then allows the camera to help out as you track.

These settings should be correct from the camera side of things but the story doesn't end here.

Your 100-400mm L lens has a 2 mode IS system, with more predictable objects, you should use mode 2.

For tracking moving subjects it is important to get a good solid stance and to track the subject as it approaches some time before you want to start shooting (trickier with birds but achievable depending on the behaviour of the species). You should also follow through after you shoot (much like when shooting a shotgun).

You need your feet about shoulder width apart and need to be facing across the path of travel of the object you're tracking:

Object -------------------------------------------->

               ^ 
              You 

Hold the camera with your right hand as normal and with your hand a good way down the lens (similar to holding a rifle or shotgun). Keep your elbows tucked in, possibly resting against your torso to provide extra support and twist your torso from the hips upwards to meet what you want to track. Track it in the viewfinder, ideally if you can switch eyes so if you're tracking right to left, look through the viewfinder with your left eye so you can keep your right eye open to help keep track of the object (especially if it moves erratically) - do the opposite for left to right.

Start tracking the object, and smoothly pan with it. If you are shooting birds, turning off IS altogether is probably a good idea as the movement is likely to be too erratic for the IS to keep up with (even in panning mode 2) - aircraft are steady enough to use IS mode 2 with usually.

Squeeze the shutter release, don't press it (just like with a rifle) or it will jolt the camera and try to keep the object in the viewfinder as you get glances between shutter releases. Carry on tracking as the object moves away.

Getting a smooth torso twist is paramount and takes practice but getting the stance right gives you the foundations to build upon.

Answered by JamWheel

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Is focusing problems on Sigma lenses still a problem on a Canon body?

Question

I am considering purchasing the Sigma 70-200mm f/2.8 EX DG HSM II Macro Lens. The problem is that many reviews point out that the lens has front focus problems, at least on a Canon mount. But those reviews date back to 2008, so things could have changed.

I have also been told that newer Sigma lenses (2010 and later) should have dealt with their focus problems.

Is that so? Can I purchase the lens and trust it to focus correctly? I have the 500D which does not have micro-focus adjust.

Asked by Pete

Answer

SOME Sigma (and indeed all 3rd party) lenses have problems on SOME Canon bodies. IF the lens is under warranty when this happens, afaik Sigma will fix the problem (which has to do with Canon's constant changes of their AF logic body/lens interface, requiring Sigma to change their chipset with nearly every new body Canon releases. It's a subtle trick by Canon to push people into buying Canon lenses...

Answered by jwenting

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Why does my lens focus better/faster with a speedlight attached to the camera?

Question

I have a Nikon d90 and have often found that the camera reaches focus faster, with less re-adjusting and missing the focus, when I have a speedlight (the Nikon sb-600 in my case) attached.

Why is this the case? I don't know enough about autofocusing systems inside the camera but I recognise that a red light is emitted from the speedlight while focusing and that this is likely involved.

Asked by Michael Allen

Answer

The phase detect autofocus on your D90 works by measuring the displacement between two 1-dimensional images formed on the autofocus sensor. By detecting features in these image strips and measuring their offset, the distance to the subject can be estimated. In low light it is difficult to for the camera to identify features and make a measurement as the image strips will be very dim.

Most modern speedlights (such as your sb-600) have an "auto assist lamp", which is usually implemented as a red LED matrix that projects a grid pattern over your subject. The projected grid lines are easy for the AF system to pick up and measure displacement as you get a sharp peak on your image strip where a grid line crosses.

Answered by Matt Grum

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What causes auto focus to misfocus?

Question

If I understand correctly, autofocus works with contrast detection or phase detection, and both of these systems are in the camera rather than the lens. Surely then, the camera just needs to control the focus motor until its detection system reports a peak focus value.

If that's the case, why do lenses have front/back focus problems, or exhibit even more complex focus issues that are a function of aperture or subject distance?

Asked by hertzsprung

Answer

Contrast detection autofocus systems work in a closed feedback loop and so the lens shouldn't cause misfocus. However phase detection AF is not completely closed loop (it's also not fully open loop), several measurements are taken and the lens instructed to move a certain amount calculated to correct the observed phase disparity. The amount the lens moves is thus subject to calibration errors. When only a small phase disparity is detected the lens is instructed to move to the final position and no further measurement is taken, and this is where focus errors arise.

It's worth pointing out that the way phase detect AF systems work is not all that well understood. Most of the literature describes to phase detect systems as being purely open loop (no feedback), however people have devised experiments to prove that this isn't actually the case (e.g. you wait for the lens to move and then block it with your hand, the camera will continue to try and focus and wont give you the confirm beep until you take your hand away, proving it's waiting for additional measurements).

It's unclear to me exactly why phase detect systems aren't purely closed loop either, as that would seem to put all the "my lens front focuses" issues to bed. If it were for speed reasons (i.e. the extra measurement delays the firing of the shutter in many cases when the lens is actually correctly focussed) that doesn't explain why there isn't a slower but more accurate AF setting for when speed isn't a priority.

Focus shift: another source of focus problems arise due to the fact that the camera focuses with the lens wide open, and then stops down to take the image. In lenses which suffer from pronounced uncorrected spherical aberration the plane of sharpest focus can shift when stopped down. A good example of such a lens is the Canon EF 50mm f/1.2L. The shift occurs between f/1.4 and f/3.2, from f/3.5 onwards the increased depth of field makes up for the focus shift. This problem is only really remedied by knowing about the problem and compensating or manual focusing whilst stopped down.

Answered by Matt Grum

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How important is auto focus and aperture control in a macro setup?

Question

I'm thinking of starting some amateur macro photography, and I'm not sure I want to go with macro lenses, due to their price.

I currently have the following lenses:

• Canon EF 50mm f/1.8 II
• Canon EF 24-85mm f/3.5-4.5 USM

And I was thinking of buying either:

• Macro Extension Tube
• Reverse Adapter

My questions are:

1. What is the difference between the tube and the reverse-ring if I want to use one of my lenses for macro?
2. How important is auto-focus in macro-photography?
3. How important is the ability to change the aperture macro-photography?

Answer

The answer to your first question is answered here. Both are good low-cost ways to get started in macro photography.

Autofocus is not very effective as you near 1:1 magnification. Most AF systems will struggle to find focus, you have such a narrow DOF anyway that holding focus is very hard. With anything that is not moving, you are far better to set up a tripod and use live view to focus, if you have it. Or manually focus, then move the camera to and fro to get the subject in focus.

If you have a macro lens, you may not use it at 1:1 as often as you think, in which case AF can be very handy. I use it when shooting bees - it's very hit and miss, but for me slightly more successful than manual focus.

If you use extension tubes or reversal ring, you will be forced to be a certain range of distance from your subject (you can no longer focus over a wide range of distances), so AF is not really useful at all.

Aperture is important. You have a very narrow DOF, so you will often want to stop down in order to get as much DOF as possible. Or if you are shooting hand-held you may need a wide open aperture to avoid blur. If you reverse a lens, or have manual extension tubes and aren't able to control aperture, it's not a killer blow, but it isn't ideal.

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How does the Nikon D40 decide which focus location to use?

Question

How does the D40 decide which focus location to use? It always seems to choose the wrong focus area.

Answer

There is a pretty good, in depth article about this at kenrockwell.com.

Essentially, it tries to use the closest subject it can find without extra help from the photographer.

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How does color affect auto-focus?

Question

I've noticed that the subject color seems to affect auto-focus, and am curious if I am just imagining this, or if there is a reason for it.

For instance, I shoot a lot of figure skaters, and synchronized skating teams. The background is constant (a rink, and boards, sometimes the stands). My auto-focus seems to work differently depending on what color dresses the teams are wearing.

Blue, purple, and red seem to give rock solid auto-focus. White or grey (and even sometimes black, which surprises me given the background is white/grey) seem to not always focus correctly.

However, even more surprising, I will sometimes find a team wearing a colour that causes my auto-focus to hunt continuously (I can hear/feel the motor thunking back and forth). This has happened with pastels (light blue in particular), grey, or even black.

I'm using a Nikon D90, and a Sigma 70-200mm F2.8 (though I'm not sure that's relevant).

If there is a color/auto-focus correlation, I'd love to know the reasons why too.

Answer

Autofocus sensors are essentially complex arrays of light-sensitive strips, and pairs of strips can detect when focus is "out of phase". Some sensors only detect phase shift in one direction, some in two, some in two diagonal directions, each with successively higher sensitivity to contrast differentials and more phase sensitive strip pairs in various arrangements. To my knowledge, AF sensors are simply tonal in nature, and do not factor in color. This is in contrast to metering sensors, which these days on prosumer and professional grade cameras, usually DO factor in color and possibly a variety of other factors. AF points also usually only "see" a fairly small area of the overall image frame, and as they are designed to sense phase shift, targeting an AF point on a surface of relatively uniform color (regardless of what that color may be) is unlikely to produce enough phase shift to lock onto and adjust focus, or if at most start AF hunting, but lack the necessary contrast to lock focus at a certain point.

While AF sensors effectively see in monochrome, and see an entirely different world than image sensors or the human eye sees, they likely have similar color-sensitivity ranges. (If there is any kind of filtration, either deliberate or as a result of the materials used in the AF sensors themselves, they may have unconventional color sensitivities as well.) Most photo-sensitive devices are less sensitive to blue light, and to some degree red light, than to "middle" colors, such as yellow, green, and very light blues. That could introduce less sensitivity to contrast within those color ranges. I wouldn't suspect that such differential in sensitivity would affect AF nearly as much as a lack of contrast at the selected AF point, but it may have an impact in some cases.

The key to AF is focusing when the AF point is very near areas of high contrast. The edge of a dark figure skater against the bright ice will likely lock every time, where as the center of the dark figure skater is likely to result in a bit of AF hunting until it locks onto something, and that something may not actually be what you intended. Using multiple AF points can assist in locking AF somewhere useful, however it broadens the area factored into AF, and its entirely possible you will still lock focus on something you did not intend.

It may also help to understand how the AF system works, as there is a world of variety in AF modules, and they all offer different capabilities. I recently researched the AF capabilities of a lot of cameras while debating whether or not to get a 7D. The D90 has a decently advanced AF system that has the ability to predict and track subject motion while focusing. If I remember correctly, its motion detection and tracking work differently depending on whether you are in a single-shot or continuous-servo AF mode. In single-shot mode, I do not believe the D90 will actually lock focus on a moving subject, and will wait until the subject has stopped moving to actually lock focus. In servo mode, the D90 will track the subject that it believes is in focus, but as far as I can remember, there is no actual focus lock since the camera is continually adjusting focus so long as the shutter button is at least half-way down. If you are using single-shot AF mode, you might not be getting AF lock because your subjects are moving. You might want to try switching to continuous-servo mode and see if you have more success with getting in-focus shots. I would offer that this is much more likely to be your issue than the color of your subjects. The D90's AF system is quite configurable, and I believe there are ways to "tune" it (for lack of a better word). You might want to hit the D90 manual or do some research on how to control the D90 AF, and see if you can get it working better for the way you use it.

For reference, here is the article I read on D90 AF (the article states D200, however the AF module is the exact same one):

Understanding the Nikon Multi Cam 1000 AF Module

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