How to measure distance to subject in Depth of Field calculator?

Question

I used Depth of field calculator here.

I should specify subject distance.

How to measure distance to subject?

In theory this is a distance to front principal plane or point. What is in practice?

I tried to calculate expected image size and then made shot with camera. In calculation I measured the subject distance from front lens of photo lens. This result that "theory image" about 1.5 times larger then "practical image".

Asked by sergdev

Answer

According to the FAQ for the particular calculator you're using, the calculations are performed for a thin lens, and hence the front nodal point of the lens should be used. The author recommends using the front surface of the lens, on the assumption that the front nodal point is somewhere inside the lens, and this assumption will yield a conservative estimate of the DoF.

By the way, note that in lens specifications, focusing distances are specified from the film/sensor plane. This location is often marked with a special symbol on the camera body. So these measurements are not exactly comparable to what the DoF calculator uses.

Answered by coneslayer

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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

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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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What does it really mean that telephoto lenses “flatten” scenes?

Question

This is another perspective distortion question. I understand that perspective distortion is primarily a function of capture and viewing distance, but I am still working on fully understanding the causes of the "flattening" of scenes that is credited to telephoto lenses. This seems to refer to the fact that our ability to discern the differences in size attributable to differences in distance from the lens for subjects beyond a certain distance from the lens. If this is correct, then is it just that with the longer distance from the subject when using a telephoto lens, this compression happens closer to the foreground, rather than in the mid- or background with a wide lens? Is it fair to say that the compression effect occurs at a certain distance from the photographer regardless of lens, but the field of view influences how close to the foreground this effect takes place? Can you share any information to help people like me understand this?

Answer

The flattening or compression effect is not caused by a particular kind of lens, it applies to all lens in the same way. Actually, this property of lenses applies to our own eyes as well. The factor that affects flattening is the distance from the camera to the subjects.

Consider the following exercise:

Place two friends 1 meter away from each other. Place yourself in line with them, so that you are 1 meter away from one of them, and 2 meters away from the other. Move a bit to the side, so that you can see both. From this position, you can easily estimate what the distance between your two friends is. Now walk 100 meters back along the imaginary line determined by your two friends. Again, move a bit to the side so that you can see both. Can you really tell now how far away from each other they are? Will you see your friends in a significantly different way if one of them moves an additional meter apart from the other? You won't, because from 100 meters away, a difference of a meter is not significant.

The thing is, our eyes have a fixed field of view, so in addition to flattening, we experience a scale reduction as we move farther away from the subjects. With a camera you can magnify the effect by using a long lens. But the compression will be exactly the same no matter what lens you use.

There are a few good examples on the Wikipedia page on Perspective Distortion.

From that page, I like this animation of a cube that goes from extreme extension to extreme compression, when changing the focal length and distance to camera:

_{Image copyright 2007 SharkD, licensed CC-BY-SA 3.0}

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How to micro-adjust distance for focusing in macro photography?

Question

I'd like to take macro photos, but I have a problem with the focus.

I put the object on a (light) table, I put my camera on my tripod and take a few shots. Since the depth of field is very small usually I realize that I have to change the distance between the object and my camera. Changing this distance is quite difficult if I have to put my camera 0.5mm closer. Is the a way to easily (and continuously) change this distance? Is there a tripod accessory for this?

I use a reverse ring and a prime lens, but the question probably applies for other types of macro photography.

Answer

In this sort of situation, the normal approach is to use a focusing rail which allows fine and controlled adjustments to be made. There are several available on the market, with some under £50, but it would be possible to make one yourself, if you have the right tools.

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