What does “N stops” mean when describing an image stabilizer?

Question

If you ever purchased any lens with IS then you have heard this term like "3 Stop Stabilization" or "4 Stop Stabilization". Now, I am familiar with this Stop for ISO and Aperture. But this I don't understand — how does that apply to the stabilization?

Asked by Hasin Hayder

Answer

In this context, a "stop" refers to exposure duration. As an approximation, the distance a handheld camera moves during an exposure is directly proportional to the duration of the exposure: double the time the shutter is open and you double the movement. That (again approximately) doubles the amount of blurring in the image.

Normally, to achieve an acceptably small blur, you have to limit the exposure duration. (To give you a sense of what is acceptable, one popular rule of thumb is that the upper limit on the time is 1 second divided by the focal length as expressed in millimeters.) Now, suppose that IS can reduce the total amount of camera travel to 1/x times its former amount. This means you can probably afford to keep the shutter open x times longer than you used to and still achieve your standard of sharpness. That change in exposure is converted to an f-stop equivalent: each doubling is one f-stop.

(Notice how this way of expressing IS is highly personal: if you are a steady shooter with low sharpness requirements, maybe you're already using much longer exposure times than other people, but even so, you still get the full multiplication by x. The improvement is always relative to your standards and your skills; it's not an absolute.)

It's now easy to work out what the claims are trying to imply: 3 stops of stabilization is 3 doublings in exposure time, or x = 2*2*2 = 8, and 4 stops is 4 doublings, or x = 16. You are meant to think "wow! Whereas before I was limited to (say) 1/125 second for my 125 mm lens, now I can hand-hold it down to 8*1/125 = 1/15 or 16*1/125 = 1/8 second" (as the case may be). That's basically right, but don't forget your subjects might be moving too, and no amount of IS is going to eliminate (or even reduce) the blur from their motion independent of the camera.

Answered by whuber

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Why do compact digital cameras not have the aperture range of DSLRs?

Question

Why is it that on compact digital cameras the aperture never seems to go any smaller than about F8 ? Even on high-end compacts such as the Cannon G10 or Panasonic LX5. Is there some practical or physical limitation due to the size of the camera or the CCD which prevents this from being possible?

Asked by mdresser

Answer

Although the relative aperture numbers — the ƒ stops — are the same regardless of format, the actual focal lengths of the lenses on small cameras are quite low: 5mm or 6mm at the wide end. That in turn means that the real aperture is small, which is why the diffraction limit kicks in sooner.

The smaller format also means that depth of field is hugely increased — even wide open at f/2.8, a camera like the Canon G10 has an infinite depth of field if you're focusing farther away than a few feet. So, there's not much difference in that aspect of changing aperture, so from that point of view there's no point even bothering. And that's presumably why there's usually not many choices besides wide open and one closed-down stop like f/8. (Because everything is small, and competitive price pressure significant, adding the mechanics for more intermediate stops is easily deemed not worth it.)

The other aspect of a smaller aperture is, of course, controlling exposure in bright light, without artificially dropping ISO beyond the sensor base or using very high shutter speeds. Some compact cameras actually use a dark neutral-density filter instead of closing down the aperture, specifically to avoid issues with diffraction.

Answered by mattdm

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Why are the area of aperture, focal length, and amount of light specified in these terms?

Question

So I'm looking at the wikipedia article on Aperture, and I'm a bit confused. I understand aperture to be the diameter of the hole that lets light in. In the article, someone states that

The amount of light captured by a lens is proportional to the area of the aperture, equal to:

Area = pi ( focal length /(2 * f number) )^2

But in the f number article, they define f number as

f = focal length / aperture

It then seems trivial to substitute in:

Area = pi ( focal length / (2 * focal length / aperture) )^2

Area = pi ( focal length * aperture / (2 * focal length) )^2

Area = pi ( aperture / 2)^2

Area = pi ( radius )^2

So, my question is: Why did they bother putting something so blatantly obvious into the article? Didn't we already know that the area of the aperture was pi * r^2? Is this just me reading too much into a wikipedia article?

Asked by Chaosphere2112

Answer

The focal length and f-number are photographically relevant quantities, so the formula is expressed in terms of those convenient variables. Photographers don't generally know the radius of their lens aperture for every f-stop.

Answered by coneslayer

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Where does the term “f-stop” come from?

Question

Does anyone know the origins of the term?

Asked by Casebash

Answer

This is a matter of some debate, and the truth is that you can pick up any number of photography books (or visit any number of photography websites) and read a variety of answers to this question, the most common of which tend to be:

• Focal Stop
• Focal Ratio
• Factorial System
• Fractional Stop
• Fractional Diameter
• Finestra Stop (Finestra is Italian for 'window')

As with many historical 'facts,' the truth is quite a bit messier than many of these sources would seem to indicate, and the real reason that we use 'f-stops' today to some extent defies a quick explanation.

The Wikipedia F-Number article actually has a rather complete (and well cited) history of how we got here, but some of the 'high points' are:

• In the late 1800's there were a variety of aperture systems which all operated more-or-less the way we are all familiar with, which is to say that the concept of letting more or less light through a lens via an aperture system was being used extensively, even if the different systems used different naming conventions. (Well-known systems at the time included Sutton and Dawson's 'apertal ratio,' and Dallmeyer's 'intensity ratio')
• In 1858 John Waterhouse invents a system of metal discs with different sized holes to act as the aperture which literally get dropped into a slot in a lens. He called them 'Waterhouse Stops' because the discs were literally stopping light from entering the camera. This is likely the first instance of the use of the word 'stop' as it relates to aperture.
• In 1895 John A. Hodges first champions the 'fractional number' system (which he abbreviated to 'F-number') in defiance of the Photographic Society of Great Britan's use of the 'Uniform System (U.S.)' This is the first recorded instance of the 'fractional number' and is likely the original meaning of the 'f' in F-stop.
• In 1901 C. Welborne Piper first proposes a unified system of describing aperture marking called the 'f-diamater' (or fractional diameter) after observing similarities between a half-dozen of the more popular methods of the day.
• From the early 1900's through about 1920, the most common way to refer to Piper's unifying system was as the 'f-number.'
• In 1961 the American national Standards Institute (ASA) officially adopted the 'f-number' as the specification for photoelectric photographic meters. Essentially codifying the term and making 'f-number' the common phrase used to describe aperture for camera manufacturers of the day.
• While the phrase 'f-number' has morphed somewhat since 1961 to the now more commonly used 'f-stop' we use today, there is currently no standardized and generally accepted definition for the 'f' in f-stop. In fact, it has variously been referred to as focal, factorial, fractional, and many other designations by a wide variety of sources over the last 120ish years to the point where even though the system itself has been standardized, there simply is no singular recognized designation for the 'f' in f-stop.

Answered by Jay Lance Photography

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Does focus breathing make a lens slower when close focusing?

Question

I've heard that focal length of some lenses will become noticeably longer when focusing to a close distance, an effect called "focus breathing". Since f-number is focal length divided by diameter of physical aperture and aperture size does not change, it seems logical to conclude that such lens should become slower when focusing close.

Is that really so, or is there something I'm overlooking?

Answer

That is true, and very noticeable in macro lenses. For example a Nikon 105mm f/2.8 VR (at infinity) is f/4.8 at it's closest focus distance of 30cm or so.

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What shutter speed to use when shooting video?

Question

My camera is Canon 60D and I will be using it to shoot a short parkour video, that means:

• a lot of motion, moving subjects, moving camera
• focal length usually from 24 mm to 75 mm, but there will also be a scenes shot at 200 mm
• a lot will be shot in 1080 at 24 fps (or should I use 25 fps or 30 fps ?)
• some scenes will be shot in 720 at 60 fps

Accessories:

• I do have a steadicam
• I have got 1.8 lenses (24 f/1.8, 35 f/1.8, 50 f/1.8) so I can use low f-stops.

Questions:

• What shutter speed should I use? I have read that it should be double the FPS. Why so? What if I break this rule?
• When I use low shutter speeds (1/120) and I'm shooting during the day, I need f-stop at around f/12. Does this mean that my f/1.8 lens are useless in this case and I can use any bundle f/4 lenses instead?
• Any tips on improving whatever I wrote above?

Answer

The reason that many people will recommend to shoot at double your framerate is because then the 'shutter angle' is 180 degrees.

This is back from when film cameras were used. To cover the negatives whilst the next frame was 'wound on', a disc would spin, blocking the light as the negative moved. When the negative was in place, this disc would spin open again. Wikipedia has a good animation of this here.

The amount of covering the disc provides and the framerate give this 180 degrees that is as close as you get to a 'standard' in filmmaking. For further reading on this (or a better explanation!) I'd suggest looking over here.

As for a specific recommendation of shutter speed, it depends on the look you want. For example, some action films (using these as an example because they have lots of movement within the frame) will use a very fast shutter speed for sharp images, but being professional films they are still shooting at a framerate (or 23.976). Others will use the slowest they can get away with, and this will give them lots of motion blur within the shot. A good film that shows examples of both of these is The Bourne Identity.

TL;DR - It's a creative decision.

As for the problem with your F-stops, this is (unfortunately, because you have some real nice lenses) just something you're going to have to live with. Consider whether a slow shutter speed is absolutely necessary for these scenes. Using a faster one could allow you to use your f1.8. Also, depending on the camera you're shooting on, sometimes setting it to 'Auto ISO' can actually let the camera reduce the ISO below the minimum (100, normally) using some metadata hacks or something. (This may just be for stills, though!)

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How many stops can a digital camera capture?

Question

How many stops can a digital camera capture?

I wondering if some one know, or have measure, the stops also for the negative or positive film, the human eyes, the monitor, the television, etc...

Thank you in advanced.

Answer

The answer will most probably change in time.

Current top cameras are said to capture around 10-11 stops at base ISO, less at higher ISOs, see DPReview tests of Nikon D3X for example. As a sidenote - you won't probably like the pictures that are processed to measure the maximum dynamic range, they'll simply lack contrast you'd expect from "normal" picture.

Negative film is said to have up to 9-10 stops of latitude and reversal film around 5-6 stops. What you're actually able to see also depends on the medium used to present the picture - prints from negatives are usually limited to what the paper can reproduce, slides to what can be projected and digital images to what the monitor is able to show.

Update: I've made a DR test on my 5D (the old one, not mk2) and with default settings I get 9 stops of usable range, with special processing 11+ (there is probably more room on the shadows' side):

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What is an easy way to remember the full stop scale?

Question

If you were teaching someone new to photography the full stop scales, is there a better way then flat out memorizing these values? Does anyone have an easy way that they remember the scale? Would it make more sense as a type of mathematical equation without getting overly complex?

Aperture Full Stops:

1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45, 64

Shutter Full Stops:

1/1000s, 1/500s, 1/250s, 1/125s, 1/60s, 1/30s, 1/15s, 1/8s, 1/4s, 1/2s, 1s

Obviously the shutter stop scale is very easy to remember, but how can I use the square root to determine the aperture easily in my head?

Answer

F-stops deal with doubling/halving the amount of light hitting the sensor. Everything revolves around twos.

With the shutter speed, it's easy to understand, as you say. Every shutter f-stop is (roughly) half/double the amount of time as the previous one. Personally, I don't even bother paying attention to the numerator ("1/") part of the shutter speed; I've drilled it into my head that bigger denominator = faster = less light = darker exposure.

Note that shutter speeds aren't exactly doubles/halves. I think that this is just because manufacturers think people like to see "round" numbers. At the fast end, that means 1000, 500, 250. At the slow end, you need more accuracy, so you have true halving of speed (1, 2, 4, 8). Then, they have to make the numbers meet in the middle, so they start to fudge the numbers a bit (15 is almost 8 * 2, 125 is almost 60 * 2). (I'm a programmer, so personally, I'm fine with seeing a shutter speed of 1/1024s :-) )

Aperture is a bit trickier. Double the light means doubling the area of the aperture, which is where the squares/roots come into play (Area of a circle = pi * r^2). That's a pain to mentally calculate, but there is an easier trick to consider: every two stops represents a doubling (or halving) of the aperture's f-number:

1, 2, 4, 8, 16, 32, 64.

If you know those, then you can guesstimate the in-between stops by calculating slightly less than half of the average of the surrounding f-stops:

1.5 -> 1.4, 3 -> 2.8, 6 -> 5.6, 12 -> 11, 24 -> 22, 48 -> 45.

As with shutter speed, bigger number = smaller aperture = less light = darker exposure.

Something similar happens with ISO. Each doubling of the ISO value represents a stop, which you can trade off (with consequences) with stops of shutter and aperture. Note that this transition is reversed though: bigger number = more sensitive = more light = brighter exposure. The common ISOs are:

50, 100, 200, 400, 800, 1600, 3200, 6400, 12800

And just to be complete, there's another similar scale with flash power:

1 (Full power), 1/2 power, 1/4 power, 1/8, 1/16, 1/32, 1/64, 1/128

This is very much like shutter: bigger denominators (forget the numerators) = less power = less light = darker exposure. (Note that true powers of two is fine here).

Honestly though, I don't bother with any of these mnemonics myself. I usually do "three clicks of my control wheels on my camera" when I want to go up/down one stop. (My camera, and many others, set one click of the control wheel to be 1/3 of a stop.) The absolute numbers aren't usually as important as the amount of change relative to "where you are now".

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