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I was about to ask why the new Canon 24-70mm F/2.8L II doesn't have IS, but then I came across the answer to Why there is no Canon 50mm *IS* lens? and it got me thinking...

(The short version of the linked question's answer: a wide 50mm doesn't need IS)

Let's assume I'm shooting at 200mm and to be "properly exposed" the shutter is set to 1/250. If I frame the same image at 70mm or even 24mm does that mean that I can shoot 1/1000 or faster to achieve the same exposure?

Part of this question comes from the fact that there are multiple versions of the 70-200 with IS and no 24-70 with IS (the 24-105 has IS, but is also F/4 where as the 24-70 is F/2.8 so either the longer focal length or the smaller aperture justifies the need for IS).

Edit: before I can accept an answer I need a clarification and maybe someone should tell me if this should be a different question. Light coming from the scene is constant regardless of the focal length, but apertures are represented as a function of focal length and we usually just refer to the denominator. So if F/4 at 24mm is 24/4 versus F/4 at 200mm is 200/4, does that mean that the amount of light actually touching the sensor is different or did I screw up math again?

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Updated my answer for your edited question. –  Vivek Sep 27 '12 at 14:27

4 Answers 4

up vote 9 down vote accepted

Edit: before I can accept an answer I need a clarification and maybe someone should tell me if this should be a different question. Light coming from the scene is constant regardless of the focal length, but apertures are represented as a function of focal length and we usually just refer to the denominator. So if F/4 at 24mm is 24/4 versus F/4 at 200mm is 200/4, does that mean that the amount of light actually touching the sensor is different or did I screw up math again?

The whole reason to state aperture as a fraction of focal length is to remove focal length from the computation of exposure.

A white wall is reflecting a certain amount of light per unit area. A 50mm f/4.0 lens captures the entire area of the wall, about 12 meters by 8 meters.

A 200mm f/4.0 lens captures a 3m by 2m section of the wall. Now only 1/16th of the light given off by the wall is actually recorded by the camera. So you might expect the image to be darker. However the physical aperture (actually it's the entrance pupil but that's another story) of the 50mm lens is 50/4 = 12.5mm across, whereas the physical aperture of the 200mm lens is 200/4 = 50mm across. The 200mm makes more use of the light coming from the wall due to having a bigger hole, so the exposure ends up the same for the same shutter speed, as both lenses were f/4.

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Doesn't this also explain why a 200mm F4 lens is more expensive than a 50mm F4? The physical aperture is much larger in the former case. –  Kartick Vaddadi Jan 5 at 13:39

No, focal-length has no impact on exposure and is not part of the exposure-triangle. One can add flash to the equation but that is not generally applicable.

The shutter-speed needed to get a sharp image hand-held though is proportional to focal-length. If your camera is on a tripod or other stable platform, then this does not matter.

The common rule of thumb is to use at least 1/50s for a 50mm lens and 1/200s for a 200mm lens, etc. You use the effective focal-length and shoot at 1 over that. High resolution in recent cameras are pushing this limit and people suggest using 2 or 3 times for the effective focal-length now to get a completely sharp image. Again this is just a rule of thumb, it varies depending on you, how tired you are, how much coffee you drank, etc.

Personally I consider stabilization useful at all focal-length and I bought systems with built-in stabilization for general use in addition to a system without for sports photography.

For general photography, one will always encounter a situation with less light and having access to slower shutter-speeds without using a tripod is always better, particularly when the other option it to raise the ISO which lowers dynamic-range.

For any type of action photography, the usual goal is to freeze action and therefore you need a high shutter-speed regardless, so buying an unstabilized lens on an unstabilized body is a good option and can save money.

Portrait photography fall in between with most posed photos needing about 1/90s to get a sharp image and avoiding blur from people blinking and breathing. For this purpose, you can say that a telephoto lens would require IS, in order to get a sharp image at 1/90s from a 200mm lens and a 50mm would not.

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In terms of exposure, no. Given the same ISO and aperture, your shutter speed will be the same, no matter the focal length.

When folks claim that wide/normal lenses don't need stabilization, they're usually talking about the ability to more successfully get sharp images hand-held with wider lenses (because the lower magnification helps mask any camera shake). The common axiom is that you can get an acceptably sharp photograph at a shutter speed of 1/(focal length) seconds. So, 1/50s for a 50mm lens, 1/200s for a 200mm.

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No, exposure comprises of three sides: Aperture, Shutter and ISO. Read: Exposure Triangle.
If you are somehow able to frame the same image in 24mm, 70mm or even 200mm, the light from the scene to your sensor would still be the same.

Edit: Focal length is just the angle of view of your lens. There is no mathematical relation between focal length and aperture. There is only a relation between size of the lens and aperture. Typically larger the aperture (f2.8, f2, f1.4...) larger is the size of the lens.

If you have to focus on the same scene with different focal length, you physically need to move either closer (@24mm) or further (@200mm) from the scene.

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