Aperture Question
Very stupid question: Is an aperture of the same value the same for all lenses? Meaning, if I have a 50mm prime lens set to an aperture of 2.8 in an indoor shooting situation and find that sufficient, will 2.8 on a zoom lens at, say, 200mm going to be just as bright as a 50mm at 2.8? Or, think of it this way, if 2.8 on a short lens at 50mm yields a histogram that bunches in the center, will 2.8 on another lens at 200mm in the exact same situation move the histogram to the left?.

Wondering if my question will make any sense to anyone but me............

Comments (7)

It is true that this is a question that may look silly but it's just a misunderstanding..

First of all you must understand that the aperture is not 2.8 but f/2.8 and 2.8 is called the f-number or f-stop. So at 50 mm you have 50/2.8=17.9 mm aperture and at 200 mm you have 200/2.8=71.4 mm. At the same f-stop (f/2.8) you will have the same parameters (ISO speed and shutter speed) both on 50 mm lens and 200 mm lens. But depending on the scene the light may not be the same because the FOV is smaller at 200 mm than at 50 mm so this will be exactly true just for a gray card that fills the frame at 50 mm.VictorBucuresti, Romania

Comment #1

The answer, in brief, is yes. Sorta. F/ 2.8 will expose the same regardless of focal length..

The question is whether you are doing something like shooting a white person with dark clothing on in the 50mm, vs only shooting their light face with the 200mm, in which case average metering will show a different exposure value.

The blog:

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Comment #2

Thanks for the replies! I've been studying Understanding Exposure, but it's tough to wrap my mind around some of it still....well, okay, a LOT of it!.

As Chevy Chase once said, "It was my understanding that there would be no math!"..

Comment #3

In reality f/8 is always f/8. But if you purchase different lens since they are a different focal length the f/8 will be different if you look at the lens when it is stopped down. But the f/8 amonunt of light is what hits the film or what ever is the recording medium. In reallity if you have a 200 mm lens and set it to f/8 and a 50 mm lens and set it to f/8. What you have to do is divide the F/ stop you choose into the focal lenght and you will get the actual diameter that that f/ stop is. The size will change with each lens depending on the focal length...

Comment #4

Disney Jim,.

One way to look at your question is to test on a zoom lens that goes from 50-200mm. Set camera in "Manual" with aperture set at say f/5.6 (available at both 50&200mm) + shutter and ISO set to suit - then take two shots (by focusing on same spot) one at 50mm and the other at 200mm..

Will depend on what you're shooting ie what's actually in each picture as to what the effects on the histograms will be..


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Comment #5

F/2.8 is always F/2.8 however most modern zoom lenses do not have a constant maximum aperture. A hypothetical 50-200 zoom may have an f/2.8 maximum aperture at 50mm but have a maximum aperture of f/4.5 at 200mm. On the other hand f/5.6 will be the same exposure whether your at 50mm or 200mm...

Comment #6

The value of f-stops is that it provides a constant reference for the intensity of light striking the film or sensor, regardless of focal length. Light intensity falls off with longer focal lengths, so a wider diaphragm setting is needed to compensate for this. When you see f/2.8, it's referring to a ratio in which the focal length divided by 2.8 yields the diameter of the lens opening. Your 50mm lens at f/2.8 would have an effective opening of about 18mm, while the 200mm lens would need about 72mm to give the same light intensity. Below is a link that describes this and also does the math for you.

Comment #7

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This question was taken from a support group/message board and re-posted here so others can learn from it.


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