Do I need to change exposure when I zoom to a different focal length?
Asked 9/1/2018
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If ISO and shutter speed stay the same, do I need to change aperture or exposure just because I change focal length on a zoom lens? Is there a chart that tells you how many stops to allow for a change in focal length, similar to ND filter charts?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
8y ago
2 Answers
3
MC's answer points out how the fstop comes to be. But, how does it affect light?
Let's take the first example of a 70-200 f/2.8 lens. At 70mm, the entrance pupil is 25mm in diameter, while at 200mm it is 71.5mm in diameter. (I know you said no math, but bear with me).
The ratio between 70 and 25 is 2.8 while the ratio between 200 and 71.5 is...also 2.8!.
The lens is designed so that, in order to compensate for the light loss effects of increasing focal length, a wider entrance pupil is used. The beauty of this and of using the fstop system is that we don't have more math to do - we can simply calculate exposure based on f/2.8 while at 70mm or at 200mm and know that it's the same amount of light.
Originally by user67377. Source · Licensed CC BY-SA 4.0
user67377
8y ago
0
Generated from our catalog & community — verify before relying on it.
Usually, no chart is needed. Exposure is determined by the lens’s f-number, not focal length by itself.
If your lens stays at the same f-number when you zoom—such as a constant-aperture zoom like 70-200mm f/2.8—then the amount of light reaching the sensor stays the same at 70mm and 200mm. So with the same ISO, shutter speed, and f/2.8, exposure does not change.
If you have a variable-aperture zoom, the maximum aperture gets smaller as you zoom in (for example, f/3.5-5.6). In that case, exposure changes only because the f-number changes, not because focal length itself needs its own separate stop chart.
So the practical answer is:
- same f-number after zooming: no exposure compensation needed
- different f-number after zooming: adjust exposure by the normal aperture-stop difference
There isn’t a separate “focal length to stop compensation” table for ordinary exposure. Just use the aperture shown by the lens/camera meter.
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