Why does a variable-aperture zoom change f-number when I zoom, and will Aperture Priority still expose correctly?
Asked 9/2/2012
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I’m using a Nikon D90 with a Tamron 28–80mm f/3.5–5.6 lens and I’m confused about how aperture works on a variable-aperture zoom.
In Aperture Priority mode, if I set f/11 and then zoom from 28mm toward 70–80mm, the lens’s maximum aperture changes with focal length. Does that mean the actual aperture is no longer f/11 even though I selected it? If the lens changes while zooming, will the camera still meter and expose correctly, or do I need to compensate for anything myself?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
13y ago
2 Answers
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The maximum aperture (minimum f stop) of your lens is different at different lenghts. For this reason, if you set up 3.5 at 20mm and then zoom to 80mm it will be automatically reduced to 5.6 (the maximum possible).
If you set up an f stop high enough for your maximum focal, it will not change as you zoom: have you tried it in practice? What will change with the focal length is the depth of field, for a given fixed number.
What could be happening is that the camera tries to handle iso and shutter speed to avoid going to shutter speed too high (which are difficult to handle, as a rule of thumb you can hand-hold a lens until the shutter speed is 1/focal) or to ISO too high (which are prone to show visible noise).
I will add that there are zoom lenses with the same maximum aperture at all focal lengths (for instance f/4 or f/2.8).
Originally by user5032. Source · Licensed CC BY-SA 4.0
user5032
13y ago
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Generated from our catalog & community — verify before relying on it.
Yes, the camera will still expose correctly.
On a variable-aperture zoom like 28–80mm f/3.5–5.6, the changing numbers refer to the lens’s maximum aperture at different focal lengths. At the wide end, the lens can open as wide as f/3.5; at the long end, only as wide as f/5.6.
If you set f/11, that setting can usually be maintained throughout the zoom range because f/11 is smaller than the lens’s maximum aperture at both ends. The lens and camera communicate, and the lens adjusts the iris appropriately for the chosen focal length so that it is still actually f/11.
The only time the camera will force a change is if you choose an aperture wider than the lens can provide at the new focal length—for example, setting f/3.5 at the wide end and then zooming in. In that case, the camera/lens will shift to the widest available aperture at that focal length.
So in Aperture Priority, you normally do not need to compensate manually. What will change as you zoom is the maximum available aperture, and also depth of field and shutter speed behavior.
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