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I'm a little bit confused regarding variable focal length lenses.

I have a Nikon D90 DSLR. I'm using a Tamron AF-28-80 mm F/3.5-5.6 lens.

Now, if I select Mode A (aperture priority) I can set the aperture I want to use and the camera automatically selects the shutter speed. Suppose I choose f/11 with ISO 200 and then I zoom in on an object using a 70 mm focal length, but as I'm zooming in from 28mm to 70mm the lens also changes the f stop because the f stop also depends on focal length.

When the lens gets to 70 mm it will be using a f stop different from the one I previously chose (it was f/11).

If I'm right, now the camera will calculate exposure for a f/11 stop while the real f stop on the lens will be different. How to make up for this difference?, does the camera take into account the changing in f stops although it was set to a fixed f stop? I hope that my question is clear enough.

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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).

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The short answer is that the lens knows how the effective aperture size varies with focal length so it always closes the iris the correct amount to get an actual f/11, regardless of the focal length, ensuring you get the correct exposure.

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