How to calculate equivalent f/stops on Zoom lenses

I stand corrected.

And as for the shutter opening, I should meant diaphragm opening. (I really need to stop typing when I'm tired.)

sandpiper wrote in post #14909201
err, no. It's the focal length.

The "f" in f/8 stands for "focal length". f/8 is literally "focal length divided by 8" the same way as a quarter (1/4) is 1 divided by 4.

The opening (aperture) at f/8 is NOT the same at 90mm zoom and 35mm zoom. At 90mm the opening will be 11.25mm (90 / 8) and at 35mm it will be a 4.375mm opening.

Oh, and the shutter opening is a fixed size and is at the back of the camera, the aperture is in the lens, a totally separate mechanism.

chauncey wrote in post #14908059
I might take issue with that...you are correct if you assume the same field of view, however...if you zoom in/out without moving, you are changing the field of view, and the exposure, by doing away/adding possible dark/light areas.

No, field of view also has no, direct bearing on exposure. There is a good reason that the exposure triangle does not include focal length; only shutter speed, aperture and ISO/film speed.

The light falling on/reflected off of your subject is what determines your exposure; if you meter a proper exposure for a subject, and the light doesn't change, then the exposure doesn't change either. You could zoom in, zoom out, change lenses or walk across the street and the settings for exposure on that subject will remain the same.

It (zooming) has a subjective effect of, potentially, giving the photographer some additional elements to consider in their exposure, which could cause them to want to adjust +/- on their original metering. This is where your 'additonal light/dark areas' come in.

All of those things will confuse the CAMERA, when shooting in Av/Tv/P modes, and cause it to indicate a different exposure. Which, in turn, causes people to make the mistake of "chasing the needle" and trying to keep the needle centered on the meter, when there was no reason to actually change the exposure to begin with.

If you put a white card and a black card, side by side, in the same lighting and set your exposure for one of those cards, the same exposure is correct for the other one as well.

Alternatively stated, if you take a single, grey card and place it in one, constant light situation, you can set your exposure for that card, using a 17mm lens. Then you could turn off that camera, walk 30ft away and grab my camera, put a 200mm lens on that camera, dial in the same exposure settings and simply take the shot. The exposure would be the same for that card both times; the only thing that would cause that to change is if the amount of light on the card changed.