How to calculate equivalent f/stops on Zoom lenses

Okay so I have searched the internet quite a bit and can't find an answer. I know that zoom lenses have different maximum apertures usually listed on the lens. What I want to know is, how do I calculate an equivalent reading if I am shooting something at 35mm at f/8, then zoom to 90mm? Obviously the photo is underexposed if I shoot at 90mm at f/8, so is there a way or an equation to figure out the correct equivalent exposure?
Thanks!

You may need to clarify your question. Yes, *some* zoom lenses have different maximum apertures at the two extreme focal lengths, but 35mm f/8 is the same exposure as 90mm f/8. Exposure will be determined by f-stop + shutter speed + ISO, and not focal length.

Dave

Yes, just to make this clear, the f-stop is a ratio, so the aperture (size of the lens opening) will be larger at 90mm than at 35mm for the same f-stop giving the same exposure.

I think I got the right definition of aperture there, but in any case the principle is correct.

f/8 is f/8, so the focal length is irrelevant. A correct exposure at 24mm will likely be a correct exposure at 90mm if you carry over the same f-stop, shutter speed and ISO.

Just to add, it is all relative to the size of the lens. Ie: a bigger focal length requires a bigger aperture but its in a direct relation to the lens to achieve the same light input. What is underexposed on one lens at F8 is for simplicity sake equally underexposed on any lens at f8 using the same shutter speed and ISO.

read this:
http://www.punitsinha.​com …perture_focal_l​ength.html

As for figuring out proper exposure based on aperture you can try to use the sunny 16 rule:
http://en.wikipedia.or​g/wiki/Sunny_16_rule

pinbingham wrote in post #14903396
Obviously the photo is underexposed if I shoot at 90mm at f/8,

Where did you get that idea?

Focal length has no appreciable effect on exposure.

pinbingham wrote in post #14903396
Okay so I have searched the internet quite a bit and can't find an answer. I know that zoom lenses have different maximum apertures usually listed on the lens. What I want to know is, how do I calculate an equivalent reading if I am shooting something at 35mm at f/8, then zoom to 90mm? Obviously the photo is underexposed if I shoot at 90mm at f/8, so is there a way or an equation to figure out the correct equivalent exposure?
Thanks!

f8 is f8, focal length is immaterial. If the image is underexposed at 90mm, then that portion of the image will be underexposed at 35mm.

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.

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.

Yeeees, OK, if you say so. I'd say thats pretty unlikely to occur each and every exposure!

I'm with the majority here, in that the aperture size is the same if it reports as the same. The camera is not stupid, it knows what lens you are using and the reported metering info in the viewfinder includes the f/no.

I imagine the OP is thinking about a lens like the 100-400 L which offers f/4-5.6 depending on the focal length. Its a non issue, as the metering suggestion copes with it and you the photographer should always exercise those brain cells before blindly firing anyway.

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.

The changes in exposure you're talking about aren't a result of changing focal length per se.

Sirrith wrote in post #14908191
The changes in exposure you're talking about aren't a result of changing focal length per se.

Agreed, if one happens to own a fixed aperture then their exposure should remain the same. But to save money usually you lose a stop or two with the average telephoto lens.

Point of clarity: f-stop refers to the ratio of the shutter opening to the *diameter* of the lens not the focal length. Therefore f/8 at 90mm zoom is the same opening as f/8 at 35mm zoom. The light falling on the sensor or film will be the same (provided the re-framed composition has similar lighting/contrast).

essvee wrote in post #14909086
Point of clarity: f-stop refers to the ratio of the shutter opening to the *diameter* of the lens not the focal length. Therefore f/8 at 90mm zoom is the same opening as f/8 at 35mm zoom. The light falling on the sensor or film will be the same (provided the re-framed composition has similar lighting/contrast).

No, in f/4 on a 200mm lens, the aperture is 50mm...aperture/FL is 50/200 is 1:4 is f/4

In optics, the f-number (sometimes called focal ratio, f-ratio, f-stop, or relative aperture[1]) of an optical system is the ratio of the lens's focal length to the diameter of the entrance pupil

essvee wrote in post #14909086
Point of clarity: f-stop refers to the ratio of the shutter opening to the *diameter* of the lens not the focal length. Therefore f/8 at 90mm zoom is the same opening as f/8 at 35mm zoom. The light falling on the sensor or film will be the same (provided the re-framed composition has similar lighting/contrast).

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.

sandpiper wrote in post #14909201
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.

Sandpiper,

I owe you one! You have increased my photo knowledge. I never knew that the "f" in f/no was shorthand for focal length. Yes, I knew the ratio bit, but for some reason I never asked the obvious question.