Aperature Question

[minatophase3]

This may be a dumb question but I'll ask anyway. Does the diameter of the lens make a difference as to how much light is let into the camera at a given aperature?

What I was wondering was this: I have a 70-200 f4.0L lens, it has a diameter of 67mm. The 70-200 2.8L has a diameter of 77mm. So at f4.0, will the 2.8L let in more light because it has a larger diamter or doesn't it matter?

Thanks,
Tim

[FrenchAmateur]

The quantity of light only depends of the aperture you have selected.
The light is the same for instance at F: 5,6 with the f:4 70-200 or the f:2,8 70-200.
The large diameter of the front lens is only the indication that a wide aperture is available IF necessary.

[minatophase3]

That's kind of what I thought but I wasn't sure. Thanks for clearing that up :D .

Tim

[SoCal69]

It doesn't matter. f-stop is a ratio of focal length/actual aperature size, so at the same focal length, whether you are using the 4.0 lens or the 2.8 lens, the aperature diameter (not the lens diameter) should be the same. Note that the diameter of the physical lens doesn't matter, it's the diameter of the aperature in the internal diaphram. I think I got this right...if I didn't feel free to jump all over me :P

[hmhm]

f/4 is f/4 is f/4, the same amount of light comes in no matter what lens you're using, as long as the lens is really accurately "doing" f/4 as it's told to (which assumes, of course, that it's _capable_ of f/4). f/4 on a 70-200/2.8 at 70mm lets in the same amount of light as f/4 on a 70-200/4 at 70mm which lets in the same amount of light as f/4 on a 16-35/2.8 at 16mm. In other words, f/4 is just as bright regardless of the focal length and regardless of the max aperture of the lens (in effect, that f/4 number is a ratio, and it already "accounts" for focal length).

Hand-held incident exposure meters measure the brightness of the light and input from you the ISO and the aperture, and they spit out a recommended shutter speed to use, without needing to ask you what kind of lens you'll be using or what focal length.

Note that for a lens to be capable of a particular max aperture at a given focal length, the lens barrel must be some minimum diameter (roughly focal-length/f-number, e.g. f/4 at 200mm requires a diameter of at least 50mm). As you set the aperture to be smaller and smaller (higher and higher f-numbers), a diaphragm in the lens closes down smaller and smaller. To see this, put the camera in Av mode, hold down the depth of field preview button, look into the lens, and dial in different apertures.

So you can think of f/4 on an f/4 lens as being close to "letting in all the light", while f/4 on an f/2.8 lens requires the diaphragm to close down to an opening that only lets in about half the light, making a roundish opening comparable in area to that max f/4 lens wide open.
-harry