Question about diameter and aperture.

[Omnis]

I'm guessing the answer is no, but this question popped into my head anyway:

If you have two f/2 lenses, but one lens is 77mm in diameter, and the other is 63mm (or smaller), then will the 77mm pick up more light even though both lenses are the same aperture?

 

[Pupik]

Technically, the 77mm will pick up more light, as the f-stop number is a ratio of opening and focal length.

See: http://en.wikipedia.org/wiki/F-number for a better explanation.

 

[Omnis]

Well, I know it's a ratio, but I thought it was the ratio of the opening at the back, which, as far as I know, is standardized among lens mounts.

So, I guess I should clarify by asking whether the diameter of the front opening makes more light reach the sensor when at a constant aperture.

 

[GilesGuthrie]

The lens is configured so that the exact size of the aperture opening will deliver the same amount of light to the sensor irrespective of the barrel size. It's just that in general, because the aperture is expressed as a ratio of the focal length, the longer lenses need wider barrels to produce the same aperture as shorter lenses.

 

[Omnis]

The lens is configured so that the exact size of the aperture opening will deliver the same amount of light to the sensor irrespective of the barrel size. It's just that in general, because the aperture is expressed as a ratio of the focal length, the longer lenses need wider barrels to produce the same aperture as shorter lenses.

Ah, yeah, that makes sense. So, in other words, am I correct in thinking that extra width (as in, beyond what is needed for aperture and lens length) does not have an effect on capturing extra light?

 

[GilesGuthrie]

It could be to prevent vignetting. When Canon updated the 16-35 F/2.8L, one of the things they did was to increase the filter thread size by 5mm to 82mm for that very reason. Makes filters expensive though. £180 for a CPL.

 

[Omnis]

It could be to prevent vignetting. When Canon updated the 16-35 F/2.8L, one of the things they did was to increase the filter thread size by 5mm to 82mm for that very reason. Makes filters expensive though. £180 for a CPL.

Oh, I didn't even think of vignetting. Well, that pretty much satisfies my curiosity about front-element size. Doesn't do anything for speed, but decreases light falloff. 👍