Front element size and light gathering

[dxqcanada]

The f-number (or strictly the T-stop) is the determining factor for image brightness. The diameter of the entrance pupil is the determining factor for light gathering (the focal length then comes into play to determine the image brightness, because of its effect on magnification).

For two lenses of the same focal length and entrance pupil diameter (and hence the same f-number) but different front element diameters, the light gathering ability and image brightness will be the same.

The diameter of the front element is determined by the relative positions of the entrance pupil and front element*, and by the angle of view. Ideally the entrance pupil should be fully visible through the front element over the entire angle of view of the lens. This means that the closer the front element is to the entrance pupil, the smaller it can be; and the narrower the angle of view the smaller the front element can be when compared to the entrance pupil. There are telephoto designs for which the entrance pupil almost fills the front element, and wide angle designs for which the front element is very large compared to the entrance pupil.

*There are cases where the front element is also the entrance pupil (simple lenses, telescopes), thus the size of the front element determines the entrance pupil diameter.

Best,
Helen

I agree.

That's what I was trying to say with my funnel question ... though I don't think it worked :thumbup:

 

[480sparky]

I'm confused with this also. A telescope that has a bigger lens lets in more light so why not a camera lens...

Well you can, it just ends up looking like this.

:lmao::lmao::lmao:

Pffft.

 

[reedshots]

If you have a 12" funnel with a 1" small end and a 6" funnel with a 1" small end.
Take the 12" small end and put it in front of your left eye.
Take the 6" funnel small end put it in front of your right eye.

If they are equaly bright then it would stand to reason that a Camera front glass lens of 77mm F2.8 would not be any brighter than a 58mm F2.8 lens as the sensor sees it.
The light intensity will be the same, but on the other hand if you use a wide angle (say) 10mm outside day time shot at F8 and use a 200mm at F8 same center target. (much reduced field of view)
This is when you could say the wide angle lens sees more light there for you have to slow down the shutter speed with the 200mm, not because it is not getting the same intensity of light, but because it is not getting as much reflected light.

 

[Garbz]

If they are equaly bright then it would stand to reason that a Camera front glass lens of 77mm F2.8 would not be any brighter than a 58mm F2.8 lens as the sensor sees it.

I can see where you were trying to go with this but it is a somewhat false example as when you look through the funnel there's no adjustment optics. As you look through a funnel nothing outside that 1" area is contributing to the image, however at widest aperture all of the 77mm or 58mm of the front element would usually contribute something to the image.

 

[FunkyRider]

I've been puzzled by the same thing, but this website seems to explain it pretty clearly:

Misconception 10: When a lens is used at a small aperture, only a small part of the front lens is used.
When a lens is set to a small aperture, each image point utilizes a small part of the front element. That part has an area that approximately equals that of the lens entrance pupil. However, different image points depend on different parts of the front lens. The image center uses a small part around the optical axis, other areas of the image use portions that are displaced from the axis. In total a substantial area of the front lens is needed to illuminate the entire image. It is true that the very margins of the front element are often sidelined at small apertures, the more so as the focal length increases.

(Misconceptions in photographic optics)

Have a thought about it, hold a lens in your hand, when viewed from different angles in front, you will see the aperture opening via Different part of the front element, that's why bigger front element doesn't give you more light, it only spreads the light further apart. However, one good thing a large front element might provide is less Optical vignetting, because when viewed from far corners of the lens, front element does not block the aperture as much as a lens when smaller front element.