35mm still camera lens and 16mm movie camera lens both 20mm at the back

[Stephen Perera]

...so out of interest I measured the rear element lens of the LEICA R 28mm lens and it measures 20mm...the same as the Cooke Varokinetal 9-50mm rear element lens.....compared to the Zeiss 10-100mm lens which measures 16mm......all measure crudely with a normal ruler........what gives?....can you explain what I'm seeing? never given this any thought....

Edited August 3, 2017 by Stephen Perera

[Dom Jaeger]

There is some correlation between rear element diameters and the size of the image circle (format coverage), but it's not definitive. Speed also plays a role, but different designs can be wildly different. Telecentric modern designs need a rear element at least as large as the format while older designs could be much smaller, some older 35mm format cine lenses have tiny rear elements.

[Stephen Perera]

There is some correlation between rear element diameters and the size of the image circle (format coverage), but it's not definitive. Speed also plays a role, but different designs can be wildly different. Telecentric modern designs need a rear element at least as large as the format while older designs could be much smaller, some older 35mm format cine lenses have tiny rear elements.

 

what do you mean by 'speed also plays a role'....the Cooke is T2.8 and has a 20mm rear element diameter....

[Dom Jaeger]

I'm not a lens designer but a faster lens needs a larger maximum aperture, which in turn needs larger elements. The front element size is directly related to the focal length and maximum aperture (or more correctly the entrance pupil) but the rear element size as I understand it is determined by other factors as well, including format coverage and distance from the sensor plane.

 

But experience with lenses tells me that although larger format lenses are often physically larger (compare medium format lenses with 16mm C mounts for example), the rule is not hard and fast. I've come across tiny lenses for cine and stills 35mm and relatively large lenses (particularly zooms) for 16mm, and rear elements can be different sizes even for similar speed focal lengths designed for the same format, so individual lens design also plays a very large role.

 

As I mentioned before, modern telecentric lenses (more often "near" telecentric) designed for digital sensors tend to have larger rear elements than older designs to allow edge rays to hit the sensor more perpendicularly.