# Random Question: Rear Lens Mount Design



## unfocused (Oct 2, 2015)

Okay, this it totally random, but it's something I've always wondered about (and since it looks like we are months away from any decent rumors anyway, might as well talk about something) – I've always wondered what determines the shape of the mounting behind the rear lens element on Canon lenses.

If you look at the back of the 24-105 "L" there is a little rectangle with rounded corners behind the lens element (closest to the camera)

One the 70-300 "L" it's just a round circle. Same with the 100-400 "L" I and the 100 "L" Macro.

The 70-200 II IS has a squarish plate that is also rounded in the corners. 

The 17-40 "L" is round with a flat bottom. 

So, what's up with that? Anybody know? Is there a reason behind the different shapes or is this just Canon designers screwing with us?


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## chromophore (Oct 2, 2015)

The focal length and f-number of the lens puts design constraints on the size and position of the rearmost element relative to the image plane. So for example, a telephoto prime like a 300/2.8 will tend to have a more deeply recessed element compared to a fast-aperture retrofocus lens like a 35/1.4.

The shape of the rear baffle is also deliberate. Lenses like the 24-105/4L IS will have a rectangular baffle in the rear because the f-number is relatively slow (f/4 is slow), and the complexity of the zoom design means that controlling non-image forming light will be important for this focal length range. You will also see this in longer telephoto zooms.

A deeply recessed element will tend to have the space in the barrel flocked. A slower-aperture lens can afford to have a rear baffle to block out light that will not form a useful image on the sensor. The overall design considerations are not arbitrary and are related to the question of, "how should the rear baffle be shaped so that as much unnecessary light is blocked while allowing all necessary light to reach the sensor, for the fastest aperture in the design and for all focal lengths in the design?"


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## neuroanatomist (Oct 2, 2015)

Mostly the wide and normal lenses are round. When they're not, it's for a reason. For the 24-105, presumably it's a baffle to reduce reflections (that lens also had a recall of early units for a flare issue). The flat bottom of the 17-40 is actually part of the gelatin filter holder (you see the same thing on other rear-filterable lenses like the 8-15L fisheye). 

The tele lenses seem to have that 'squarish with rounded corners' and the rear element is recessed relative to the mount (my 135L and 600L are like that). The rear element of the retracted 70-300L is not recessed, I guess that's why ithe mount opening is round.


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## unfocused (Oct 2, 2015)

Thanks guys. An interesting bit of trivia.


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