So I've heard it before a while ago, both here and CR and other forums, and it's come up again lately in the 1dx vs 5d3 thread. The question is whether an EF-S über-tele would have any size/weight (but most importantly) price advantages over the regular EF-mount lenses (let's just focus on build size, not L-lens quality/sealing/etc).

Now, the "entrance pupil" is what determines aperture. So a 300 f/2.8 has an entrance pupil of 300/2.8 = 107mm, a 600 f/4 has 600/4 = 150mm.
But because of different lens designs, the "entrance pupil" is not (always) at the front element, it's a "virtual" pupil that could be anywhere. The best guess is that it's just behind the front element (as used in panoramic photography), if you rotate around the entrance pupil you get parallax-free stitching.

So here's the thing that's confusing me. I've got a Zeiss Sonnar 180mm f/2.8 lens. Originally made for Pentacon Six, frame size of 56 x 56mm, image-circle diameter is 80mm (compare FF image circle is 43mm, 7D is 27mm). Now, it's got a 180/2.8 = 64.3mm diameter entrance pupil. The front element has a filter ring of 86mm, and the lens barrel is pretty much that diameter for most of the lens until it tapers to the mount (inside diameter of the mount is about 55mm, the rear element is about 35mm dia). The front element, at infinity focus, is pretty much bang-on 180mm from the sensor (ie, it's not a true 'telephoto' in the lens-design sense of the word, it's just a "long focus lens").

But here's the thing.
I put it on my 7D, and I can mount any number of Step-Down rings, down to my smallest of 37mm diameter, and it doesn't vignette at all. But it's still a 180/2.8, so the entrance pupil must still have a diameter of 64mm. Does this mean that the entrance pupil is outside the lens? Or that the pupil is inside the lens, light rays are passing the 37mm step-ring, then being bent 'outwards' inside the lens, to the pupil-size of 64mm? (that doesn't make sense to me, but it could happen)

Maybe that lens is a fluke?
Let's try my other long-fast (ish) tele. Jupiter 250mm f/3.5. Alleged entrance pupil size of 250/3.5 = 71mm. Again, step rings to 37mm and it doesn't vignette, infinity or mfd, f/3.5 or f/22, any combination or anywhere in between. The lens is once again about the same as its focal length, 25cm front-element to the sensor.

OK, lets forget the old East german and soviet glass.
EF 70-300 f/4-5.6L. Front element 67mm.
@70mm, f/4.0, entrance pupil = 17mm. Step rings down to 37mm do nothing.
@300mm f/5.6, entrance pupil = 53mm. step ring to 37mm, again, no vignetting.
This one's a zoom, so a bit different, length at 70mm is about 15cm, at 300mm length is 25cm or so.

How about something faster?
EF 100mm f/2.0, pupil = 50mm, front element 58mm. 37mm step-ring? No vignetting.
FL 55 f/1.2, pupil = 46mm, front element 58mm. Nada.
Samyang 35 f/1.4, pupil = 25mm, front threads 77mm (element a bit smaller). Finally, it vignettes. At 52mm step ring, I could possibly see some, but it may just be the edge-of-sensor phenomenon. With 37mm (i've got no sizes in between), f/1.4 it's a bit worse than my ef-s 15-85 @ 15mm f/3.5. At f/22 it looks like a bad fisheye, the sides are blackened a bit too.


So the results get less valid for the wider focal lengths. But the confusion remains the same. How can I have (effectively, using step rings) a front-element diameter of less than the entrance pupil? If the Zeiss Sonnar was remade as an EF-S long-skinny thing, could it have a front element of 37mm (like maybe filter threads of 52mm)? Do I just not have lenses long enough to test (like 4-500mm+), is the theory of "ef-s long teles aren't any smaller" only hold above 400, 500, 800mm? Does anyone want to post me an über tele to try it out? Or test yourself, i know lots of people here would have long-fast lenses and at least one crop-camera. (front elements are so huge you're probably stuck with cutting holes in black paper to emulate front element sizes).

I'm not a lens designer, i've got more chance of building my own diy sensor than building a lens, so maybe i've missed something completely obvious. But still, could we (theoretically) have long-lenses in EF-S mount that are smaller, lighter, cheaper than the EF-mounts? (It's not going to happen from a marketing perspective, at least until the 7D2 or 3D come out with APS-C 45pt f/8 AF, ie never), but still, i'm intrigued (and confused)...


(And if i've learnt one thing from this exercise already, it's that I do up my step-rings too tight, and if you pay $2 for filter wrenches off ebay, they last just as long as you'd expect them too...)