Diameter of rear element for fast lenses?

[steve_chan5]

Out of pedantic curiousity, I'm trying to figure out the

shallowest angle of incidence for light hitting the sensor on the

Canon 1DS. I don't know if anyone has that answer handy, so I was

hoping that folks could help me out with a few measurements?

 

I'm curious to know 2 values (in millimeters):

 

1) The radius of the largest rear element on a Canon lens - I'm

guessing that the 85/1.2 may be the winner here. But maybe there are

lenses with bigger rear elements? Failing that, just having the size

of an F1.4 or F1.8 lens would probably be helpful.

 

2) The back focal length for the Canon mounted wide angle lens.

I'm guessing that this is going to be about equal to the lens

registration distance. I think it is 44mm.

 

Since the CCD is full frame, it should be 24mm x 36mm. It should

be pretty straightforward to calculate the angle of incidence assuming

the light is coming from the outer edge of the rear element and

striking the edge of the CCD diametrically opposite.

 

Thanks,

 

Steve

[minh_thai]

Steve<p>

I can't help you w/ any of the measurement, but out of curiousity, are you asking this question in the light of the Olympus E1 system?<p>

Regards, M.

[ron c sunshine coast,qld,a]

Kind of interesting question,allthough these days i am more interested in fixing the damage done by those that think sensors MUST have light rays hitting them perpendicular to the surface.Do you have 1Ds? It would be an interesting experiment to try some weird lenses to see what kind of angle COULD be acheived without vignetting or other problems

[steve_dunn2]

<p>My 50/1.4 USM's rear element is about 30mm in diameter. The inside diameter of the lens mount is about 48mm, and the electronic contacts extend about 3mm inside that, so assuming it would be symmetrical, I'm guessing that the largest rear element* should be about 42mm. The 50/1.0L and 85/1.2L would probably both be in contention for this one; the block diagrams for both show large rear elements mounted very far back, right near the lensmount. I've seen a picture of the rear end of the 50/1.0L somewhere and the rear element really does fill up the back end of the lens barrel.</p>

 

<p>Can't help with you question #2; sorry.</p>

 

<p>*: that is, for lenses whose rear elements are right at the back of the lens. Some of the telephoto lenses' rear elements are probably a fair bit larger, but they're mounted a long way up the barrel, ruling them out of the sweepstakes for the shallowest angle of incidence.</p>

[steve_chan5]

Thanks for the responses. I'm asking because I'm trying to evaluate the Leica claim that the short back focus of Leica M lenses makes the angle of incidence too shallow, leading to unacceptable light dropoff at the edges of the frame.

 

So, if I didn't screw up the trig, it looks like with a 30mm rear objective, a 36mm wide CCD, and a 44mm back focus length, the angle of incidence is about 53 degrees.

 

The way I set it up is i = angle of incidence and tan(i) = 44/33. The 33 is half of the CCD frame, plus half of the rear objective 18mm + 15mm = 33mm.

 

If we conservatively take 40mm as the maximum rear element diameter, then we get tan(i) = 44/38, i = 49 degrees.

 

Does that sound right?

 

Steve

[marc schneider dc metro]

Steve -

 

Yes, your trig checks out correct.

[ron c sunshine coast,qld,a]

Mmm,i also wonder about the back focus (or film plane to lens element distance) .In theory it could be as little as 37mm -most eos camera mirrors clear the lens mount by 8mm.