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“Telecentricity” and the Canon EF-S mount

Discussion in 'General Equipment Chat & Advice' started by ChrisNewman, Mar 16, 2014.

  1. Benchista

    Benchista Which Tyler

    But Nikon make no attempt to put the rear element closer to the sensor with DX lenses - so they don't need to. Nikon's choice is to maintain the pretence of compatibility within the F mount, Canon's to allow potentially improved quality. I say "pretence" simply because of the maze of issues that have inevitably grown up over the years; it's a matter of personal opinion as to if the use of APS-C lenses in crop mode on full frame cameras is worth the lens design limitations that ensue.
     
  2. daft_biker

    daft_biker Action Man!

    Bummer....might have to get a proper job again so I can buy new toys :eek:
     
  3. ChrisNewman

    ChrisNewman Well-Known Member

    I’m trying to explore whether there are serious lens design limitations in maintaining compatibility between APS-C format lenses and full frame DSLR bodies. The very poor edge and corner performance of the Leica Elmarit-M 24mm on the Sony Alpha 7 seems a dramatic demonstration that digital systems can benefit from a greater distance between sensor and rear element than is necessary for film. And I’m not aware that Nikon DX lenses are generally considered to perform worse than Canon EF-S.

    Chris
     
  4. Benchista

    Benchista Which Tyler

    You're barking somewhat up the wrong tree, I think. What the article in the mag shows is that digital cameras benefit from having lenses designed for digital, particularly when the back focus is short; film-era Leica wideangles, not being retrofocus, have very little degree of telecentricity and consequently present light at a very oblique angle towards the edges of the sensor, particularly on full frame sensors; Leica, being aware of that, have a microlens design able to deal with it, and in-camera processing to assist as well. The A7/R doesn't; that's pretty much all there is to it. It's not the distance from sensor to rear element per se, but the optical construction of the lens, the arrangement of the microlenses, and the processing of the camera. These were the issues that delayed Leica launching a digital M in the first instance, and making the M8 an APS-H sensor when they did; it took a little longer to engineer a working solution for full frame.
    Incidentally, a slightly different but broadly similar scenario was behind Nikon's insistence that full frame was unnecessary for some time - the relatively small opening of the F mount makes it harder to design telecentric lenses for full frame.
     
  5. ChrisNewman

    ChrisNewman Well-Known Member

    I appreciate that there is more to projecting an image well onto a sensor than just the size of the back focus. But I think it must be important. If the M8 sensor has microlenses set up to collect oblique-angle light efficiently, I suspect its performance with near-perpendicular light will suffer to some extent. I assume Leica telephoto lenses will have a much longer back focus than the wide angles.

    From lenses with relatively long back focus, presumably all light will strike the sensor at a near-perpendicular angle, and at a similar angle to other lenses that also have long back focus. But of course achieving this would require a strong retrofocal design for wide-angle lenses, which would create its own optical challenges.

    The article about Leica Elmarit-M 24mm was a revelation to me, and thank you for helping to set it in context. It’s an interesting point you make about full frame on the Nikon F mount. I assume it was inconvenient even with film!

    Chris
     
  6. RogerMac

    RogerMac Well-Known Member

    I have always believed that Leicas were limited to telphotos of 135mm because that was the limit of accurate range finding with their rangefinders. Yes I know there is a curious reflex finder available but it does not invalidate my point. It is therefore possible that there are no big teles that need to be accommodated.

    No doubt somebody who knows a lot more about Leicas will put me right!
     
  7. PeteRob

    PeteRob Well-Known Member

    The comparitive time to market of the 10-2? mm zoom between Canon and Nikon for the crop sensors would give an indicator of the time taken to solve such technical issues.

    At the end of the day same mount/different mount will have been a commercial decision, made before 135 sized sensors were available and when film cameras were mainstream. Possibly Canon saw EF-S as a market differentiator for digital vs film - one can only wonder.

    The Leica comparison in AP says nothing new.
     
  8. Benchista

    Benchista Which Tyler

    Not necessarily; it depends on the shape of the microlenses.

    Largely true, I think, which is why the mirrorbox of a DSLR isn't really the disadvantage that the mirrorless brigade would have you think.

    It was certainly an issue with wide aperture lenses; Nikon's engineers have done a staggering job over the years to produce such good lenses despite the sizeable limitations of the F mount. Again, it's interesting to contrast Nikon and Canon's approach to lens mounts - Nikon's compatability is rather analogue, Canon's digital - if a Canon SLR lens fits, it works fully, whereas a Nikon SLR lens WILL fit (with only a handful of exceptions) but may not work fully - Canon's engineers have had it a lot easier since 1987, Nikon's have been working pretty hard since 1959.
     
  9. Roger Hicks

    Roger Hicks Well-Known Member

    That's right. There were allegedly a couple of longer Japanese lenses but I never saw them. Zeiss intially coupled the 180/2.8 "Olympic" Sonnar but then switched to a reflex housing for the same reason. The Visoflex wasn't all that bad, though: see http://www.rogerandfrances.com/subscription/worst dslr.html

    Cheers,

    R.
     
  10. Benchista

    Benchista Which Tyler

    Roger, was the 153mm for the Kodak Ektra rangefinder coupled? I can't recall; I think it was. I believe they had a 254mm planned.
     

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