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“Telecentricity” and the Canon EF-S mount

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

  1. ChrisNewman

    ChrisNewman Well-Known Member


    I was fascinated by the article “Classics to use - Leica Elmarit-M 24mm f/2.8 Asph” in AP 15 March, and in particular by the very poor edge and corner performance of the lens on the Sony Alpha 7. Until now, I had assumed that referring to lenses as “designed for digital” was little more than a marketing ploy.

    The article piqued my curiosity about the Canon EF-S mount. There seem to me to be two fundamental differences between the Canon and Nikon DSLR systems. Firstly, Canon use exclusively electrical connections between the body and lens, whilst Nikon retain some mechanical communication. Secondly, Nikon retain full compatibility between full frame and APS-C sub-systems, whilst I understand that Canon EF-S lenses cannot be fitted on full frame bodies.

    The reasons for the body-lens connections seem straightforward. When designing an autofocus system, Canon presumably decided that it would be best to start with a clean slate, and gambled (apparently successfully) that the change would not alienate too many customers. (This happened a few years after I had bought my film SLR, but fortunately I chose Pentax, not Canon!) Meanwhile Nikon chose to adapt their existing system by adding a focusing motor to the body and a shaft drive connector to focus the lens. This has since been superseded by focusing motors in their lenses, but the mechanical aperture lever has been retained.

    When DSLRs were being introduced, it must have been apparent to both companies that bodies with full frame sensors could not be produced at a cost that would attract mass-market consumers, so both introduced APS-C sensors, and both introduced a range of smaller-format lenses that could be lighter and cheaper, and would enable shorter focal lengths that could compensate for the cropping effect of the smaller sensor. Nikon chose to retain full compatibility, which must require a strong retro-focal design for the wider-angle lenses. Canon took advantage of the smaller APS-C mirror to reduce the distance between the sensor and rear element, forcing the lenses to be incompatible with full-frame bodies. With a smaller-format film body, this would have given a clear advantage when designing the wider-angle lenses. But after reading the “Classics to use” article, I wonder whether it was a mixed blessing with digital sensors. Canon must have been well aware of the difficulties of imaging light at lower angles onto digital sensors. Can anyone shed light on Canon’s decision?

    For me this is only a matter of curiosity. Having upgraded from an APS-C Nikon D90 to a full frame D800, I am taking full advantage of the compatibility by using a full-frame Tamron 24-70mm as my principal lens, whilst carrying DX ultra-wide angle and telephoto zooms, and a macro lens, to save weight. But both my camera bodies have focusing motors which have never been used, as all my Nikon-fit lenses have their own focusing motors, and I have had problems with the aperture lever of my Tamron lens, so perhaps Canon’s all electrical connections have their advantages. Anyway, I don’t want this thread to be about whether Canon or Nikon is better - I am just curious about the EF-S mount.

    As this thread arose from an AP article, I had intended to post it on the forum “News - Discussion: A place to discuss AP News stories”, but that does not seem to be permitted.

  2. daft_biker

    daft_biker Action Man!

    IIRC the S in EF-S is for Short back focus......the lens can poke further into the body because smaller sensor cameras have smaller mirrors so there's more clearance before the mirror whacks into the back of a lens.
  3. PeteRob

    PeteRob Well-Known Member

    I always wondered if, at the beginning, the crop sensor as we now know it was expected to become institutionalised or whether it was expected to fade away. My memory is that in the early days there was a lot of fuss about image fall off in the edges of the frames with existing lenses. Perhaps the EF-S mount came about as an engineering solution to that problem and to prevent damage to the stock of film cameras at the time. Then it turned out that making full frame sensors was more difficult and expensive than thought and so the mount became entrenched.
  4. daft_biker

    daft_biker Action Man!

    The way I remember it EF-S simply came about at a time where there were plenty of us looking for more and better wide and standard zooms and smaller, lighter telephotos for APS-C. And that's what we got :)

    I know I'd rather have one of the EF-S standard zooms with IS than a slow and expensive (for a first lens at least) 17-40mm from Canon. Before EF-S standard zooms 3rd party lenses may have had more appeal as a "kit" lens.
  5. ChrisNewman

    ChrisNewman Well-Known Member

    Thanks guys for the background. But I was particularly curious as to why Canon went for a design that couldn’t be mounted in their full-frame bodies.

    The compatibility of Nikon DX lenses with full frame mirrors forces the rear element of the APS-C lens to be as far from the sensor as a full frame lens, so that light must arrive at the sensor fairly close to perpendicular. But the EF-S mount allows the rear element to be closer to the sensor, presumably giving much the same scope for image fall-off as a full-frame lens on a full-frame sensor. If it was thought necessary to keep the light fairly close to perpendicular, I don’t see much advantage in an incompatible mount.

  6. Roger Hicks

    Roger Hicks Well-Known Member

    Telecentric designs have drawbacks as well as advantages: as I recall, worse colour correction (and therefore lower resolution) for a start. This is however on the edge of my recollection of discussions a few years ago with a Zeiss lens designer. I remember something was a disadvantage: I just don't recall what.

    Crop-frame sensor chips are easier to make (being smaller) and use the sharpest and mos vignette-free (central) area of a lens.

    It's easier (and therefore cheaper) to make sharp, small, fast, light lenses that cover smaller areas. This surely explains the attraction of lenses designed for crop formats only. Why make a lens that covers more area than you need?


  7. Sejanus.Aelianus

    Sejanus.Aelianus In the Stop Bath

    I remember a statement, in a book about optical design techniques, about design being easier for smaller exit pupils. It was a long time ago and I don't have the book to refer to but I think the general point is that the smaller the area to be covered, the easier the corrections are and the sharper the outcome. If I recall correctly, a Minox lens has a far higher lp/mm count than a Hasselblad lens, because of this, something like 140 versus 50. I'm sure someone will put me right if I'm missremembering.
  8. daft_biker

    daft_biker Action Man!

    Possibly because they sell more expensive lenses for FF? That and it's not very user friendly to be able to mount lenses that might damage the camera whilst allowing more freedom in lens design for the smaller format.
  9. PeteRob

    PeteRob Well-Known Member

    Indeed, I hypothesised it might have been a recognition that the format was here to stay leading to a commercial decision to make a dedicated lens line. I waited until a 135 version came on the market before going digital and it seemed to me it would just be a matter of time before all cameras were FF. I am no longer waiting for the affordable 120.

    I also seem to remember discussion that Canon EF mount lenses produced (or could produce because of the mount throat) a relatively big image circle and this aided the development of the full frame sensor. I don't have a feel for how many EF lenses have been recomputed/replaced since, say, 2003.
  10. Malcolm_Stewart

    Malcolm_Stewart Well-Known Member

    We can't really discuss APS-C and Canon without mentioning their recent EF-M mount and small range of lenses. I have the kit zoom, 18-55 STM and the EF-M 22 F/2 STM. The EF-M 22 has the same coverage as a 35mm on full frame, and interestingly, has a concave front element, just like some of the varieties of 35 F/2 Canon FD lenses from the 1970s.
    At the moment, I have three Canon 35 F/2 FD lenses, (2 with a concave front element and I assume, thoriated glass, and one with the more common convex) and an FD to EF-M adapter, and am hoping to do some comparisons. Unfortunately, as there's no lens-element free adapter, I can't try my FD lenses on my Eos bodies, without accepting an unknown degree of compromise.
  11. Benchista

    Benchista Which Tyler

    Most of them, actually - among the zooms, I can only think of the 100-400 L that hasn't been replaced. Pretty much all of the superteles have. There are a few primes left - the oldest lens still in the range must be the 50mm f2.5 Compact Macro.
  12. Benchista

    Benchista Which Tyler

    Probably the most interesting EF-M lens is the 11-22; that is a remarkable performer and takes full advantage of the mount design.
  13. daft_biker

    daft_biker Action Man!

    1987 according to the Canon Museum and I'd still rather have one over their newest 50mm!

    Although 1991 was a good year too - I reckon the TS-E 45mm and 90mm are still worth having.
  14. ChrisNewman

    ChrisNewman Well-Known Member

    Of course, but I am curious as to why Canon modified the mount of their EF-S mount so that the lenses cannot be mounted on a full frame camera, without producing a different mount optimised for APS-C; full frame lenses can be mounted on Canon’s APS-C bodies.

    Thanks Roger, that could be a valid reason why a lens with its rear element closer to the sensor might have a better overall performance even if it was not imaged so clearly by a digital sensor. It might also give Canon’s own-brand EF-S lenses an advantage over third-party lenses, which will be designed to allow space for a full frame mirror on a Nikon DX mount!

  15. Benchista

    Benchista Which Tyler

    Yup, still a fine lens, and the last survivor of the original EF lenses - the 15mm FE, 28mm f2.8 and 35mm f2 all having been replaced relatively recently, and the 135mm f2.8 SF having been quietly dropped.

    That they are, although the rumours say they're due for replacement this year by L versions similar to the 24.
    Other lenses from 91, the first USM wave, are the 28mm f1.8 and 85mm f1,8 IIRC.
    Other older lenses that haven't yet been replaced are the 20mm f2.8, 35mm f1.4 L, 50mm f1.8 II, 50mm f1.4, 100mm f2, 135mm f2 L, 200mm f2.8 L II, 300mm f4 L IS and 400mm f5.6 L, and the 180mm f3.5 L Macro and, of course, 65mm MP-E.

    Of those, the first 4 and the 180 are probably the closest to being replaced.
  16. Roger Hicks

    Roger Hicks Well-Known Member

    Dear Chris,

    This sounds completely logical to me, if I am understanding you aright. A lens that will cover full-frame will also work on APS-C, so it makes sense to allow it to be used on APS-C. A lens that will only cover APS-C is of very limited use on FF, so put in a gizmo to block its use on FF...


  17. ChrisNewman

    ChrisNewman Well-Known Member

    The possibility of damaging the camera would only arises because Canon opted for a design that would allow the rear element to be closer to the sensor with APC-S than with full frame. Seeing the very poor edge and corner performance of the Leica Elmarit-M 24mm on the Sony Alpha 7, I wondered why Canon had not chosen, as Nikon appears to have done, to maintain the same minimum distance between sensor and rear element, increasing the distance relative to sensor size, for digital-only lenses.

    I can’t believe that retaining the standard EF lens mount for APS-C lenses would have reduced sales of full frame lenses, because the APS-C format only produces a good image on less than half of the sensor. Despite already owning the Nikkor DX 85mm macro, I have now bought the Nikkor 105mm macro for my D800, for when I think macro shots are likely, to get the benefit of all 36MPx. But I still carry the 85mm in my bag most of the time, on the off chance that I might see something small and interesting, because it is only half the weight of the 105mm and gives an acceptable 15MPx.

  18. Benchista

    Benchista Which Tyler

    You've answered your own question; so that EF lenses can be mounted without an adaptor.

    In theory it can; the place where this is best seen is with the EF-M mount and the 11-22.
  19. ChrisNewman

    ChrisNewman Well-Known Member

    Dear Roger,

    I disagree with you about that. The usual contents of my camera bag are my D800 and full-frame Tamron 24-70mm, which I use for most shots, but also DX ultra-wide angle and telephoto zooms, and a macro lens. These DX lenses widen my photographic scope. Their results are of course inferior to what I could get from good full-frame equivalents, but I am not prepared to carry the weight of those full-frame equivalents, because doing so would make my photography far less enjoyable.

  20. ChrisNewman

    ChrisNewman Well-Known Member

    I’m afraid you’ve misunderstood me. Nikon DX lenses can be mounted on FX bodies and vice versa without needing an adaptor, because Nikon did not modify the DX mount to prevent it being used on an FX body.


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