Film/digital: An odd concept...

Discussion in '35mm Cameras' started by Longfellow, May 7, 2005.

  1. Longfellow

    Longfellow Guest

    Talking to an owner of the 20D. He said that his lenses all performed
    better on the digital body because the sensor size was using only the
    best part of the lens. Looked at the MTF graphs for some of Canon's
    lenses and they show a common fall-off towards the edge of the circle of
    the projected image, just as one would expect; this was the basis of the
    fellow's statement.

    It occured to me that I could achieve the same effect by simply cropping
    full-frame image to the same size as the sensor and get the same
    improvement in image quality. Somehow, this doesn't sound right, though
    for other identifiable reasons.

    Has anyone considered this sort of thing, and if so, what sort of
    results and/or conclusions have been drawn?

    Thanks,

    Longfellow
     
    Longfellow, May 7, 2005
    #1
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  2. Longfellow

    Colin D Guest

    That's true, but your limit would then be the resolution/grain
    characteristics of the film when cropped to sensor size. The trade-off
    would be worse than the small loss of definition from the lens at full
    35mm size.

    Colin
     
    Colin D, May 7, 2005
    #2
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  3. It's grainy!

    Instead, put 645 lenses on 35mm bodies - that, I imagine, would have a
    similar effect?

    Duncan.
     
    Duncan J Murray, May 7, 2005
    #3
  4. Longfellow

    Chris Brown Guest

    [cropping the centre portion of the lens circle]
    People with 4x5 cameras do it all the time when they use a rollfilm back.
    With 35mm, the problem is that your film area is at a premium when you
    start, so cropping even more of it away reduces your enlargement potential
    somewhat.
     
    Chris Brown, May 7, 2005
    #4
  5. While this is true, you have to remember that the smaller format will need a
    higher lines per mm for a given level of detail.

    So a 10 lp/mm feature on a full frame 35mm will require 16 lp/mm on a APS-C
    DLSR.

    There will be some MTF rolloff from 10 to 16, how much depending on the lens
    quality.

    Lester
     
    Lester Wareham, May 7, 2005
    #5
  6. Longfellow

    Matt Clara Guest

    "Duncan J Murray"
    Except medium format lenses as a general rule don't resolve as much detail
    as 35mm lenses.
     
    Matt Clara, May 7, 2005
    #6
  7. Longfellow

    Matt Clara Guest

    I don't believe the 20D resolves as much detail as the finest grained 35mm
    films do, ergo, it's somewhat of a moot point.
     
    Matt Clara, May 7, 2005
    #7
  8. Longfellow

    JohnJ Guest

    Absolutely...
     
    JohnJ, May 7, 2005
    #8
  9. He is about half right.

    It is true that with the crop factor, you do get to use the sweet spot
    of the lens. However you got to use that on the 35mm and you go to use the
    outer 30-40% of the lens as well that now you are throwing away.

    In reality you will loose something compared to a 35mm, but it will not
    be as much as might be expected. Because of the difference between silver
    and electronic it is not really possible to give an exact comparison and
    lenses are also different some are almost as good at the edge as center.
     
    Joseph Meehan, May 7, 2005
    #9
  10. Longfellow

    Alan Browne Guest

    Sure. Many images just look a bit crisper in the middle/softer to the
    edges.

    However, it is in enlarging the image that you will run out of the
    benefit of the middle sharpness. Eg: if you cropped a 36x24mm negative
    to 24x16, then an enlargement to 10" would be 10.5:1 instead of 7:1. So
    that small gain in sharpness on the film would be completely lost in the
    enlargement.

    Better to get closer to the subject and shoot as large a negative or
    reversal as possible regardless of the rolloff to the corners.

    Cheers,
    Alan
     
    Alan Browne, May 7, 2005
    #10
  11. Longfellow

    Longfellow Guest

    Agree. You cite the "identifiable reasons". I'll suggest the fellow
    read this NG and look up this thread ;)

    Longfellow
     
    Longfellow, May 7, 2005
    #11
  12. Longfellow

    Tony Guest

    It really is not that large a dropoff. I doubt you could see it on most
    lenses even if you were looking for it (if you happen to own crap lenses
    like Vivitar -- never mind). Why stick yourself with using only part of the
    frame because of a graph?
     
    Tony, May 7, 2005
    #12
  13. Longfellow

    ian lincoln Guest

    Well it seems to work and thanks to digital printing and unsharp mask my eos
    300D is sharper than my eos 50E with the same lenses. More to do with the
    fact that the digital image to printer is a first generation image whereas
    film to filmscanner then digital image to printer is a 2nd gen image.
    Despite the fact that pixels are larger than grain and the digital image is
    enlarged more times are still get sharper, more detail and no grain. The
    turn around time is also a fortnight faster than when shooting slide and
    then scanning and retouching 4 frames per hour.
     
    ian lincoln, May 7, 2005
    #13
  14. It occurred to me once that if the film could be plastered to the inside of
    a hemispherical bowl, then the images might be sharper at the edges/corners.
    Or, at least, it might be cheaper to build lenses, because they wouldn't
    have to compensate for the longer distance to the corners of the film. Then,
    I realized that a CCD plane could be built that way. i.e.: spherical instead
    of flat. Perhaps they are, and perhaps this is why the lenses on a digital
    camera are cheaper. (are they cheaper?) At least, there is no inherent
    reason why digital images shouldn't be just as sharp in the corners as they
    are in the center, assuming that the corners of the image are in the same
    focal plane as the center, which is probably not usually the case.......
     
    William Graham, May 8, 2005
    #14
  15. Longfellow

    Gordon Moat Guest

    Fairly common for most lenses to fall off a bit in either contrast,
    resolution, or exposure (or all three) from the centre to the edges. Some
    fisheye lenses have more even exposure from centre to edge, though then
    distortion is more of an issue than using perhaps a superwide lens. Some
    lenses have enough fall off in exposure that using a centre filter can help
    the exposure become more even. I suppose it might be thought that there
    could almost be a centre filter for nearly all lenses, though in practice
    it is usually tough to see much difference from centre to edge performance
    .. . . at least in good lenses.
    Maybe crop slightly to avoid any vignetting. As someone else mentioned, you
    could use a medium or large format lens on a smaller film area. Again, the
    difference is very slight most of the time.
    I considered film and direct digital, and the interaction of printing
    enough to write about it. There was a great deal of research, and only a
    little might address your question, but if interested take a look at:

    <http://www.allgstudio.com/technology.html>

    You might find some of the links interesting as well. However, as others
    have stated, the differences are very slight. In practice, you learn to not
    even consider minor differences across a lens, and just intuitively use a
    lens to best advantage for a given scene or subject.
     
    Gordon Moat, May 8, 2005
    #15
  16. Longfellow

    PhotoMan Guest

    Your reference is to what is commonly called 'curvature of field', wherein
    the actual plane of focus is spherical.
     
    PhotoMan, May 8, 2005
    #16
  17. Longfellow

    Alan Browne Guest

    Carefully shot slide and negative film still has more detail than an 8
    mpix image if the subject detail is high, and the contrast high enough.

    Cheers,
    Alan
     
    Alan Browne, May 8, 2005
    #17
  18. Longfellow

    Alan Browne Guest

    First off, the image plane is flat, not a sphere section. [In
    disposable cameras using cheap lens elements, the film plane is curved
    to help compensate for the cheap optics.]

    Corner sharpness loss is simply a question of economics. You could have
    much sharper corners with larger diameter lenses projecting a larger
    image circle.

    Sensors are very, very flat, and this is a benefit to digital shooting
    for focus accuracy across the image. Making a curved section chip would
    be a major, major change in semiconductor fabrication.

    "digital" lenses are not cheaper if you consider the image circle is
    smaller. Another advantage of 35mm lenses on a cropped sensor is you're
    working in the sweeter spot of the lens.

    Cheers,
    Alan.
     
    Alan Browne, May 8, 2005
    #18
  19. Longfellow

    Alan Browne Guest

    But is it really curved? My impression remains that the focus plane is
    flat and that the image plane is flat.
     
    Alan Browne, May 8, 2005
    #19
  20. Longfellow

    PhotoMan Guest

    In varying degrees depending on lens quality, every lens test I've read that
    addresses field curvature indicated its presence. It's the primary cause of
    edge softness. A popular spec in tests of film cameras is 'flatness of
    film', which is controlled by the pressure plate, and can have the same
    effect as 'flatness of field' as projected by the lens.
     
    PhotoMan, May 8, 2005
    #20
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