Digital Lenses, Purple fringing, etc.

Discussion in 'Digital Photography' started by J. B. Dalton, Jun 3, 2004.

  1. J. B. Dalton

    J. B. Dalton Guest

    It's time for a few personal observations.

    [Preddy, et al, please ignore, as I ignore you.]

    Having a big collection of 35mm lenses isn't a big incentive to buy a
    matching brand of DSLR, unless it has a full-frame sensor. Those lenses
    were designed for max resolution (min. aberrations) in an annular region
    with a radius a bit less than about half frame height. The center and the
    corners are allowed a bit of drop off from the best region in many if not
    most designs.

    Smaller sensors often are confined to the central "blur donut hole" and
    most of their area isn't as well resolved as the lens' best regions. Many
    good 35mm lenses are not able to get much contrast at the pixel level with
    sensors of 4 MP and above, particularly on a 2/3 or smaller sensor.

    What is worse, the digital sensors don't handle off-perpendicular light
    very well. Silicon has such a high index that a lot of the off-axis rays
    may be reflected away, and some vignetting from the recessed cell edge is
    inevitable. This means a very special lens design, known as "telecentric"
    is best for cmos and ccd sensors. Hence the "D" series for the Nikons, and
    the new Oly lenses for the 4/3 systems like the E1.

    If you really want resolution matching your sensors, don't count on your
    old 35mm lenses to do all you need. Nice to have an assortment of "just
    adequate" lenses, they will never give all the resolution and image
    uniformity you have learned to expect with them, when used with film.
    Expensive waste, IMHO.

    Worse than using old 35mm lenses is to get a camera without interchangeable
    lens (which is all most of us can justify) and discover that it has
    Chromatic Aberration (CA) and gives the dreaded "purple fringes."

    Much discussion here lately of Bayer and processing causes for it. In
    actual use, the stuff I have seen is 99.9% due to the lens design and the
    way it is used. It is more obvious in digital because it is so much easier
    to blow it up and examine it. There are two kinds of CA. The regular
    longitudinal kind that causes some colors to focus in front of or behind
    the plane of best focus for others. Then there is the lateral kind, which
    tends to be radially symetric. It happens when the image size is different
    for different colors.

    Lateral CA can be corrected in a digital darkroom process, and maybe even
    in the camera's processor. No processing can do much for longitudinal CA
    without taking separate exposures at all colors with focus adjustment in
    between them. I don't think we are ready for that.

    The bottom line, for those of us who must buy a camera with a fixed lens
    attached is to be extra careful that the lens is as good as the sensor and
    processing, for you are stuck with it for the life of the camera.

    That's my thoughts for today.

    J. B. Dalton, Jun 3, 2004
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  2. J. B. Dalton

    bagal Guest

    Thanks for sharing your wisdom and experience JBD - it is much appreciated

    bagal, Jun 3, 2004
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  3. Hope I am not among et al :)
    Now - this is news to me. I have only read that the smaller sensor
    uses the best parts of the picture. According to my memory that is
    also how the MTF curves I have seen looked. No "donut hole" comes
    to my mind. Not a big and important one at least.
    Yes - tele centric lenses is an advantage. Maybe the off-axis rays
    reflection is a problem. I don't know. This could be dealt with in
    software though. Much more problematic is the off-axis rays hitting
    the micro lenses and the Bayer mosaic filter though. There you will
    get color and other strange errors.

    Roland Karlsson, Jun 3, 2004
  4. You obviously know quite a bit about imaging with lenses. Calling 35mm
    lenses expensive waste is a bit too much, though. Most people bought them
    when digital either did not even exist or did not provide nearly what it
    does now. And we all have different standards, the final image size we are
    aiming for and amount of aberrations we are willing to tolerate. There seem
    to be even some "pixel-peepers" at least in dpreview Canon lens forum who
    seem to be happy with their lenses, which are mostly old 35mm stuff. They
    are using typically a 10D, but even 1DS pops up now and then. Thus I really
    do not find it futile to at least try using the old lenses. Some serious
    photographers with high technical standards have noted that even the high
    quality tele zooms aren't good enough for sensors with most resolution. So
    they use primes instead. But I have not heard any of them say that a
    completely new lens lineup would be needed.

    I became seriously interested with photography less than four years ago.
    Even those days obtaining good photography equipment at sensible price meant
    purchasing film gear. Since then I have of course acquired some additional
    objectives, not all cheap. And I do have plans for them. I really hope my
    technical standards are not too high... :-|

    Markus Malmqvist, Jun 3, 2004
  5. A 6 Mpixel sensor that is a factor of 1.5 smaller than full frame 35mm cannot
    resolve more than 63 lp/mm. If you look at MTF graphs for 35mm lenses,
    there is quite a bit of performance at 40 lp/mm (unfortunately they
    don't measure higher than 40 lp/mm). 60 lp/mm should be no problem.

    After all, for film you want high resolution lenses, because you don't
    want to lose any of the limited resolution of film (at low contrast).
    Philip Homburg, Jun 4, 2004
  6. J. B. Dalton

    J. B. Dalton Guest

    Not a chance. I seek out and enjoy your posts very much.
    It isn't something advertised in MTF releases. but is well known to most
    lens designers. If you truly optimize the center, it is likely that one or
    more aberrations will eat you alive in the corners. Better to get a good
    average over the desired field, and let the center and corners both trail
    off just a little bit.

    No, it isn't as big and important as people spending huge amounts for a
    fast 300mm macro and then stopping it down to f 32. to get back some field
    depth. :)
    I don't really, either. I was just guessing, because silicon has an index
    of 3 or more, as I recall. The microlenses and filters may actually help
    there, a bit, to match indexes.
    Possible polarization spookiness, too, at very wide apertures. :) A
    telecentric-leaning design sure looks like a good idea, to me.

    Unfortunately that tends to greatly reduce the size advantage of systems
    like 4/3, because a telecentric lens needs such a big exit aperture.

    J. B. Dalton, Jun 4, 2004
  7. J. B. Dalton

    bagal Guest

    aren't R&D teams at Canon researching this very possibility?

    telecentric lenses I mean

    bagal, Jun 4, 2004
  8. That would only apply to diffraction patterns, mainly of slightly out of
    focus images. Since in-focus images also display the fringing, as do large
    and small sensors, it seems to be not the general reason.

    To me, that still looks like a more likely reason, combined with 'some' true
    optical chromatic aberrations (since smaller apertures do make a (positive)
    That would cause vignetting, not "lateral-CA".
    That would add a design constraint. However, it would be similar to
    suppression of vignetting, and it's behavior is good for the better lenses.

    Bart van der Wolf, Jun 10, 2004
  9. SNIP
    Correct. Some of them may not be optimal for sensors other than film, but
    the better lenses are still more than 'usable'.

    As said, there's not much wrong with the better lenses, especially when the
    low-pass anti-aliasing filter is adequate for the sensor pitch and

    Correct. Zoom-lenses are *always* a bit of a compromise, and sensors can
    demand a lot from lens design. However, good prime lenses and an adequate
    AA-filter wil produce good images.

    Bart van der Wolf, Jun 10, 2004
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