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  1. #11
    DrPablo's Avatar
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    Quote Originally Posted by Snapshot View Post
    I must admit I'm quite skeptical of individuals claiming their DSLR superior to film because their 10MP camera is supposedly capable of resolving 200 lp/mm. Doubtful, especially when their lens can barely resolve more than 50 lp/mm.
    To be fair many 35mm lenses can resolve well over 50 lpm.

    Just to answer the issue re: DSLRs, it's easy to calculate the theoretical maximum resolution of a digital sensor (in terms of lpm). I mean a grid-shaped sensor can resolve nothing more than one detail per photosite, so a line pair would require two photosites. So the Nikon D2x, with the highest of all pixel densities (12.8 MP on APS-C) can theoretically resolve no more than 89 lpm, because it only has 89 pixel-pairs per mm. This, of course, will be limited by the antialiasing filter, low contrast subjects, Bayer algorhithms, etc.

    The thing is, people (including former film users) who proclaim superiority over film often mention non-optical considerations, such as convenience, versatility, and cost-benefit factors. These can't be argued, I mean they are what they are. They also mention, fairly commonly, application-specific circumstances where digital may not equal film but it's good enough. That's pretty fair, I mean it doesn't take many megapixels to fill a wedding album with small prints. But a lot of this debate is idealism as well, and selective ignorance of the factors that make a "film versus digital" debate fairly trite and irrelevant.

    Anyway, if you take a 35mm film camera and use a fantastic lens, you may or may not have greater resolving power depending on the film you use. Keep in mind that with digital sensors that resolution stays constant at all ISO speeds, whereas with film a higher ISO film is basically a different sensor. If you use a phenomenal lens, like a Leica or Contax-Zeiss lens (which claim 200 lpm resolutions sometimes) and a film like Tech-Pan, and avoid diffraction limits and motion or mirror-blur, then you can have extraordinary resolving power in 35mm format.

    Now, small format shooting is quickly limited by a few factors, including its diffraction limit (which often happens at around f/11 or f/16) and its circle of confusion (in other words, the requirement for extremely fine resolution in order to enlarge a smaller film image). This is true whether film or digital, but these factors certainly favor 24x36 over 15x24. And the diffraction limit becomes a nearly insignificant issue by the time you get to large format, because your enlargement factor is so small that subtle losses in resolution are trivial.

    There was a great series on print sharpness in the Sept/Oct and Nov/Dec issues of View Camera. What is clear to me from these articles is that the degree to which diffraction limits sharpness depends highly on your depth of field requirements. If you need a huge depth of field, because of critical foreground and background detail, then this can limit sharpness much more than diffraction. On the other hand, if your only important subject is at infinity and your DOF requirement is trivial, then excessively small apertures will make diffraction your limiting factor.

    Thus, you want to operate below the diffraction limit when possible, which would basically make film/sensor characteristics and your intrinsic lens resolution (and its aberrations) the ultimate delimiters of system resolution. But sometimes it's not possible to operate below the diffraction limit, because your DOF will be too small for a sharp print if you do.



    Now, there are many people on this forum who use digital cameras -- it's just not discussed here. But what I can offer, for what it's worth, is that debates over 'better vs worse' are just not worth wasting your precious brain cells. I mean on another forum I just witnessed a whole bunch of Nikon users claim they were switching to Canon when Canon's new DSLR was introduced -- then a rumor circulated about a new Nikon pro DSLR that prompted Canon users to do the same -- and then when the Nikon rumor proved false, the pendulum swung to the other extreme again. I swear, sometimes watching the digital world is like being a Red Sox fan (and trust me, I am a Red Sox fan) -- just too many highs and lows, too many emotions, too much posturing.
    Paul

  2. #12
    los
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    paul may I?


    THE END

  3. #13

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    There isn't a "diffraction limit", image quality simply gets worse the smaller the aperture is. Other things affect the image quality at various aperture, things that I don't really understand.

    With large format cameras the loss of quality becomes less significant because the focal lengths are longer. With a 50mm lens at f/22, the diaphragm opening is 2.3mm, while at 150mm, f/22, the lens opening is 6.8mm which results in lot less loss of resolution due to diffraction.

  4. #14

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    It probably depends on the film. With fast films the film is probably the limiting factor whereas with higher resolution, slow films the lens' limitations might be the more significant factor.

    David.

  5. #15
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    Quote Originally Posted by reub2000 View Post
    There isn't a "diffraction limit", image quality simply gets worse the smaller the aperture is. Other things affect the image quality at various aperture, things that I don't really understand.
    Yes, there IS a "diffraction limit". It is part of the design criteria that determines the smallest (numerically highest) f/stop the manufacturer will allow. Beyond that, the image quality will be degraded (by diffraction) to less than the manufacturer will allow. It is determined mathematically - and not influenced by lens design. I have the formula around here - somewhere - but it has not drawn my interest for long, long time - ever since I got out of the lens manufacturing business.

    In short, it is not something I'd EVER be concerned with - the manufacturer has already done all of that that is necessary - or even, reasonable - unless one really WANTS to be anal to a fault.

    .
    Carpe erratum!!

    Ed Sukach, FFP.

  6. #16
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    I want to thank everyone that contributed to this discussion thus far. It has been informative (as usual) for me. Hopefully, I can digest what has been discussed here and put it to good use.
    "The secret to life is to keep your mind full and your bowels empty. Unfortunately, the converse is true for most people."

  7. #17
    Ole
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    "Diffraction limited at f:8" means that at f:8, the resolution of the lens is so good that only the increasing diffaction limits the total resolution on film.

    So a lens that is diffraction limited at f:8 is a lot better than one that is diffraction limited at f:16, and this is likely to be visible all the way from wide open to f:22!
    -- Ole Tjugen, Luddite Elitist
    Norway

  8. #18
    Ed Sukach's Avatar
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    Quote Originally Posted by Ole View Post
    "Diffraction limited at f:8" means that at f:8, the resolution of the lens is so good that only the increasing diffaction limits the total resolution on film.

    So a lens that is diffraction limited at f:8 is a lot better than one that is diffraction limited at f:16, and this is likely to be visible all the way from wide open to f:22!
    Whooo .... You LOST me there, Ole!!


    The aperture where diffraction does not degrade the image more than allowed by the manufacturer is ... where?
    Carpe erratum!!

    Ed Sukach, FFP.

  9. #19
    Ole
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    Quote Originally Posted by Ed Sukach View Post
    Whooo .... You LOST me there, Ole!!


    The aperture where diffraction does not degrade the image more than allowed by the manufacturer is ... where?
    That's where you got lost - by assuming it's "allowed by the manufacturer"-

    Simply put, an ideal lens would have the best resolution wide open, and only diffraction would reduce the resolution as the aperture is stopped down.

    With real lenses, the aberrations and "stuff" makes the wide-open resolution poorer. And those faults decrease as the aperture is decreased.

    At some point, the diffraction will become the main limitation on resolution. This can be described as "diffraction limited at..." whatever fstop.

    Think of it as two crossing lines in a diagram: One, "lens fuzz" starts high and decreases with aperture. The other, "diffraction fuzz", starts low and increases with aperture. The diffraction-limited value is where the two cross each other.

    It's not quite as simple, and the lines aren't straight, and so on. But it's a simple way to see it...

    A lens which performs to the limit on resolution determined by diffraction at a large aperture (say, f:4) is very good. another, diffraction limited at f:16, won't perform anywhere near as well at any larger aperture than f:16!
    -- Ole Tjugen, Luddite Elitist
    Norway

  10. #20
    Ed Sukach's Avatar
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    Quote Originally Posted by Ole View Post
    That's where you got lost - by assuming it's "allowed by the manufacturer"-
    Uh ... I did not "assume". That is the standard - and gerearlly accepted description of "Diffraction Limit".

    Simply put, an ideal lens would have the best resolution wide open, and only diffraction would reduce the resolution as the aperture is stopped down.
    With real lenses, the aberrations and "stuff" makes the wide-open resolution poorer. And those faults decrease as the aperture is decreased.
    At some point, the diffraction will become the main limitation on resolution. This can be described as "diffraction limited at..." whatever fstop.
    I understand what you are writing here. This is the first time I've ever heard of "diffraction limits" described in that way. The idea that "Nothing else really matters except diffraction beyond a certain aperture", is to me, strange. Do you have any references to this use?
    I have never seen the characteristics of a lens system described in this manner ... "Diffraction limited at f/ - whatever". Can you direct me to an example?

    BTW - All manufacturers ... except those who convert the bottom of Coke bottles into lenses (not necessarily "bad" - see "Holga") DO pay scrupulous attention to design characteristics - resolution is only one of many.

    For REALLY "fine" (standard definition of resolution is the minimum distance where two points can be seen as two points - not "lines per millimeter" - although L/mm is a useful derivation), look to Aerial Mapping and Military Recon.... OOPS! Forget that last one.
    Carpe erratum!!

    Ed Sukach, FFP.

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