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  1. #21
    DrPablo's Avatar
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    Quote Originally Posted by Ed Sukach View Post
    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?
    All manufacturers publish it in their MTF charts. You see lens resolution peak at a certain aperture (often f/8 or f/11 for 35mm lenses), beyond which resolution progressively decreases with narrower apertures.
    Paul

  2. #22
    Ed Sukach's Avatar
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    On further thought...

    Isn't that "Diffraction Limited" definition saying,

    "Diffraction has no effect on the resolution of the lens until it is stopped down to an aperture of f/... whatever"?

    I could have an abysmally poor lens - will not resolve 1 (one) line per millimeter. It does not get worse until it is stopped down to f/4 (the smallest amount of diffraction imaginable has a negative effect on its performance).

    Or - given the opposite, "This astoundingly **wonderful** lens is not affected by diffraction until it is stopped down to f/32" ...?

    Do I have it "right"?
    Carpe erratum!!

    Ed Sukach, FFP.

  3. #23
    DrPablo's Avatar
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    Ed, the "limit" part of "diffraction limit" simply describes where lens resolution becomes limited due to diffraction, as opposed to being limited by aberrations or other things intrinsic to the lens.

    So diffraction does affect the lens, including when it's wide open, because you're still passing light through a little hole.

    But the salubrious effects of stopping down, like decreasing the effect of aberrations, make the increase in resolution outweigh the decrease caused by diffraction. At some point, however, the effects of diffraction become more important than any beneficial effect of stopping down, and that is where it becomes diffraction limited.

    As I said in my first post, at that point you have to decide whether your resolution will be limited by diffraction or by depth of field (because, obviously, you can have critical subjects out of focus if you don't stop down enough).

    As to your example, diffraction per se is not intrinsic to the lens elements, so it is not a feature of good or bad lenses. It's that the theoretically perfect lens will never benefit from stopping down -- so if you stop down from f/2.8 to f/4, diffraction will limit resolution rather than a smaller aperture improving performance.
    Paul

  4. #24

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    Not to my way of thinking.

    At F/4 the highest perfect lens is going to be close to 400lpmm. So most lenses won't be made worse by diffraction at this point.

    OTOH at some point the best possible resolution will be lower then what the lens can get more open.

    To me the point a lens becomes diffraction limited is the point stopping it down reduces resolution. Of course some times the smaller F/stop is more important then the resolution.

  5. #25

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    I need to type faster

  6. #26
    Ed Sukach's Avatar
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    Quote Originally Posted by DrPablo View Post
    All manufacturers publish it in their MTF charts. You see lens resolution peak at a certain aperture (often f/8 or f/11 for 35mm lenses), beyond which resolution progressively decreases with narrower apertures.
    ... And where do we determine that this effect is invariably - and only - caused by diffraction?

    A lens is DESIGNED for optimum performance under certain circumstances. Fast 35mm lenses "peak" at larger apertures (the assumption is that they will be used more frequently at larger apertures and in low-light situations than slower lenses) - NOTHING - at all to do with "diffraction". Enlarging lenses are designed to give optimum performance in the middle of the aperture range - but they CERTAINLY do not "go to hell" at the extremes, either... and neither do cameara lenses from any of the reputable manufacturers.

    Of all the concerns I have when photographing, using a lens to the limits of its design is going to be the least.
    Last edited by Ed Sukach; 03-06-2007 at 12:03 PM. Click to view previous post history.
    Carpe erratum!!

    Ed Sukach, FFP.

  7. #27
    Ed Sukach's Avatar
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    I have the Carl Zeiss Planar 2.8/80 T* - S/N 7210829 on one of my 'Blads at the moment. I also have the MTF charts. Can anyone tell me where this lens is "Diffraction Limited"?
    Carpe erratum!!

    Ed Sukach, FFP.

  8. #28

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    The Nikon D2X(s) has the highest number of pixels per millimeter and is only 180 pixels per millimeter. The Canon 1Ds Mark II with full size sensor is 138 pixels per millimeter. Of course pixels per millimeter is not the same as lines per millimeter but it simply can not have a higher lpm rating.

  9. #29
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    Snapshot
    Resolution the lens is capable of and lens “sharpness” does not have many things in common. That term “sharpness” does not have meaning and it is actually a vague term, it means actually lens acutance. This is, simple, distance between light and dark portion of the edge and is controlled by lens design, accuracy of production and assembly, kind of glasses,….
    That 200 lpm resolution I would rather assign as important to painting, airbrushing,… not to photography. One of property of photography is not high resolution but rather blur. This is quality that no other medium have, it belongs to photography only. However this means blur controlled by photographer (lens opening, developer, film) not produced by bad lens design.
    Do not be tricked by lens manufacturers. Leica, Rodenstock, Schneder, Nikon, Canon,… they all make very good lenses that can give honor to any photographer, just need to learn how to use it.
    I like to say how good is Nikkor 1.4/85d lens. Just recently I had shooting people portraits on snow included into picture. This lens in such condition will flare (and if you like will give “low resolution picture”), so Leica summicron (or Nikkor micro 2.8/55) was my choice. This is how it works when you know your lenses. Lens resolution, when you stick to lens maker listed above, is not something to be concerned about. Good lens maker knows very well what photography need, and job of photographer is not to think about lens design but how to use the lens to achieve the goal.
    Have a nice photo day

    www.Leica-R.com

  10. #30
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    Ed -- I'll link you some reading on the subject. This article explains very well how the theoretically perfect lens appears on an MTF curve, and how that differs from an imperfect lens. Much is as I've explained above, but it may be more helpful when you see the graphs.

    http://www.bobatkins.com/photography.../mtf/mtf2.html


    Of all the concerns I have when photographing, using a lens to the limits of its design is going to be the least.
    Well, I don't think it's hard to operate within the strengths of a lens, and I also don't think it's wise to dismiss these factors. This is particularly true for 35mm shooting. I mean why would you stop down to f/32 on a 35mm camera if you're focusing on the moon? All you'll do is limit lens performance by stopping down out of proportion to your DOF needs.


    Quote Originally Posted by Chan Tran View Post
    The Nikon D2X(s) has the highest number of pixels per millimeter and is only 180 pixels per millimeter. The Canon 1Ds Mark II with full size sensor is 138 pixels per millimeter. Of course pixels per millimeter is not the same as lines per millimeter but it simply can not have a higher lpm rating.
    LPM is line pairs per mm, i.e. the ability to resolve two separate lines from one another. So when calculating that on these DSLRs, lines per mm is actually 1/2 the pixels per mm, because it takes two pixels to distinguish two details. Thus, the D2X is about 89 lpm and the 1DsII is about 79 lpm.
    Paul

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