Diffraction question

Discussion in '35mm Cameras and Accessories' started by michael_r, Jan 9, 2011.

  1. michael_r

    michael_r Subscriber

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    This applies to any lens in any format for cameras or enlargers so I wasn't sure where to post it.

    I've always thought of diffraction as an unfortunate fact of life for photographers, a limitation in light gathering, which cannot be corrected for. To me, this was presumably why a theoretically perfect optical formula is said to be "diffraction-limited" - ie in any lens diffraction increases as the aperture decreases in size. So I always thought of diffraction as being strictly/mathematically related somehow to the size of the circle through which light passes. Maybe the focal length has something to do with it, not sure.

    Recently though I came across a lens test in one of the magazines (unfortunately I can't remember which magazine or which lens, but it was pretty recent) in which the lens in question was compared to alternate offerings from other manufacturers, and one of the comments made in favor of this lens was that diffraction was better controlled at smaller apertures. I'd never come across a statement like that before so I'm puzzled. How is diffraction controlled in the first place? Besides the aperture size, what are the factors? For example, is it something in the optics, or perhaps something with the diaphragm itself - rounder? thinner blades?

    Thanks
    Michael R
     
  2. hpulley

    hpulley Member

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    Sounds like a misinformed author. Diffraction cannot be controlled. It is simply the size of the aperture that causes it, nothing more. Nothing you can do about it.

    I suspect some other aberration was better controlled but they messed up.
     
  3. Hikari

    Hikari Member

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    Diffraction is a property of an aperture. The shape of that aperture impacts the diffraction. And because of that, mechanical vignetting impacts the shape of the aperture off axis at large apertures--small f-numbers. As hpulley also stated, aberrations can impact diffraction. And because nothing is simple, lenses can be optimized for resolution or contrast, but not both at the same time. This mix between the two will also affect diffraction.

    But I concur with hpulley, he probably did not understand what he was saying.
     
  4. hpulley

    hpulley Member

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    On second thought: With obstructed telescopes and mirror lenses you can have bad diffraction effects due to the secondary mirror and its holder but in most normal camera lenses it is not an issue. I suppose if you had reflective aperture blades you could have similar problems with them but I've never heard of anything like that. Perhaps circular aperture blades could help reduce those sorts of diffraction effects like diffraction spikes but I've never seen them outside of astronomy where there are point source lights. Perhaps in scenes with lots of specular highlights? Still sounds like stuff you won't encounter often.
     
  5. ic-racer

    ic-racer Member

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    I'd be puzzled also.
     
  6. phaedrus

    phaedrus Member

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    This tells it very well, even including the unfortunate interaction some modern recording media have with the Airy disks that result from diffraction. Nothing to worry for film users, though. For us, diffraction only depends on f-stop and rear focal length.
     
  7. Q.G.

    Q.G. Inactive

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    Diffraction creates patterns (of light distribution) which depend on the shape of the aperture (which is why in the printing industry, making half tone plates, they used a choice of shapes). The Airy disc may be a triangle lozenge, pentagon, or whatever form, and those forms wil create uneven 'density' in the pattern, distinct directions.
    So, as Hikari already mentioned, the shape of the aperture may have an effect.
     
  8. alanrockwood

    alanrockwood Member

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    One interesting feature of diffraction is that the image of a point source that is imaged by a system with a circular aperture is typically a large central lobe and a series of concentric but less intense rings (lobes). Interestingly, if the aperture were not a hard stop, but instead were composed of a graded density ring of gaussian transmission profile the image of a point source would not have rings but would instead be a gaussian blur, i.e. one central lobe with a rapidly decreasing intensity when moving away from the main part of the lobe.

    Laser physicists and chemists actually produce good approximations to gaussian beam profiles in the laboratory, but their methods are not useful for producing two dimensional images.

    I just thought you might want to know that bit of optical esoterica.
     
  9. michael_r

    michael_r Subscriber

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    Interesting. Thanks everyone.