Using Small Apertures

Discussion in 'Exposure Discussion' started by David Jones, Aug 31, 2010.

  1. David Jones

    David Jones Member

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    I have seen a difference in centre sharpness between using some lenses at full aperture and stopping down to, say, f5.6 but how about small apertures? Between F11 and F22 I can't see any difference in a 10x 8 print. I am talking centre sharpness rather than depth of field. Is it just with very large prints that this is a worry? I suppose it is to do with refraction. I use 35mm and medium format.
    thanks
    Dave
     
  2. williamtheis

    williamtheis Member

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  3. ic-racer

    ic-racer Member

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    Ralph Gibson (early 35mm work) and Joel Meyrowitz (Cape Light) both used an aperture around 3mm. A little fuzzy for me, but certainly an 'artist intented' effect.
     
  4. Sirius Glass

    Sirius Glass Subscriber

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    Most if not all lens manufactures do not put apertures that show strong diffraction.
     
  5. David Jones

    David Jones Member

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    Thanks for those replies. Diffraction is what I meant! Just curious if anyone had noticed this phenomenon.
    Dave
     
  6. Gerald C Koch

    Gerald C Koch Member

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    Because lens design is a compromise between several optical effects, lenses have an optimal resolution at only a single aperture. This information can be obtained from the maker. Resolution will fall off on either side of the optimal stop. The further you differ from this stop the poorer the resolution.
     
  7. ic-racer

    ic-racer Member

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    I think nearly every lens I own with an adjustable aperture will stop down to fuzz. Most notable, a 5mm Bolex lens with a minimum aperture of f22!
     
  8. pgomena

    pgomena Member

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    The rule of thumb is that best resolution is two stops down from wide open. Consider, however, that on an f/1.4 50mm lens on a 35mm camera, your sharpest aperture would be f/2.8. If you go by the rule of thumb, you have a whole lot of stops left over that would give poorer quality images. Hogwash. Use the aperture you need to get the depth-of-field and exposure you need. Just be aware that f/16 or f/22 may give you the depth of focus you need at the expense of some sharpness in some circumstances.

    I've really noticed diffraction effects only on extreme close-up photographs. A photographer in a studio I used to work in was having trouble getting a good, sharp image of an ornate ring. He was using an RZ67 with a macro lens stopped down as far as it would go, bellows racked way out. I suggested he open the aperture a stop or so to reduce diffraction and that did the trick. Use what you need to get what you want. Also, the more you enlarge an image, the more any defects, including diffraction effects, will show.

    Peter Gomena
     
  9. Sirius Glass

    Sirius Glass Subscriber

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    The minimum aperture for maximum DOF and low diffusion varies with the format.
    35mm ~ f/11
    120 ~ f/16
    4x5 ~ f/22

    This is a rule of thumb, YMMV as some lenses are better than others. You can use this as a starting point, test it if you want or ignore it. This is not an invitation to start a religious flame war. Please note the use of "~" before you start to blather.

    Steve
     
  10. ic-racer

    ic-racer Member

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    Agreement and simplification:
    linear aperture around 4 to 5mm for all formats
     
  11. bobwysiwyg

    bobwysiwyg Subscriber

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    Newbie here. I've been following these threads and they may go a long way to explaining some of my less than desired results on some shots as far as sharpness goes. I'm a bit of sucker for flowing water shot at 1 sec. or so. Using ISO 100 film I can sometimes get this by stopping way down. Now I see it would be better to use a more optimal f-stop and ND filters. Now to my question, if, for example f22 is optimal for 4x5, why are there additional f-stops way beyond f22?? Is it simply a matter of making fuzziness available for those who like it?
     
  12. RalphLambrecht

    RalphLambrecht Subscriber

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    Open up a stop for each, and it matches my tests.
     

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  13. Q.G.

    Q.G. Inactive

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    Yes.
    But it comes at a price.

    A bit more fuzziness in parts that else would be, uhm..., a bit less fuzzy, in exchange for a bit less fuzziness in places that else would be more fuzzy (i.e. increased depth of field).
     
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  15. RalphLambrecht

    RalphLambrecht Subscriber

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    The depth-of-field requirements may demand it. Selecting film speed, aperture and shutter speed is full of compromises. Ultimate sharpness is not always the guiding factor.
     
  16. bobwysiwyg

    bobwysiwyg Subscriber

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    That makes sense, thanks.
     
  17. Donmck

    Donmck Member

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    This question comes up every couple of years.

    Leaving out the mathematics,a 2mm iris opening is the sweet spot
    between difraction and coma --as a general rule. The f stop will change
    according to the focal length of the lens. I imagine there may be modern very well corrected(and expensive) lenses where this rule may not apply.

    Lynn (Jones?) is pretty good at explaining how/why this works
    give him a google.

    Don
     
  18. Donmck

    Donmck Member

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    forgot to mention--

    the iris in your eye will stop down to a minimum of 2mm--

    I assume that'a a built in technology.
     
  19. RalphLambrecht

    RalphLambrecht Subscriber

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    Don

    I don't think that works very well. If that's true my 6x6 normal lens would have a sweet spot at f/45 and my 4x5 normal lens at f/64. You are deep into diffraction with both lenses at that point. See the graph in my post above.

    Actually, I don't think it's an absolute measure.
     
  20. Martin Aislabie

    Martin Aislabie Subscriber

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    Diffraction is IMO, a much over-rated concern for MF and Large Format unless you are producing exceedingly large prints.

    If you look at Ralfs’ excellent graph of performance v aperture, for a MF Lens it gives you a working range of f11 to f16 for critical work and f11 to f22 for standard observation.

    If you have a 5x4 the operating range is even greater f16 to f32 for critical work and f16 to f45 for standard work.

    If you are at all concerned about the effect diffraction has on image quality you could perhaps try looking at some of John Sextons work.
    Fabulous images of the very highest quality, which tend to be shot at very small apertures (typically f22 to f45).
    John kindly provides technical data for each of the shots in his books.

    It is worth considering, if you are concerned when looking at the chart, that photographic paper can only resolve about 5 l/mm

    Martin
     
  21. Donmck

    Donmck Member

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    Hi,Ralph-

    I believe that is correct, though maybe, somewhat misleading.

    Light passing through a 2mm aperture will have the same diffraction
    regardless of what camera it's mounted on-----however as you move up in format size, the film plane moves farther away from the iris, increasing the effects from diffraction.

    At the same time diffraction is increasing,film size is increasing-----at a linear rate ,keeping the signal to noise ratio the same.
    So, a print from 135,6x9,and 4x5 should all be "as sharp."

    So, I should have said,if your using small f stops, below 2mm is where diffraction becomes a big problem.

    Don
     
  22. RalphLambrecht

    RalphLambrecht Subscriber

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    Photographic paper resolves well above this figure. I don't have targets fine enough to measure it's capability, but I know from contact printing negatives that Ilford MGIV for example, resolves at least 50 lp/mm. And why shouldn't it? Paper emulsions have a very fine grain, much finer than film.
     
  23. RalphLambrecht

    RalphLambrecht Subscriber

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    Don

    I believe this is not the case. You can combine the absolute aperture measurement and the film distance to the lens aperture and, in fact, show that the diffraction limit does not depend on a fixed aperture value.

    The diffraction limit is dictated by the Airy disk. If the lens is focused at infinity, the calculations for the radius of the Airy disc simplify to:

    r = 1.22 * l * N

    and the maximum lens resolution (diffraction limit) is then given by:

    R = 1/r = 1 / (1.22 * l * N)

    where ‘l’ is the wavelength of light, and ‘N’ is the lens aperture in f/stops.

    As you can see, the diffraction limit depends on the wavelength of light and the f/stop and is not limited by an absolute aperture value.

    Not the same diameter but the same f/stop (regardless of film format) creates the same amount of diffraction.
     
  24. ic-racer

    ic-racer Member

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    Some people contact print 4x5 (in which case Airy disk size can be acceptably large). The lens manufacturer doesn't dictate how you will use the negatives so you have to know what you are doing when you move that aperture lever :wink: (unless you use a Minox, in which case Walter Zapp specifies the aperture for you :smile: )
     
  25. ic-racer

    ic-racer Member

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    A practical interpretation of what Ralph posted can be summarized as follows: diffraction is proportional to aperture diameter (in millimeters) across formats, whereas it is proportional to aperture number within a format.

    So, one way to understand it is that within a format a long lens magnifies the Airy disk, so you need a larger aperture diameter (but it works out to the same aperture number) and a wide lens shrinks the Airy disks, so you can use a smaller aperture diameter (but it works out to the same aperture number).
     
  26. RalphLambrecht

    RalphLambrecht Subscriber

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    The equation for the diffraction limit, posted in #22, is totally independent of film format. The f/stop and the wavelength alone control the diffraction limit.

    Diffraction is an optical phenomenon not limited to photography. Original research was done for telescopes and astronomy. The diffraction limit is a 'perfect' lens characteristic and has nothing to do with film formats.
     
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