Adox CMS 20 and lens resolution

Discussion in 'Black and White' started by willscarlett, Sep 13, 2007.

  1. I was reading on last night and caught this bit under the section for Adox CMS 20 film. In short,
    it says:

    If you want high resolution pictures you need to open your lens aperture to one stop below maximum
    opening. Otherwise the lens defraction will lower your lenses resolution down to half of what this film can
    capture. Best lenses are F1,4 high speed high quality lenses like Nikkors, Summiluxes, Rokkors or Canon
    lenses. This puts you effectively to between F2 and F4. Any normal winter day 2 hours after sunrise and
    up to two hours before sunset will give you something like a 125th or a 250th of a second at F 3,5. More
    speed would force you to stop down too much on a sunny day, letting slip what your lens can actually

    So I'm wondering, is the sweet spot for all lenses a stop under closed down from wide open? Does it
    depend on the film as well? Different format, but when I used to shoot 16/Super16 with Zeiss lenses, we
    were always told the sweet spot of those lenses was around a 5.6.
  2. Most lenses have to be stopped down several stops from wide open to hit their optimal sharpness, before the diffraction limit takes over. There are a few very pricey lenses that may hit optimum nearer to their largest aperture.
  3. JP, a pretty good rule of thumb for optimal sharpness, is f/5.6 for lenses faster than 1:2, and f/8 for slower lenses.
  4. I read a mathematical theory once,on lenses and angle of difraction or some such thing ,that I don't pretend to fully understand----but IIRC, the gist of it was that most(not all) lenses are sharpest when the iris is open 4mm.

    Some more tests.
  6. It's very easy to determine the sweet spot yourself. Put the camera on a tripod and shoot the same scene at all apertures. DOF will vary, of course, so be sure to focus on something of interest. You can examine the negatives on a light table with a magnifier. Look for tiny details and notice how they vanish at small apertures.

    I did this on a Pentax 67 some years ago. I determined that diffraction wasn't noticeable (for me, anyway) until I got down to f22. Prints later confirmed this.

    In the real world, you have to balance the "sweet spot" against getting enough depth of field. The slight loss of sharpness to diffraction at f22 is much preferred to parts of the picture being out of focus.

    I wouldn't get too excited over Adox's marketing blurb. Loss of sharpness due to diffraction (or sloppy technique)is visible on practically any film.

    What they're really saying is, "You gotta be a real man to use Adox CMS 20! Only the best photographers can take advantage of what we're sellling!" Which of course, implies that if you buy some, you'll be the best. :)

    Happy shooting!
  7. Hi John Paul,

    Ignore this statement from the film maker. Sharpness related to diffration is related to diameter of aperture (and in the case of DOF, diameter of aperture and distance to the subject).

    If you wish to explore the science, see Dawes and Dawes Limits of resolution.

    In testing lenses for various purposes, there is a tremendous difference between visual resolution and film resolution (much less). In tests for some superb 210mm LF lenses, the film resolution at f32 was always 1/3 as much as at f 5.6, however, the film quality at f32 was such adequate to make mural sized enlargements. Remember, it is diameter of aperture, not f stop.

    Diffraction is sort of like watering your flowers with a hose, if you just point it, the water come our in the same shape of the hose, if you put your finger into the stream of water it splays out in that direction. Something like that takes place in the bundle of light entergy as it passes by the lens aperture, the aperture blades cause the energy to be splayed out around, pretty much the same way regardless of the diameter of the aperure. The smaller the aperture, the higher the percentage of interference with the bundle of energy. The fact that the aperture is not really round but a series of lines and v slots, the light is in wave form and hits in different places and so the the interference can be greater in smaller apertures. In some macro testing with pinhole sized apertures in fine quality lenses, the images were so unsharp that you couldn't even focus them.

    Practically speaking, an aperture of about 1/8" (.0125" or about 3mm)or smaller will reduce sharpness due to diffraction. For the same reasons, some lenses are pegged so that they can't be stopped down because of sharpness reduction. We see that often in some P&S digital cameras where the lens can't be stopped down smaller than f8 or f11 due to diffration.

    Often, much will be published about the "sweet spot", of stopping down for 1.5 to 2 stops for better quality. Theoretically this should not be true, but, the spherically based lenses need to be stopped down because of slight differences in the focal length of the edges of lenses, and improving the Cos4th characteristics. Photographers simply have to make the compromises for DOF, shutter speed, and diffraction.

  8. I don't claim to know squat personally but when I bought my M7 with 50mm Summicron, I e-mailed Leica and asked them about the optimum aperture. They said that f5.6 gave the highest overall resolution but that f4 gave the highest contrast. I've also noticed that where all my Contax lenses have their highest MTF averages at f 5.6, the very highest MTF numbers can occur in the dead center of the lens at a wider aperture.

    I haven't researched this but I seem to recall that the Popular Photography "shootout" to determine the best 50/1.4 lens on the market (in 1999?)showed the highest single score for resolution to be 95 lp/mm for the Leica at f2.8, although they gave the "best overall lens" award to the Contax.

    Clearly, this is for when DOF isn't a consideration - but it seems that the very highest resolution can indeed occur a stop or two wider than the "optimum aperture".
  9. DOF definitely cross my mind when I read the article. Neal has also mentioned to me that when shooting at 1/60 sec or slower, sharpness can suffer.

    Lots of good stuff here, thanks to all.
  10. There is an earlier discussion on diffraction, relative aperture and aperture diameter here: (link)
    Best, Helen
  11. 1/60 and slower can suffer serious effects hand held. you can get lucky with a steady hand and good timing (in between heart beats and body motion) at that speed.

    unless it was a bright sunny day, i would never shoot a slow-film like CMS 20 without a tripod, and i'd probably use a tripod anyway. if you're going to suffer from motion blur, you'd have been better off using a 100 or 400 speed where at least things are sharp. going to the trouble and expense of using a speciality film might as well pay off :)
  12. Hi John-Paul, sweet spot for lenses is often also at a given distance. For example on a distance of 1 or 2 meter my Canon EF 1.2/85 L Mk 1 was not as sharp as its very high old.photodo rating would led to believe. I add a popphoto diagram of one of the very sharpest lenses: Tamron 3.5/180 Di Macro (similar diagrams have the 50/1.4 lenses) even sharper (even at close focus distance) is the 6x6 Zeiss Sonnar CF (CFE) 5.6/250 Superachromat it's sweet spot is FULL OPEN (f/5.6) also certain Leica APO ASPH lenses are said to have the sweet spot already FULL OPEN. however there are about 10 variables to "control/master" apart from the aperture. I wish you lots of fun trying it out.

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