Pentax 645N Hyperfocal

Discussion in 'Medium Format' started by hugh_brown, Jun 5, 2000.

  1. Gidday

    Some four months ago, I purchased a Pentax 645N AF camera. I have
    three prime lenses. I have been using manual focus as I shoot
    primarily landscapes and have been using the hyperfocal markings for
    the purpose of optimising depth of field. The challenge however, is
    that when I look through the viewer, part of the field is blurred. I
    have been ignoring that blur and taking two shots: one using the
    hypeerfocal markings, and the other using the viewer as my guide. I
    do not yet have a good loupe so am interested in whether I should be
    trusting the hyperfocal markings or the viewer. Any assistance would
    be most appreciated.

    Best regards,

    HUGH BROWN
     
  2. The hyperfocal markings in medium format are there just for orientation. Usually they are one or two f/stops off, which means that you must look for the hyperfocal distance for f/11 when working with, say, f/22. Otherwise it is logical to trust the finder, it is always so that what you see is what you get. And get a good loupe, that's crucial. Look for Schneider, Rodenstock or NPC. I have a 10x russian Horizon loupe (heavy as a tank, but with corrections and multicoated glas) for sharpness checking and a Rodenstock 4x for "general impression". Without a loupe you cannot evaluate the slides properly.
     
  3. david_henderson

    david_henderson www.photography001.com

    Difficult to say. Are you convinced that the blurring on the viewfinder is focus-related? Are you stopping down the lens to the selected taking aperture via depth of field preview before concluding that part of the field is blurred? The poster above is correct to point out that most dof scales are optimistic- and some widely so.

    Subject to these you should be able to trust either technique though I guess I'd expect to use them in different circumstances. By the way if you don't have a good loupe, your standard lens will make a good substitute- look through the rear element at the slide on a lightbox.
     
  4. Hugh, to use the viewer, you have to activate your depth of field preview button or lever. Otherwise, you're seeing the DOF as it would be if you shot wide open, because the lens is wide open for viewing.

    In practice, however, I find this just dims the image to the point that I can't easily judge the sharpness.

    An alternative is, as you've done, to use the hyperfocal distance to maximize DOF. But keep in mind that, regardless of the aperture or focusing technique, best sharpness is achieved at only one distance, the one at which you focus. Objects at all other distances will be less sharp to one degree or another. DOF is merely a way of expressing what is "acceptably out of focus," and even that varies with degree of enlargement and personal judgment.
     
  5. The hyperfocal markings on lenses are always too optimistic. I always stop it down at least one more stop (for macro lenses, this is often difficult since the markings are so close to one another). I know that the DOF tables for my MF lenses were computed at CoC=56 um (micrometers). I believe this is pretty typical for MF (for 135, often 30 um is used, and for LF, 100 um is used). In practice, I find the MF/LF CoC's to be about a factor of two too large. The best way is to compute your own DOF tables. A factor of two smaller in CoC diameter corresponds to twice the hyperfocal distance!

    PS: one potential pitfall of relying strictly on the viewfinder is that viewfinders (even waistlevel finders in the 'Blad and Rollei) aren't exactly 100%..more like 95% (and prisms often show even less). So even if the entire image on the viewscreen looks sharp, there's some of the image you can't see that's likely to be slightly out of focus. Invest in a top-end loupe to evaluate your transparenices (my favorite is the Zeiss 5x triotar), and take notes on what apertures were used for what lens (to learn how accurate the DOF scales are for each lens...differs from lens to lens).
     
  6. You can get my program that generates DOF tables here.
     
  7. Getting accustomed to using the DOF lever will solve the "blurring" problem. It just takes time to acclimate yourself to judge sharpness when the viewfinder is darker.
    As a general rule, the Ptax lenses are roughly one stop overly optimistic in their hyperfocal markings. I've always had excellent results using one stop less than the lenses specify.

    Rob
     

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