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Thread: Focusing

  1. #11
    Ed Sukach's Avatar
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    </span><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (tschmid @ Dec 13 2002, 07:30 AM)</td></tr><tr><td id='QUOTE'>Ed,

    although your suggestion is based on common experience, you will find that more than a few enlarger lenses will show a significant focus shift when stopped down. Although enlarger lenses are designed for flat field projection, the practical performance is always a compromise within the variety of possible magnification factors and apertures.

    Apart from that, the performance of an enlarger lens can be bad wide open making it difficult to find any sharp grain. In my experience, it is always better to use the grain magnifier at working apertures. If the grain is sharp, your picture will be sharp, too, no matter how many degrees you can turn your focus knob around this. However, if you check the edges, too, you will see that there is usually not that much tolerance.

    </td></tr></table><span id='postcolor'>
    I have heard this idea of focus "shifting" at different apertures many times, but from my training in physical optics it is difficult to explain. It is interesting to note that few claim that that phenomenon also applies to camera lenses, and the last time I looked the same optical principles applied.

    My experience does not cover every enlarger lens ever made; it is largely restricted to Rodenstocks and Schnieder (on the turret of my Omega now) and in times past, Elgeet , Federal (Rodenstock??) and a few others ... I have never observed that "focus shift" in any of them, although I will be the first to admit to the possiblity of having "lucked out" to a major degree.

    Also, in a few (actually more than a "few") hours of lens testing on the Optical Bench (dating myself here&#33 I&#39;ve found various optical flaws and manufacturing errors - but I cnnot recall a "shifting of focus" as one of them - and come to think of it - I cannot ever remember a criteria of "Stability" (?) of Focus at Various Apertures.

    I am familar - very - with the difficulties of lens design, and just why it is that large-aperature capacity lenses are optimized for certain apertures (read: best compromize) but the parameters existing for enlarger lenses are far more restricted.

    Personally, if I ever did find an enlarger lens with anything aproaching a "focus shift", I might conduct an experiment: find out how many times I could make it "skip" across the water of my local pond.
    Carpe erratum!!

    Ed Sukach, FFP.

  2. #12
    David A. Goldfarb's Avatar
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    I&#39;ve seen it with a Componon-S and old Dagors have a notorious shift.

    I think the explanation is not that the focal length of the lens actually changes, but that it is an illusion resulting from uncorrected spherical abberation being corrected when the lens is stopped down.
    flickr--http://www.flickr.com/photos/davidagoldfarb/
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  3. #13
    clay's Avatar
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    >>>It is interesting to note that few claim that that phenomenon also applies to camera lenses, and the last time I looked the same optical principles applied.

    Actually quite a few of the old dagors and protars (view camera lenses) will shift focus significantly as you stop down. I have a box full of negatives to prove it. This has been discussed ad nauseum in many of the online discussion groups.

    Clay


    I just want to feel nostalgic like I used to.


    http://www.clayharmon.net - turnip extraordinaire

  4. #14
    Ed Sukach's Avatar
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    </span><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (David A. Goldfarb @ Dec 13 2002, 03:23 PM)</td></tr><tr><td id='QUOTE'>
    I think the explanation is not that the focal length of the lens actually changes, but that it is an illusion resulting from uncorrected spherical abberation being corrected when the lens is stopped down.</td></tr></table><span id='postcolor'>
    I agree. Having worked in Optical Quality Control, I have been *very* sensitive to careful identifcation of optical properties and specific errors. *Apparent* quality can shift significantly as the parameters of testing change. A smaller aperture will significanly increase the
    "depth" of focus- properly known as "hyperfocal distance", and edge-to-edge resolution, by restricting the area open to both spherical and chromatic abberations, curvature of field... (and a few other factors) until diffraction effects kick in.

    Incidentally, one of the most valuable accessories I own are 40.5mm "Soft" filters. I bought two of them solely out of curiosity from a "bargain box" as they were the precise size to fit my Rodenstock enlarging lenses. Indispensable for printing Portraits... they do a remarkable job in eliminating minor (and some not-so-minor) skin flaws. I *do* have to remember to remove them when I focus.

    I prefer to keep the sharpness in the negative (i.e. not using "Softars") and "adjust" as necessary in the enlarger.
    Carpe erratum!!

    Ed Sukach, FFP.

  5. #15

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    </span><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (Ed Sukach @ Dec 13 2002, 11:01 PM)</td></tr><tr><td id='QUOTE'>I have heard this idea of focus "shifting" at different apertures many times, but from my training in physical optics it is difficult to explain.</td></tr></table><span id='postcolor'>
    Ed,

    this is not a real focus shift, it is an apparent focus shift. Some aberrations can actually be diminished by a slight defocus (i.e. an offset to the extension computed by the exact focal length and the exact magnification ratio/subject distance). Some of these aberrations are influenced by the aperture diameter. So stopping down a lens can shift the apparent focus towards real focus (usually not the other way round). David probably meant the same thing. Depending on design goals, lenses show different behaviors here. The primary design goals of enlarger lens are: resolution and flat field. Camera lenses usually have other or additional design goals, resulting in different focus-shift behavior, too

  6. #16
    Ed Sukach's Avatar
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    </span><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (tschmid @ Dec 14 2002, 02:26 AM)</td></tr><tr><td id='QUOTE'>
    This is not a real focus shift, it is an apparent focus shift. Some aberrations can actually be diminished by a slight defocus (i.e. an offset to the extension computed by the exact focal length and the exact magnification ratio/subject distance). ...</td></tr></table><span id='postcolor'>
    Again, I agree.

    You may have noticed that I wrote about "Optical Bench" testing - that dates me... I think the last lens testing I did was around 1990. Now, most lens quality is defined in terms of Modulation Transfer Function (MTF).

    An excellent article is "Understanding Lens Contrast And the Basics of MTF", by Mike Johnstone, available at The Luminous Landscape web site, http://www..luminous-landscape.com under "Tutorials", "Understanding Series"...

    Luminous Landscape is a good site, but strongly tilted towated "digital" imaging.
    Carpe erratum!!

    Ed Sukach, FFP.

  7. #17
    Ed Sukach's Avatar
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    </span><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (Ed Sukach @ Dec 14 2002, 05:54 AM)</td></tr><tr><td id='QUOTE'></span><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (tschmid @ Dec 14 2002, 02:26 AM)</td></tr><tr><td id='QUOTE'>
    This is not a real focus shift, it is an apparent focus shift. Some aberrations can actually be diminished by a slight defocus (i.e. an offset to the extension computed by the exact focal length and the exact magnification ratio/subject distance). ...</td></tr></table><span id='postcolor'>
    Again, I agree.

    You may have noticed that I wrote about &quot;Optical Bench&quot; testing - that dates me... I think the last lens testing I did was around 1990. Now, most lens quality is defined in terms of Modulation Transfer Function (MTF).

    An excellent article is &quot;Understanding Lens Contrast And the Basics of MTF&quot;, by Mike Johnstone, available at The Luminous Landscape web site, http://www..luminous-landscape.com under &quot;Tutorials&quot;, &quot;Understanding Series&quot;...

    Luminous Landscape is a good site, but strongly tilted towated &quot;digital&quot; imaging.</td></tr></table><span id='postcolor'>
    Correction: One too many periods in the address - Should be:
    http://www.luminous-landscape.com.
    Carpe erratum!!

    Ed Sukach, FFP.

  8. #18
    clogz's Avatar
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    An old trick to help you to focus: take an old, preferably dense, negative; take a ruler and a needle and make a nice scratch on the emulsion. You can focus on that.
    Digital is best taken with a grain of silver.

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