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  1. #1
    pellicle's Avatar
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    large format lens testing (method confirmation question)

    Hi

    I have a chart here by Norman Koren which is for testing lenses. I wonder if this is applicable to use with my 4x5 lenses by applying it in the exact same manner as his site documents. For example

    For the target's lp/mm scale to be calibrated correctly it must be at the correct distance from the camera (though the distance doesn't have to be precise— small errors of 1 or 2 percent are not significant). The equation details are here.

    His materials seem to be aimed at 35mm lenses but I thought that a 90mm lens is a 90mm lens irrespective of its coverage so I could just apply his methods directly. IE:

    Suppose you are testing a 200 mm lens. With the 5 mm targets the nominal magnification (the magnification where the lp/mm scale reads correctly) is 50. The lens-to-target distance is d1 =(M+1) f = 51*200 mm = 10.2 meters.
    so does this mean I use 51 * 90 = 4.59 meters

    my Fujinon 90mm f8 seemed to get about 45lp/mm using this method.

    thanks
    Theory: you understand why it should work but it doesn't
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  2. #2
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    That sounds about right. I assume that is the center resolution, and for the lower contrast comparison? I've used his test charts, too, although not at the prescribed distance but at 1:20. Thats because I don't have a wall or door around big enough to fill the frame with the patterns tacked on and I wanted to see both the corners and the center of the neg.
    Last edited by acroell; 02-09-2009 at 03:22 PM. Click to view previous post history.

  3. #3
    df cardwell's Avatar
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    Why are you doing this ?

  4. #4
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    The result you describe seems a bit low when compared with the hevanet LF tests done on several other LF 90s at f/11...

    http://www.hevanet.com/cperez/testing.html

    I haven't done any such testing myself, but it is a worthy pursuit, particularly if the lens hasn't satisfied you.
    "Only dead fish follow the stream"

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  5. #5
    bowzart's Avatar
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    Quote Originally Posted by keithwms View Post
    ...
    I haven't done any such testing myself, but it is a worthy pursuit, particularly if the lens hasn't satisfied you.
    I dunno.

    When I don't like a lens I find myself not using it, and it is a candidate for disposal. There is no need to test it.

    When I do like a lens, I can't think of why I would bother testing it, unless maybe I wanted to have a show of test charts - which I have considered, actually, but I'd find something really strange to do to them, I'm sure. By themselves, they aren't very interesting, IMHO.

    Maybe its just me, but it seems to me that the image is more important than numbers.

    Usually, when I say stuff like this, nobody pays any attention. So Be It.

  6. #6
    keithwms's Avatar
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    If you don't want to test your lens then don't test your lens
    "Only dead fish follow the stream"

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  7. #7
    bowzart's Avatar
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    Quote Originally Posted by keithwms View Post
    If you don't want to test your lens then don't test your lens
    Guess I won't. Thanks.

  8. #8
    pellicle's Avatar
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    Folks

    thanks for the feedback. To answer the question of why there are some reasons. Firstly I wanted to understand the process, I'm someone who likes to understand what it is that I'm doing (you know, know my tools, understand the processes).

    I also have an issue with the Fujinon 90 and I really don't know if my example is good or bad. I can get subjective about it till the cows come home, but I thought that by measureing it I'd have some idea.

    I was also in a discussion with someone who was big on opinions but only used evidence from others (which I did not trust). As a result of that discussion I put up this page on my blog.

    now that I know what I have I'll consider selling my Fujinon and acquiring a Nikon f8 because I've grumbled about it for some time now.

    About positioning, the chart was 'sort of central' towards the left a little. contrast was 'lowish' and exposure time was 1 second f22
    Theory: you understand why it should work but it doesn't
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    Here theory and practice meet, things don't work and I don't know why
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  9. #9
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    Keith, you can't compare the numbers with the ones from Chris Perez and Kerry Thalmann, since its a different target. In the Koren test target you compare the remaining contrast of your target with two calibration targets of about 20% and 50% MTF. The 20% one gives the higher resolution, but still not as high as a disappearing MTF. Read the instructions on the web site. A rough guide is to add another 10-15% in lp/mm to arrive at Chris' and Kerry's numbers.
    The best number for the center I got was over 80lp/mm in the center of a Super-Symmar HM 120mm at f/11 for 20%MTF, but that falls down quickly to 50lp/mm at f/22. I have not tested any 90mm lenses, but two 65mm ones; their best number for the center was 52 lp/mm.

  10. #10
    pellicle's Avatar
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    Hi

    Quote Originally Posted by acroell View Post
    ... In the Koren test target you compare the remaining contrast of your target with two calibration targets of about 20% and 50% MTF. The 20% one gives the higher resolution, ... The best number for the center I got was over 80lp/mm in the center of a Super-Symmar HM 120mm at f/11 for 20%MTF, but that falls down quickly to 50lp/mm at f/22. ... but two 65mm ones; their best number for the center was 52 lp/mm.
    so perhaps my figure on the 90 isn't far from what I'd be expecting. I could have used higher contrast lighting (which would assist the film's response) but I wanted to see a normal use scenario (I often use the camera in low contrast situations).

    I'd have thought that at f22 I still wasn't diffraction limited, which is why I normally pick f16 or f22 with my Fujinon 90 (besides it vignets a bit at f8)

    anyway ... the comments are helpful
    Theory: you understand why it should work but it doesn't
    Practice: it works but you have no idea how
    Here theory and practice meet, things don't work and I don't know why
    Homepages: here Blog: here

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