large format lens testing (method confirmation question)

Discussion in 'Large Format Cameras and Accessories' started by pellicle, Feb 9, 2009.

  1. pellicle

    pellicle Member

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    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 preciseĀ— 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:

    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
     
  2. acroell

    acroell Member

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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 a moderator: Feb 9, 2009
  3. df cardwell

    df cardwell Subscriber

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    Why are you doing this ?
     
  4. keithwms

    keithwms Member

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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.
     
  5. bowzart

    bowzart Member

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    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. keithwms

    keithwms Member

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    If you don't want to test your lens then don't test your lens :wink:
     
  7. bowzart

    bowzart Member

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    Guess I won't. Thanks.
     
  8. pellicle

    pellicle Member

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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
     
  9. acroell

    acroell Member

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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. pellicle

    pellicle Member

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    Hi

    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
     
  11. pellicle

    pellicle Member

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    Hi

    certainly if I was "well heeled" I'd just grab the better lens (in my case I've been eying off the Nikkor 90mm f8) but not having access to a ready supply of funds I'm trying to see if
    1. my expectations are too high
    2. if I can get any quantative measure to compare the lens I have with the tests of the lens I'm interested in
    3. by evaluating my technique seeing if the improvement can come from my technique (meaning I don't need to spend $700 on a lens)

    however ... all donations are accepted :smile:
     
  12. df cardwell

    df cardwell Subscriber

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    Scattered observations

    Diffraction limit: f/22. ALL good lenses perform the same at f/22 (as soon as you image on film).

    From a practical point of view, a PERFECT lens will image no finer detail than .0125 mm at f/22.... 80 lp/mm
    (f/32 = 55 ------ f/16 = 110)

    .............

    You are testing a camera system (meaning the flaws introduced by ground glass placement,
    focus error, and film flatness, film performance, etc.). Your RESULT does NOT isolate the Fuji lens,
    it integrates the lens into the rest of the stew.

    Have you confirmed the ground glass is in the exact place ?

    WHAT KIND OF FILM ARE YOU USING !!!!!!????????? DEVELOPER ???

    ........

    Koren's test measures the point where the SUM of the lens contrast and the film contrast becomes noise.
    If you are using a normal film at a normal development, you can't get better than 100 lp/mm from it.

    IF a perfect lens at its perfect aperture can image 100, and the film can image 100, this is your formula:

    1/R = 1/Rfilm + 1/Rlens 1/R = 1/100+1/100 R=50

    Increase the contrast of the film, you can image a higher result:

    1/R = 1/200 + R/100 R=67

    Factor in diffraction, your results are really pretty awesome.

    ..........

    There is really NO difference in the performance of a Fuji / Nikon / Schneider / XYZ design. The only difference will be from sample to sample. The best way to see where you might be losing performance in your camera system to to check the ground glass placement.

    Then, the best way to improve your 'acutance' is to use a film like TMY or TMX.

    ..........

    All 'sharpness' issues have to consider the contrast of the scene. A nice winter landscape will lack the contrast of summer sun. This means that you can't get some theoretically perfect contrast from a low contrast scene using a film and developer intend for making pictures. This is why Sharpness of Seeing is more important than the Sharpness of your Lens. If you compose an image using adjacent tones (middle gray next to dark gray) your image will look soft, regardless of your equipment. If you compose black against middle gray, it will have more snap.
     
  13. keithwms

    keithwms Member

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    Whoa wait a sec df, who shoots everything at f/22? Not I for one. Isn't the whole point of using an LF camera to be able to manipulate the plane of focus so that you don't have to stop down excessively to compensate for the much larger circle of confusion? And conversely, to be able manipulate OOF transitions so that the tightest focus is placed where you want it? My, I guess I have been doing it all wrong :wink: Then again, nobody invited me into the f/64 (or should I say f/22?) club so I am still a free spirit.

    Seriously, I just don't think anybody should assail someone who wants to learn how to test lens resolution. C'mon now, if it's not for you then don't do it. But at some point people should learn about the difference between high and low frequency information and how colour rendition affects focus and so forth. These are really basic issues that the O.P. apparently wants to learn about.

    *Of course* the image is the most important thing. So...? We should shut off our brains and just make spectacular images? Great, let's do that....

    (no offense intended, I am sarcastic after 2 cups of strong coffee...)
     
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  15. df cardwell

    df cardwell Subscriber

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    Keith

    OP tested his lens at f/22 because he thought that it would not be affected by diffraction.
    Since Diffraction IS in fact determining the outcome of his test, I brought it up.

    Testing is kinda fun. But our run-of-the-mill photo gear is so good,
    we have to be very rigorous WITH our tests to get answers that are helpful.

    d
     
  16. keithwms

    keithwms Member

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    Agreed :wink:
     
  17. pellicle

    pellicle Member

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    Hi

    df

    lots of good ones too ... I'll chew on this and reply to some of them later (should you be so kind as to be following up on this)

    Keith

    I didn't feel assailed ... I'm sorta used to terse and my thesis examiners were about the same (only that counted more for me ;-)
     
  18. Ralph Javins

    Ralph Javins Member

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    Good morning, Pellicle;

    Nice to see that someone else has discovered that there is more to lens testing than just the lens resolution numbers you get from a test target. And, of course, which target? Your observation on the limiting factor of the sensor in a digital camera is one that is overlooked fairly often.

    Nice thread. Thank you for bring it up. An enjoyable read, including your detailed comments on your own site.
     
  19. pellicle

    pellicle Member

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    Hi


    ok ... I will try with f8 and f16 to see if there is noticable advantage.

    that is true, but then all testers must use something to hold the lens and something to read the information, so like any testing procedure I have my system and they have theirs ... now your next question is a step towards confirming how well I've performed this analysis on my testing equipment.

    yes, although I have not measured it with a probe but only by exposing film.
    My procedure was to:

    1. place a ruler on the desktop
    2. with the camera looking down at the desktop at about 30deg inclination I focused on the 15cm point of the ruler (about mid way) using my Fujinon 180mm lens @ f5.6 and at a magnification of about 1:2
    3. I confirm focus with a x10 loupe and take a sheet for examination
    This rough test indicates the the center of focus is pretty darn close to where I think it is and the movement of film plane by as much as a 0.1 mm in this case will show up

    Also, my sheets are awfully flat (its surprising) which I attirbute to the humidity here in houses in Finland. (I almost never have 120 film curl for instance)

    well, there is no need to yell ... and besides there are bold and underline provided here for emphasis ...

    In this instance I was using ADOX CHS film processed in a BTZS tube using D-76 neat and rolled gently during the entire 6 minutes at 22deg C

    I've heard something like that before. I'm willing to accept that, but no matter I don't think I'm likely to get that value :smile:


    well I'm not familiar with that way of dealing with accumulation of error in the system ... but I'm not sure how to apply (my chemistry background) knowledge to this system.

    I'm more aquainted with how to include errors as %age or as plus minus values.

    however it seems rather a conservative calculation (which is good) and so should result in a worst case estimate. I'm ok with that.

    ok ... well ... that's good to know :smile:

    ok ... well, you see ... not having had one I have no basis for comparison. Which is the goal of this exersize .. a basis of comparison.

    The tests by Perez and Thalmann

    seem to show that there is a substantial difference between the Nikkor 90f8 and (for example) the Schneider SA

    its the edges which really make me interested in this lens as I like to do work with either a 6x12 roll back or a "split sheet" method like this

    [​IMG]



    it depends, for example here

    [​IMG]

    I think that the black on white edge shaprness of the willow on the snow was as important as anything for making the image.

    Strangely I was unsatisfied with the sharpness from this situation, but in a MUCH lower contrast was very happy with this lenses production

    [​IMG]

    which was my 180 (mate, that lens is great!) Looking at this, and thinking about what you say below, perhaps its the black on grey again ... as the shadow details of twigs in the murky light at the bottom are where its "gosh" for me.

    I gess that's why this image worked out so well ... black on grey

    [​IMG]

    down on the corner here ...

    [​IMG]

    anyway, thankyou for all your information and suggestions. I'll try the f8 and f16 and post results

    :smile:
     
  20. pellicle

    pellicle Member

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    Keith

    well ... me sometimes. I prefer not to go bigger than f11 and so I'm usually somewhere around that or f16 on either of my lenses.
     
  21. pellicle

    pellicle Member

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    I said above that I would compare the lens at f8 and f16, so here is that comparison. I processed the f8 / f16 image on a single sheet (using a half dark) and shifted the lens to keep the test target in the center of the image circle

    [​IMG]

    please click on thumbnail for larger view (as I didn't wanna waste anyone's screen size)

    I can see a little bit in it ... perhaps with better gear it might be clearer. Personally I'm comfortable with them being quite similar (meaning that diffraction is less of an effect than I'd have thought)

    NOTE: the lens is wide open at f8 and f64 is the smallest aperture.
     
  22. df cardwell

    df cardwell Subscriber

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    Working with your method, I don't see how ANY comparable Biogon-type lens could outperform your Fuji.

    To make a more discerning test, you would need a target with much higher image contrast. Your Koren chart probably has a contrast of 16:1, which reduces most films to a fraction of their Resolution rating. You would also get potentially higher scores if your film were developed to a higher CI.

    But if the purpose of the test is to examine how, under field conditions, your 90 Fujinon would perform, it is a good test.
    But NO properly made lens will exhibit significantly higher scores than your Fuji, even if it somehow were capable of higher tests from an optical bench. The SYSTEM buffers a miraculously perfect lens. But it also buffers your lens ! How much better is IT than a Nikkor ? No way to tell. This is why old, decrepit and worn out photographers like myself don't test any more: if it makes a GOOD picture in the field, we know it will test as well as we can test the lens outside a laboratory. Taking normal pictures presses a lens more severely than will a lab test.

    On the Thalman-Perez Tests: for decades and decades, that type of test has not seen any serious use. First, the actual test has little bearing on normal image making, secondly it is open to misinterpretation, and most importantly, while it might describe the lenses in the test, it would need a LARGE sample to infer any performance from lenses outside those tested. It is interesting, but you should not pay any attention to it. Your photographs are excellent, and you have no reason to be concerned about the quality of either your vision, or your lens.

    If you DO want to get the maximum performance from your outfit, you might use a film like FP4, with PyroCat. It will bring every bit of imaging potential from your system. I hope to see more of your pictures, they are lovely. A high acutance developer, and PyroCat has the best imaging qualities of anything available today, will make more of a difference than changing lenses.

    (forgive my SHOUTING. I have arthritis in one hand and it has been a little harder lately to make B and I and U.)

    Oh, and here is a link to a Zeiss explanation of practical photographic resolution: http://www.dantestella.com/zeiss/resolution.html
    d
     
  23. pellicle

    pellicle Member

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    Hi df


    ok ... that's nice to know. Now, all I need to do is sort out that sticky seiko shutter. It really doesn't like the cold.


    it was ... I hadn't given it much thought before, but I guess that like hifi reviews tests are often intended to make a lens 'look' better than it is.


    well I'm not good at taking compliments, but thanks :smile:


    well perhaps, but I'm not after performance per-se I'd like to get it, but other criteria (like look) are more important to me (and I use colour a lot)

    no stress ... I have to occasionally call my dad and tell him to take the caps lock off.


    handy ... thanks!

    Its a struggle working with LF but the results are often worth it. I have a permanent quest to make the best image I can when I see something which deserves pulling out my 4x5 for ...

    Thanks for your inputs and information it all helps :smile:
     
  24. Ed Sukach

    Ed Sukach Member

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    Having worked with lens design parameters, I have questions regarding the above information...

    How was this diffraction effect calculated/ Has there been consideration given to focal lengths?

    An example of the calculation/s would be very helpful.
     
  25. df cardwell

    df cardwell Subscriber

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  26. pellicle

    pellicle Member

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    Hi

    perhaps the following will help

    RN Clark's page has a good table just below where this link goes to. Take care that you get to that point on the page its really long. After page has loaded just refresh to make sure it takes you to the anchor (hes an astrophysicist and into photographing with telescopes)

    Roger (we have communicated a few times) has a few "limitations" with writing simply and for the layman. He basicaly says (buried on that page)

    where:
    • w = wavelength
    • f = focal length,
    • D = aperture diameter, and
    • f_ratio is the f/ratio of the optical system
    ,

    Diffraction spot diameter = 2 * 1.22 w * f / D

    Since the Fnumber and it is defined as:
    The ratio of the focal length divided by the diameter of the hole. It will be a constant for all focal lengths.


    so f_ratio is f / D so we can reduce this to

    Diffraction spot diameter = 2.44 * w * f_ratio,

    Roger then cites:
    Dawes limit = 1/(Fw) in line pairs per mm, where
    Rayleigh limit = 1/(1.22*Fw) in line pairs per mm, where
    50% MTF ~ 0.39 * Dawes limit.
    80% MTF ~ 0.16 * Dawes limit.

    and then provides a table where
    f16 = 100lp/mm
    f22 = 75lp/mm and
    f32 = 51lp/mm

    so its falling off fast at f32 but (in my opinion) still acceptable (all other factors considered) at f22 (which is why I pick it often enough when I can't quite fudge all parts in focus with some tilts and swings)


    also this is a good page which is a little introductoyr and has a calculator as well.

    HTH