Ctein grain focusing "gotcha"

Discussion in 'Enlarging' started by michael_r, Aug 1, 2011.

  1. michael_r

    michael_r Subscriber

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    Anybody find these big focusing errors Ctein mentions in his Post Exposure writings? I'm thinking about running the simple test he explains, just for the hell of it, but I'm wondering why nobody else ever talks about this phenomenon, or why even in workshops with some of photography's big names, I've never heard about it? Based on the way Ctein describes it (with focus for an 8x10 print potentially being off by something like 15mm (!!!)), that would be horrible. And it seems like you can't avoid it even with a high quality APO enlarging lens. Strange, and disturbing.
     
  2. Greg Davis

    Greg Davis Member

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    I read that part, but since I don't own the blue filter for my grain focuser I haven't tested it. Try it and post your results.
     
  3. BetterSense

    BetterSense Member

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    Could you please better explain this "gotcha" so that some of us can understand it? It's not very clear from your post.
     
  4. Greg Davis

    Greg Davis Member

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    You can download the whole book from Ctein's site for free and read that section if you like. It may provide a more complete explanation.

    I also remember someone posting here that they did test it and found just the opposite, that is, they had sharper prints using the blue filter. I believe they also had a blind test from 3rd parties choose the sharpest print, which was the one printed with the blue filter focus. Personally, I think it works for some people and not others due to individual ability to focus blue light with the naked eye.
     
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  5. David Brown

    David Brown Subscriber

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    I sometimes wonder how any of us ever make a print ... :whistling:
     
  6. Andrew O'Neill

    Andrew O'Neill Subscriber

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    All About "Gotcha"

    You can download Ctein's book for free at ctein.com:

    http://ctein.com/booksmpl.htm

    Go to page 76 for an explanation to the gotcha factor.
     
  7. michael_r

    michael_r Subscriber

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    Greg, there are two LCA issues discussed by Ctein. It seems like you're referring to the Patrick Gainer one where he says we should not focus with blue light. Is this the one you're thinking of?

    Here I was referring to the other issue he raises, with VC papers, in which he found depending on how much contrast you dial in (ie how much blue light you let through), the plane of focus can apparently shift by a huge amount compared to using yellow filtration. This would be due to the blue emulsion being significantly sensitive to light around the UV spectrum, and even well corrected lenses are not corrected for longitudinal chromatic abberation outside the visible spectrum.

    I wonder if you can solve the entire problem (if there is one) by simply putting a UV filter in the light path.
     
  8. michael_r

    michael_r Subscriber

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    No not that one. That's the "Gainer effect". Go to page 131. That's where it really gets ugly.
     
  9. Greg Davis

    Greg Davis Member

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    Yes, I am thinking of the Gainer effect. I'll reread the other and think about it a few minutes.
     
  10. chimneyfinder

    chimneyfinder Member

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    Hello Michael, I recently read Ctein's book, generously offered as a free download (linked from Mike Johnson's 'theonlinephotographer'), and thought that I didn't need to do a whole series of tests regarding the focus anomaly of paper, because it would be very time consumming given that I enlarge 4 different formats to 3 different sizes using half a dozen different papers. Hence, the 'simple' test to work out the paper's plane of focus as opposed to the optical one (yes, that's right folks !) would be a major undertaking to have real meaning. However, out of interest, I will try it for one set up to see if there is a significant difference: not sure when, but it is on my 'to do' list.
    Only ever under a magnifier have I noticed that a print hasn't been as sharp as hoped and put this down to the point of capture, and here lies the context in which I put the existence of focus anomaly: so far my prints are sharp enough for normal viewing, so I am not alarmed that they could be sharper. However, I am interested in this and so will, as said, explore.
    For anyone browsing this, the research by Ctein outlined in 'Post Exposure: Advanced Techniques for the Photographic Printer' showed focus anomalies on variable contrast paper due to the spectral sensitivity of papers receiving a different plane of focus to that of our eyesight. This differs between lens and paper combinations, but is a recognised phenomena by manufacturers. Basically, there is little in lens or paper manufacture to overcome this. However, as said, this is looking at critical sharpness and under normal viewing most prints are probably judged to be 'sharp enough', if they aren't then read the book.

    Regards, Mark Walker.
     
  11. michael_r

    michael_r Subscriber

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    For whatever it's worth, since the test appears to be pretty simple to perform, I'm going to try it just out of curiosity.

    My prints look pretty sharp to me. But I never need high contrast filtration (I'm usually around grade 2) so the effect might be negligable at those settings. Who knows. So I'd be curious to see if there is a noticeable effect when dialing in higher contrast on my setup with my usual paper.

    The figures thrown out by Ctein are still somewhat shocking to me. I mean, if you're making an 8x10 print from a 35mm negative (as in his example) and your focus is off by a whopping 15mm, wouldn't the print be grossly unsharp using a reasonable aperture?
     
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  12. Greg Davis

    Greg Davis Member

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    You are referring to the section in the lens testing on page 82 titles "An Unfortunate Omission". I suppose you could test for this by checking for correct focus in the extreme corners vs. the center at the extreme ends of the contrast scale. If you have a UV filter of the proper size, try running the test both with and without the filter in place and see if there is any perceptible difference. Ask other people if they can pick out a sharper print to eliminate bias.
     
  13. ic-racer

    ic-racer Member

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    Oh no, not this again...
     
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  15. Greg Davis

    Greg Davis Member

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    I know, right?
     
  16. michael_r

    michael_r Subscriber

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    Sorry about that. I guess this has been hashed and rehashed to death.

    Greg, in the detailed description he gives beginning on page 131, he suggests a simple test raising and lowering the easel from the point of visual focus and seeing if either of the "out of focus" prints are sharper than the one that appeared to be best in focus visually.
     
  17. michael_r

    michael_r Subscriber

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    Sorry I meant the discussion beginning on page 145, not 131.
     
  18. chimneyfinder

    chimneyfinder Member

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    Hello, again, Michael, (just read the previous comments). I don't think the effect can be practically solved with a UV filter because of the various conditions under which you would want to enlarge. The focus effect will shift dependent upon the filtration used, the amount of enlargement and the aperture. Also, Ctein was using specialist UV filters which only reduced the problem and cut down light by half and made grades 4 and 5 fall outside the parameters. The only real compromise, he suggests, is to make fixed focus tests with shims at 5mm and 10mm and none, under the easel to: 1) determine if you have a shift, and 2) to approximate its compensation. This being for one paper, lens, filtration, enlargement size only.
    This is how I interpret his findings, anyway. (It appears that the theory of focusing through a blue filter - which I have never done- is not a reliable method).
    Regards, Mark Walker.
     
  19. chimneyfinder

    chimneyfinder Member

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    The point of bringing this up 'again', is that for many people it will be the first time. Not all posts can be original.
     
  20. Monito

    Monito Member

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    Perhaps such experienced forum readers (and writers) can go beyond simple expressions of dismay and provide links to the relevant threads which in the past have resolved this issue (these issues), at least to the extent they can be resolved in this Enlarging forum. Failing that, at least they could point out the relevant thread titles or keywords for searches (though if they can do that then they can probably provide links to the best threads).

    How about it? A helping hand rather than a superior statement?
     
  21. Mainecoonmaniac

    Mainecoonmaniac Subscriber

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    Are we not seeing the forest for the trees? I mean not seeing the print for the grains?
     
  22. michael_r

    michael_r Subscriber

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    Thanks all. I'm not a newbie at any of this so I should probably do a search when I think of one of these idiosyncratic topics before starting a new, potentially repetitive thread. But sometimes I just jump the gun :smile:

    Anyhow, I'm going to run the simple test from the end of that chapter, but agree with Mark Walker that even if anyone has this problem there doesn't seem to be much you can reasonably do about it.
     
  23. ath

    ath Member

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    Sometime ago (before I had Cteins book) I read that prints on VC paper might be not really sharp because of the sensitivity to UV. I made a simple test: raise the head as far as possible (resulting in a mag. of 17), make a print with and without UV filter. Guess what - no difference visible (EL Nikkor N 50/2.8), case closed.
     
  24. pentaxuser

    pentaxuser Subscriber

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    I think the OP brings up a valuable point. I have just re-read Ctein's book again and confess:

    1. I didn't fully understand the section but was astounded by the figure of 15mm, thinking as Michael did that this would show up as plain as your nose
    2. I still cannot figure out why this kind of problem didn't spread worldwide alarm at the time of his publication and protests to paper manufacturers. You'd think that it would have entered into the annals of darkroom disasters and still be talked about

    3. Is it possible that whatever was causing the problem that Ctein saw could somehow have been peculiar to his set-up, unlikely as this seems? Might paper manufacturers have since done something to rectify the issue?

    Ath had no issue and neither or so it would seem, did the majority of users so what was happening in Ctein's tests?

    Unfortunately he does not go on to say why it may be that most users will never experience the issue and yet the very fact he devotes a section of his book to it indicates that he thought it a serious enough issue to put it before his readers.

    Actually there are a number of "alarms" raised by Ctein in the book which does not seem to have engaged the darkroom world such as serious issues with, I think, some makes of RC paper which he cites and which should be a separate thread. Again maybe the issues he and a fellow photographer of high standing experienced were not that widespread and have been subsequently solved.

    I haven't got the date of the book but it would appear that he was speaking of issue in the mid 1990s


    pentaxuser
     
  25. MattKing

    MattKing Subscriber

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    It seems to me that 15mm isn't all that much.

    Remember that we are talking about depth of focus at the paper plane.

    If you have ever tilted an easel to try to correct converging parallels, most likely you have needed at least that amount of depth of focus.
     
  26. chimneyfinder

    chimneyfinder Member

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    Ctein is a very clever fellow and anyone who reads his books and articles will know how scientific his analysis is. He is essentially looking at the fine margins of printing which, as with all worthwhile study in detail, has a broader implication and comes with useful insight: delving into the real nitty-gritty of what occurs in a process, for example. Ultimately, it is what creates further development and he had responses from paper and lens manufacturers on this issue (and they concurred that there is a focus factor), which is no mean feat. Furthermore, the possible solution creates another set of theoretical problems: as with most things compromise is the result, but at no great cost to ourselves, quite the reverse.
    Personally, I can live with the fact that I am not going to achieve 'perfect' pictures, but I am grateful that others are looking into this and giving me the benefit and, maybe, helping to keep alive the advancement of our silver world. (It's what the good forums, like this one, thrive on and perpetuate).
    Thanks, Mark Walker.