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  1. #51
    Marco B's Avatar
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    Quote Originally Posted by MattKing View Post
    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.
    Amen!...

    I have to admit I haven't done this much, but the few times I did this, and set an F16 on my enlargers head to get the required depth of focus, I really couldn't tell the difference between the sharpness across the plain compared to the "straight" print... I was completely satisfied with the resulting print (about 7x10 inch from a 35mm negative), see below where I did a major correction.

    But maybe I am not that much of a grain peeker...

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    "The nineteenth century began by believing that what was reasonable was true, and it wound up by believing that what it saw a photograph of, was true." - William M. Ivins Jr.

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  2. #52
    Leigh B's Avatar
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    Quote Originally Posted by Smudger View Post
    Put another way,shooters of infrared film are aware that,after reaching good visual focus,they must then offset the lens focus to bring the IR rays into proper focus on the emulsion layer.
    The focus problem with IR is due to two factors:
    1) The human eye cannot see infrared; and
    2) The optics focus IR at a different distance from the rear lens node than the light we do see.

    This has absolutely nothing to do with the subject under discussion in this thread.

    The suggestion was to focus using blue light. The emulsion is sensitive to blue light, so there's no source of error in doing so.

    - Leigh
    Last edited by Leigh B; 08-03-2011 at 06:29 PM. Click to view previous post history.

  3. #53

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    My Peak Critical Magnifier came with a blue filter, and with the various modern (mostly apo) enlarging
    lenses I use, I can't see any difference. Once I had on hand some old-style Componons which someone
    gave me, but I never actually used, and there seemed to be a little difference viewing thru the blue filter. Since I print graded, VC, and color papers - clear up to 30X40, and all come out immaculately sharp, I don't see what the fuss is about. Getting an enlarger properly aligned and making sure the film is flat is far more important.

  4. #54

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    Quote Originally Posted by Leigh B View Post
    The focus problem with IR is due to two factors:
    1) The human eye cannot see infrared; and
    2) The optics focus IR at a different distance from the rear lens node than the light we do see.

    This has absolutely nothing to do with the subject under discussion in this thread.

    The suggestion was to focus using blue light. The emulsion is sensitive to blue light, so there's no source of error in doing so.

    - Leigh
    Not really. The particular LCA issue at hand is the plane of focus for UV, not visible blue.

  5. #55
    Leigh B's Avatar
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    Quote Originally Posted by Michael R 1974 View Post
    The particular LCA issue at hand is the plane of focus for UV, not visible blue.
    Michael,

    You apparently haven't been reading the thread. Go back and read post #47.

    There is NO UV in the system. So why should anybody care about it?


    - Leigh

  6. #56

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    Well I said I'd try Ctein's test for residual UV or blue range LCA and I did.

    Equipment/Materials:

    -Saunders 4550 XL 4x5 enlarger with VCCE (B&W VC) head (halogen lamp)
    -35mm negative in glass carrier, enlarged to 8x10 paper size
    -50mm f2.8 APO Rodenstock at f5.6
    -Ilford MGIV

    Method:
    I raised the easel 10mm off the baseboard, focused with the grain magnifier at f5.6, and made exposures at grade 2.5, 0, and 5. I then repeated the same exposure series without touching focus, moving the easel to the following positions:

    -Raised 5mm from starting plane
    -Raised 10mm from starting plane
    -Lowered 5mm from starting plane
    -Lowered 10mm from starting plane

    I've attached scans showing the results. Unfortunately it is hard to tell anything from crap scans, but it's something at least. Figure 1 is at grade 2.5, figure 2 is at grade 5, figure 3 is at grade 0.

    Obviously focus degrades faster when moving closer to the lens so it makes sense I found in all cases sharpness fell off more noticeably in the upward direction. For grades 2.5 and 0, although it might not be that obvious in the scans, the prints were visibly sharpest at the plane of visual focus. With the easel at raised positions, the prints are noticeably soft. Sharpness is still pretty high when lowering the easel 5mm, but I see a small difference.

    At grade 5, I see noticeable softening when moving the easel upward. However even with my face in the print I had a difficult time deciding between the prints at the plane of visual focus, -5mm and -10mm. All three look quite sharp. Depending on when I looked at them (I shuffled them for a more unbiased visual test), about half the time I picked -5mm as the sharpest and the other half I picked the print at the plane of visual focus. I'm really not sure and it's splitting hairs. For reference on this particular issue I've attached figure 4 which shows the three grade 5 prints in question, scanned at a higher resolution (not that it really helps siince everything looks pretty bad in the scans).

    My conclusion is this: For my specific setup and my most frequently used enlarging paper, there may or may not be a slight focus shift at a high contrast settings, but there would be no visible impact on print sharpness. Everything looks tack sharp at any filter setting using my grain magnifier with white light. I also tried focusing in the grain magnifier at high and low contrast settings and found no shift.

    Here is a question though. If there is indeed a small focus shift at grade 5, is it logical the plane of focus for shorter wavelengths would be further away from the negative plane (as I might have observed in my experiment)?

    It would be interesting if someone else could try just the grade 5 test versus grade 2.5 or white light. I might try a couple of other papers too.

    Anyhow, I'm not too worried about this Ctein phenomenon with my enlarger and paper. If it does exist it will not be visible in my prints. What is troubling though is that if I would have clearly seen evidence of this, I'd be at a loss to do anything about it besides stopping down more.

    Michael
    Attached Thumbnails Attached Thumbnails Fig 1.jpg   Fig 2.jpg   Fig 3.jpg   Fig 4.jpg  

  7. #57

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    All the posts and your test seem to point to the issue being a non issue but it still begs the question: Why did Ctein see what he saw? What is different in yours and other's experience.

    Please note: I am trying to see if there is an objective explanation which reconciles the different conclusions and not implying that Ctein had simply lost his judgement. This is NOT an attack on Ctein

    pentaxuser

  8. #58

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    To me it depends on three things:

    1. The light source and how much non-visible light passes through to the paper
    2. The type of paper, which may have higher or lower sensitivity to shorter blue wavelengths
    3. How well corrected the lens is for LCA at these wavelengths

    What was inconclusive to me in my test was the grade 5 exposure below the plane of visual focus. I was thinking yesterday in order to close the issue for me, I need to re-run just the grade 5 test, and use a larger aperture on the lens (eg f4 as Ctein used). It occured to me as I was posting my results that even at a relatively large aperture like f5.6 there might be too much depth of field at the paper plane, which made it difficult to see a variation as the easel was lowered from the plane of visual focus.

    Just to close that loop in my system I will re-run the grade 5 test.

    Other than that I don't know what to make of the test Ctein made with his own setup and his results. Hopefully the problem he observed is simply not materially present in my setup.

  9. #59
    Greg Davis's Avatar
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    There may have been changes in the materials since then. If you rerun the test, try including a section of the image that has fine detail. That may make any difference more apparent than just the sharpness of the grain itself, which can be difficult to detect.
    www.gregorytdavis.com

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  10. #60

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    Thanks, will try it. But it's not hard to tell looking at the grain in these prints. I purposely used a fairly grainy film and sharp developer so that the grain would be easily seen in the prints without magnification. The scans are crap but in the real prints everything is easy to see.

    It might be a few days before I have a chance to re-run but will definitely post the results.

    How do you pronounce "Ctein" anyway?

    One other interesting by-product of the test that I found interesting (the scans show this relatively well): Even at grade 0, with a middle value tone, there is a clear difference in exposure just by moving the easel +/- 10mm from the plane of focus at this magnification. Even at +/- 5mm a difference is visible upon comparison. We all know this makes sense in theory, but it was interesting to see such a small change in distance (or magnification) make a material difference.
    Last edited by Michael R 1974; 08-04-2011 at 09:50 AM. Click to view previous post history.

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