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Thread: If the plane of focus is a half-inch at the easel...

  1. #1

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    If the plane of focus is a half-inch at the easel...

    I've read that some experiments have shown the plane of sharp focus on the enlarging easel is a half-inch.

    So, does that mean I don't need to keep a scrap of enlarging paper to put under the grain focuser?

    I've been doing that since I was a teenager and it's hard to break old habits.

    But if it's a wasted effort, I may reteach myself to skip it.

  2. #2
    Format Omnivore Brian C. Miller's Avatar
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    Re: If the plane of focus is a half-inch at the easel...

    Why don't you just make a paper step wedge, focus in the middle of it, and observe the results?

  3. #3
    Apo-Heespharm-N MC
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    Re: If the plane of focus is a half-inch at the easel...

    No not needed but I still do it... I have a sheet of fixed paper used for focusing ... Old habits die hard

  4. #4
    Steve Smith's Avatar
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    Re: If the plane of focus is a half-inch at the easel...

    We had a discussion on this on APUG last year. I contacted a few manufacturers of grain focusers and the overwhelming conclusion was that it is not required.

    Also in his book, The Edge of darkness, Barry Thornton describes how he made a print with a 1/2" board under the easel focused as best he could then made two further prints: one without the board and one with a second board (so 1/2" above and below the original print). He claimed that he could see no difference - and this is from someone who was obsessed with sharpness.


    Steve.

  5. #5
    Film and Darkroom User
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    Re: If the plane of focus is a half-inch at the easel...

    The available depth of field is a factor of the lens focal length and aperture. I suggest that you raise one side of a 16x12 easel by several inches, eyeball focus an image around the centre, and make three large prints, one at maximum aperture, one at minimum and one around the middle of the range and compare them. It is quite easy to get acceptable side to side sharpness in these situations using a small aperture.
    Regards
    Dave
    The PPC

  6. #6
    ic-racer's Avatar
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    Re: If the plane of focus is a half-inch at the easel...

    Quote Originally Posted by Bill Burk View Post
    I've read that some experiments have shown the plane of sharp focus on the enlarging easel is a half-inch.

    So, does that mean I don't need to keep a scrap of enlarging paper to put under the grain focuser?

    I've been doing that since I was a teenager and it's hard to break old habits.

    But if it's a wasted effort, I may reteach myself to skip it.
    It depends on the magnification and aperture. You can calculate it with the view camera focusing equation for defocus and diffraction or you can just raise and lower your grain magnifier to see. I can't think of any practical enlarging scenario where the baseboard side depth of field is anywhere near the thickness of a piece of paper.

    Elevation changes of the easel are kind of cumbersome. Since many enlargers have a measuring scale on the column, it is easiest to just raise and lower the head to check your focusing depth, and read off the numbers from the column.

    As an example a 4x5 negative enlarged to 8x10 at f11 would have about 2.5mm of depth of field at the baseboard.

    A Minox negative enlarged to 11x14 at f4 would have about 27mm depth of field at the baseboard.

    A piece of paper glued to the bottom of a Peak 1 grain focuser keeps it from scratching one's easel.

  7. #7

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    Re: If the plane of focus is a half-inch at the easel...

    There is a formula (there usually is for theory purposes)

    depth of focus at the paper is:

    t = 2Nc(1+m)

    where t=depth of focus, N=f-number, c= circle of confusion, m=magnification.

    circle of confusion on paper needs to give 8lpmm, I used figure for 10lpmm which is 0.05mm

    So using above formula and an 8 times enlargement of negative(which is about right for an 8x10 print from 36x24 neg) and f5.6 we get:


    t = 2*5.6 * 0.05 (1+8) = 5.04mm

    Which means your depth of focus at the paper will be at least 5mm. Putting a piece of paper under the focus finder makes no difference.
    And in fact because of the bellows extension the effective aperture f-stop would be around f40 which would make depth of focus around 36mm. And if you are only aiming for 5lpmm then you can double that to around 72mm.

    But given that perfect enlarger alignment is unlikely, your margin of error is less than depth of focus might indicate. Good job there is some margin of error cos if there wasn't you'd rarely get an apparently sharp print.
    Putting paper under your focus finder won't make any noticeable difference and besides, how do you know the designer did that when they designed it?

    p.s. not sure above formula is the most accurate but it should be close enough.

  8. #8

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    Re: If the plane of focus is a half-inch at the easel...

    Thanks tlitody and ic-racer,

    I think I may forgo the paper. I can use the time I would have spent looking for the scrap of paper to look at the edges to ensure my enlarger is correctly aligned.

  9. #9
    Drew Wiley
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    Re: If the plane of focus is a half-inch at the easel...

    Depends on the depth of field of your enlarger lens, its particular focal length, and how far it is stopped down. Do you defocus your camera shots by half an inch, or
    even a millimeter? Why do it on the enlarger?

  10. #10

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    Re: If the plane of focus is a half-inch at the easel...

    We're talking about .05 mm thickness of paper.

    At the enlarging lens stage or negative stage, .05 mm would be noticeable, but the story I am hearing is that at the paper plane there is more leeway.

    Because the subject and sensitive material are arranged differently than in camera, it's like if you are taking a picture of someone and "they" moved a millimeter.

    If doing reductions, the sharpness zone is working against me again.

    My specifics: D2 enlarger, 135mm Xenar f/4.5 at f/11 - 11x14 from 4x5. I do care about subject sharpness but allow softness at the edges.


    Drew, I've got a lot of respect for your opinion - if you say I should keep using paper I will.

    Bill

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