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  1. #71
    Steve Smith's Avatar
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    Quote Originally Posted by Nicholas Lindan View Post
    It helps that I am terminally myopic.
    Yes/ Being short sighted has its advantages. Take off your glasses for good close up vision. Particularly good for ground glass focussing.


    Steve.

  2. #72

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    Quote Originally Posted by Nicholas Lindan View Post
    ...The Ctein article on focusing was discredited long ago.
    Where and by whom?

  3. #73
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    Quote Originally Posted by Nicholas Lindan View Post
    The Ctein article on focusing was discredited long ago.
    as far as i can tell, he was spot on. paper shims under he grain focuser are nonsense. the do f is huge! and covers many focusing issues, including wavy paper.
    Regards

    Ralph W. Lambrecht
    www.darkroomagic.comrorrlambrec@ymail.com[/URL]
    www.waybeyondmonochrome.com

  4. #74

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    And Ctein's method of determining the actual focus relative to the paper position is simple and reliable. To me, the main question here is "are the grain focusers giving accurate focus on the paper?"; and this can be clearly answered with a few sheets of paper using his method. Then if things are off, at least you have some solid footing to work from in figuring out the problem.

  5. #75
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    Quote Originally Posted by Nicholas Lindan View Post
    The Ctein article on focusing was discredited long ago.
    I should qualify this: Ctein published an article in the early 90's advocating focusing through a blue filter, and claimed that not using a blue filter resulted in a 1/2" to 1" error in focus wrt the paper position. The claim was rebutted in letters to the editor and a subsequent article. I see Ctein no longer makes his original claim.
    Last edited by Nicholas Lindan; 10-27-2012 at 04:17 PM. Click to view previous post history.
    DARKROOM AUTOMATION
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  6. #76
    Bill Burk's Avatar
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    Quote Originally Posted by Nicholas Lindan View Post
    I should qualify this: Ctein published an article in the early 90's advocating focusing through a blue filter, and claimed that not using a blue filter resulted in a 1/2" to 1" error in focus wrt the paper position. The claim was rebutted in letters to the editor and a subsequent article. I see Ctein no longer makes his original claim.
    That limited refutation seems reasonable, of otherwise pretty decent information.

    I don't want to open a can of worms but I believe there could be an error of focus with respect to ultraviolet vs visible in my system... which is quite different than the norm (though not an unreasonable system). I use graded paper and an Aristo grid bulb through a Xenar lens... I may have a system that is more susceptible to focus errors due to ultraviolet exposure than the more common tungsten sources used with multigrade paper through an APO lens.

    Our Original Poster here, though, seems to have something very unusual going on, a gross error that really sounds like tampering, dropping, or possibly a bowed piece of paper.

  7. #77
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    Quote Originally Posted by DREW WILEY View Post
    What th' .... Ice Racer? Mitutoyo makes the instruments that measure the dies and machinery that
    make things like Peak in the first place. They're a precision instrument and optics mfg - and obviously
    have to been of a far higher quality control standard than the thing itself being made. I don't know what you're thinking of. You can buy something as simple as a digital caliper from them (and they
    make some of the very best) or some advanced instumentation that will measure in angstroms!
    They don't make a grain focuser. Peak 1 is the best out there. How would you improve it?

  8. #78
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    Quote Originally Posted by DREW WILEY View Post
    What th' .... Ice Racer? Mitutoyo makes the instruments that measure the dies and machinery that
    make things like Peak in the first place. They're a precision instrument and optics mfg - and obviously
    have to been of a far higher quality control standard than the thing itself being made. I don't know what you're thinking of. You can buy something as simple as a digital caliper from them (and they
    make some of the very best) or some advanced instumentation that will measure in angstroms!


    Peak 1 in conjunction with an enlarger will measure smaller distances than a consumer grade caliper.

    In terms of focusing on the paper or the baseboard, I don't have an opinion on the matter, just the optical formula that predicts about a 2mm focus spread at f2.8 at 9x enlargement. This easily encompasses any photographic paper or film of which I am aware.

    Modular transfer function focusing equation (equation #38 in http://www.largeformatphotography.in...DoFinDepth.pdf) :
    N_max ~ 20 / (1 + m) sqrt(dv)

    N-max = F number
    m = magnification
    dv = focusing leeway on the baseboard
    20 = constant for circle of confusion about 0.15mm on the print

  9. #79
    Steve Smith's Avatar
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    I think our enlargers are built the wrong way round!

    They should have the light source, negative and lens at the bottom, projecting upwards onto a piece of ground glass for focussing which is then replaced by a paper holder for exposing in the same way as the ground glass and film in an LF camera.

    Then we could check the focus with a loupe or even just our eyes without casting a shadow.


    Steve.

  10. #80
    AgX
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    Steve, on a large table/baseboard that means bending over the groundglass, maybe leaning even on it...
    I'm the killjoy again.. But actually I like that idea!

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