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  1. #21
    gainer's Avatar
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    Quote Originally Posted by ic-racer View Post
    I have experienced this when the focuser lands on a light area of the negative. When I move to a dense portion of the negative then I can focus. My negatives tend to be denser, because I put the image on the straight portion of the cuve, so there is usually a reasonable place to focus. If your negatives are 'thin' with fine grain I can see how it can be challenging. Putting the blue filter on might dim it down and might help.
    Do not use a colored filter except perhaps green. The human eye is not achromatic. There is a focus shift that increases the scatter of the focussing error and tends to bias it as well. White or green is best. It probably evolved that way through eons of hunting and living in forests. If you can find the Photo Techniques article "Haards of the Grain Focuser" you will see experimental proof. I seem to recall it was reprinted in www.unblinkingeye.com.
    Gadget Gainer

  2. #22
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    I didn't find it at the unblinkingeye, maybe it's elsewhere?
    Everytime I find a film or paper that I like, they discontinue it. - Paul Strand - Aperture monograph on Strand

  3. #23
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    Jerold, I too have a similar problem but tackled it slightly differently

    I added equal amounts of C/M/Y from my colour head to act a neutral density filter.

    I am still focusing under white light but without the searing intensity

    Martin

  4. #24
    gainer's Avatar
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    It may still be in the archives of Photo Techniques which you can search on the net. Basically, what I showed is a focus shift with color of light of roughly 10 mm in one direction for blue and the other direction for red light, as well as increased scatter on either side of green. If you want to try green with dichroic filtering, mix cyan and yellow. Focus a well defined line, such as a hair stretched across the negative carrier, with white light, with the head about where it would be for an 8x10. Mark the location of the enlarger head on the column. Now switch to red, which you can get by mixing yellow and magenta. Do not use the focusing knob to refocus, but change the column height if necessary to restore focus on the hair. Mark the new location. Now switch to blue light by combining blue and cyan. Refocus, again using the column adjuster. You should see differences between the positions of best focus. To show the pictorial difference, make a print of the ame negative from each of the focus positions. These are the experiments I did, repeated a number of times so I could calculate the mean and standard deviation of error at different colors.

    No modern lens of good quality will show the degree of color shift with color that the human eye does. This is not new knowledge. The usual dictum to focus with the color of light where the paper is most sensitive, thaereby compensating for focus shift of the lens, is not good for modern lenses, nor for VC papers.
    Gadget Gainer

  5. #25
    ic-racer's Avatar
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    If there is a focus shift of the projected image with different colored light then the enlarger lens has chromatic aberration.

    When using a grain focuser with an arial image, if the reticle is in focus on the retina then chromatic aberration of the human eye will be compensated.

  6. #26
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    Quote Originally Posted by Mick Fagan View Post
    I also use a Peak focuser and had exactly the same problem, my course of action was to use the blue filter which came with the Peak focusing unit. Not only does it reduce light by about 2 stops, maybe a bit more, it aids focusing somewhat for me with my enlarger, which is a DeVere 504 with a dichroic head and single 250W globe.
    ...
    Mick.
    I found that using the blue filter got me close enough to focus that I would then take it off the Peak focuser and spent a relatively short time fine tuning the focus. By limiting my time with unfiltered light, the retina-toasting effect isn't as severe...and I'm using a condenser head on a Beseler 45 MCRX.
    Save the Earth. It's the only planet with chocolate.

  7. #27
    RJS
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    I come late - for various irrelevant reasons. Faced with this problem using a cold light and particularly wanting to print consistently with the optimal aperture of my enlarging lens I purchased a rheostat from Aristo. This enables me to focus more comfortably, using a dimmer setting, but also to always print two stops down from wide open.

  8. #28
    gainer's Avatar
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    Quote Originally Posted by ic-racer View Post
    If there is a focus shift of the projected image with different colored light then the enlarger lens has chromatic aberration.

    When using a grain focuser with an arial image, if the reticle is in focus on the retina then chromatic aberration of the human eye will be compensated.
    That is not true. There was no measurable focus shift of the lens itself. I proved this by making prints with red, green and blue light after focusing visually with green light, using the grain focuser. None of these prints was out of focus. On the other hand, refocusing with red, green and blue light using the same grain focuser produced out-of-focus prints with red and blue light. You see, the eye is most sensitive and has its highest resolution in green light. The resolution is markedly less in blue and red. Both the image and the crosshair are fuzzier in red or blue light. The image of the crosshair cannot be in sharpest focus on the retina in red or blue light if it was set in green light because of the chromatic abberation of the eye. The combination of focus shift of the eye, which you can easily demonstrate to your self by measuring the nearest point of clear vision in different colors of light, and reduced resolution gives a wider spread in focus errors as well as a bias of the mean.

    There is no point in arguing the theory when the theory does not explain the experimental results. The lens I used was a Rodagon apochromatic which is optimized for reg, green and blue.
    Gadget Gainer

  9. #29
    gainer's Avatar
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    PS: the back issue of Photo Techniques containing my article is still available at
    http://www.phototechmag.com/back_issues.htm
    if anyone is interested. It is 1997 Vol. 18 No. 1.
    Gadget Gainer

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