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  1. #11
    Bob Carnie's Avatar
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    Hi Terry
    A bit confused with your post, are you suggesting a flat field enlarging lens works the same way as a camera lens. ie stopping down the lens will give you depth of field??
    In my experience this is not the case, maybe you could let me know what lens does this magical feat for enlarging and sharpness issues. I would be extremely happy to find this lens.

  2. #12
    Terry Hayden's Avatar
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    dof enlarging

    Hello Bob,

    Well, any lens with an aperture offers more d.o.f. at a smaller f stop.

    That doesn't have anything to do with a "flat field" or not flat field lens.

    It's basic optics.

    My point was that these flat field lenses aren't all that flat. The d.o.f.
    effect does indeed apply, and alleviate some of this problem.

    Anyone else want to chime in- I'm pretty sure that I'm right on this,
    Butt Hay - maybe I'll learn something new...

  3. #13
    Ed Sukach's Avatar
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    Quote Originally Posted by Terry Hayden
    Well, any lens with an aperture offers more d.o.f. at a smaller f stop.

    Anyone else want to chime in- I'm pretty sure that I'm right on this,
    Butt Hay - maybe I'll learn something new...
    You are absolutely right. I often wonder why some think that an enlarging lens is somehow - magically - subject to different optical laws. A Lens is a lens is a lens...

    Try it ... focus, using a grain focuser, with the lens wide open, and, first shifting so that you are again out, re-focus at a smaller f/stop. The grain will "snap" into focus much more decisively with the large aperture. That is the major source for the myth that there is a focus shift with different apertures - the lens is focused non-critically at a small f/stop, and when opening up to a larger one seems to be out-of-focus due the the reduced depth of ... not field, since this is a projection lens - but focus.
    Carpe erratum!!

    Ed Sukach, FFP.

  4. #14

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    Quote Originally Posted by Bob Carnie
    Hi Terry
    A bit confused with your post, are you suggesting a flat field enlarging lens works the same way as a camera lens. ie stopping down the lens will give you depth of field??
    A lense is a lens :-) DOF effects are the same i.e. smaller aperture will always help with any focus issues in relation to the negative and the projected image especially if you have a lense whose focus varies across the projected plane.

    The problem of course is that the smaller the aperture, the longer the exposure - so you end up with blurring caused by unstable enlarger mountswhen using 30 sec exposures at f32

    Graham.

  5. #15
    gainer's Avatar
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    Every lens has an optimum aperture for resolution. When the lens opening is too large, lens aberrations are apparent. When the opening is too small, diffraction limits resolution. It is impossible to design a perfect glass lens that will have diffraction limited resolution at all openings, no chromatic aberration, no astigmatism, perfect flatness of field, etc. The best we can do is close enough, and sometimes that costs much coin of the realm, depending on how close is close enough. I don't have a 50 mm enlarging lens that is better at f/8 than it is at f/5.6 or better at 2.8 than at 5.6.
    Gadget Gainer

  6. #16
    Bob Carnie's Avatar
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    Hi Tom Ed
    sorry guys, I dissagree.
    Your post said depth of field, which is associated with the 1/3-2/3 principle associated in a camera taking Lens.
    In enlarging we are concerened with depth of focus which is extremely minimal with an apo chromatic enlarging lens.
    I have done tests with different f stops from wide open to fully closed and I can assure you that there is no difference.
    Simple test , glass carrier, focus wide open ,, then adjust density for each apeture click from wide to close down .. make a print and tell me that the focus is better on any of the prints.
    I doubt it very much.
    In fact I have found the lens at minimum apeture to be a percieved softer not sharper ,only due to the light bouncing around trying to get through this minimum diaghram.
    My position is from practical experience and some optic theory I took years ago , I am certain their are others on this forum who can clarify this , in a thoretical manner.Better than my description.

  7. #17
    Terry Hayden's Avatar
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    dof

    Okay, it's true, in enlarging we are indeed talking depth of focus.

    However, the same principals apply.

    Try this web site -

    http://www.matter.org.uk/tem/depth_of_field.htm

    Read it with an english accent - makes it more authoritative...

    Later,
    Terry

  8. #18
    gainer's Avatar
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    Quote Originally Posted by Bob Carnie
    Hi Tom Ed
    sorry guys, I dissagree.
    Your post said depth of field, which is associated with the 1/3-2/3 principle associated in a camera taking Lens.
    In enlarging we are concerened with depth of focus which is extremely minimal with an apo chromatic enlarging lens.
    I have done tests with different f stops from wide open to fully closed and I can assure you that there is no difference.
    Simple test , glass carrier, focus wide open ,, then adjust density for each apeture click from wide to close down .. make a print and tell me that the focus is better on any of the prints.
    I doubt it very much.
    In fact I have found the lens at minimum apeture to be a percieved softer not sharper ,only due to the light bouncing around trying to get through this minimum diaghram.
    My position is from practical experience and some optic theory I took years ago , I am certain their are others on this forum who can clarify this , in a thoretical manner.Better than my description.
    That's not the test you want. You should focus stopped down and then open up. You will see the depth of field-focus when you try to focus stopped down, as the uncertainty will be greater stopped down. Also, when doing these tests, you should leave the lens-film distance constant and move the head on the column to sharpen the focus visually. Use a meter stick to measure the position of the head. After each trial, move the head on the column a small amount to disturb the focus, then see if it returns to its previous position when you sharpen the focus by moving the head.
    The depth of field and depth of focus are interrelated. You cannot change one without changing the other. If you do enough of these trials at each f-stop, you can get an idea of the mean and standard deviation of the head positions. You can repeat the whole series of tests with a different color of focusing light. There is among some an idea that one should focus by the color of light you plan to use in making the print. Unless this is green or white, you will find it is not true. Green or white is best to use for focusing no matter what color you plan to use for the print. Any lens you are likely to have on your enlarger is much better than your eye. If it is not, then you have a real problem.
    Gadget Gainer

  9. #19
    Ed Sukach's Avatar
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    Quote Originally Posted by Bob Carnie
    Hi Tom Ed
    sorry guys, I dissagree.
    Your post said depth of field, which is associated with the 1/3-2/3 principle associated in a camera taking Lens.
    In enlarging we are concerened with depth of focus which is extremely minimal with an apo chromatic enlarging lens.
    I have done tests with different f stops from wide open to fully closed and I can assure you that there is no difference.
    Simple test , glass carrier, focus wide open ,, then adjust density for each apeture click from wide to close down .. make a print and tell me that the focus is better on any of the prints.
    Please re-read my post. I was very careful to choose the term "Depth of Focus" - applying to a projection lens.

    You say you focus "wide open" and then stop down, and there is "no improvement". Of course, but that is not a measure of acceptable resolution at varying apertures, given focusing distances other than "ideal" - a.k.a. "Depth of Focus".

    Projection or Taking lens, the same optical theories apply ... unless there Is another set I don't know about. Could you be more specific?
    Carpe erratum!!

    Ed Sukach, FFP.

  10. #20
    Bob Carnie's Avatar
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    Ok, maybe I am not understanding something here and without getting pissed I will make a statement and I would like someone to tell me in practical terms why my statement is incorrect.
    When printing I focus at wide open apeture. I see grain and I know that I am sharp , I then stop down two stops to print , mainly for time applications. I also see grain when I close down.
    Is someone here saying that I am sharper at f8 than at f4????
    If I close down and focus at f16 and see grain then open up to f4 and see grain am I decreasing my sharpness.

    Sorry to be a pain in the neck here , but just do this simple test and I assure you the grain is the same at any fstop when enlarging.
    One of the posters suggested that focusing on the edges and then closing down the image would snap into place. > the only way I could see this happen is if the poster was not using a glass carrier and this snapping into focus is the negative popping out of position as the negative heats up( therefore the practice of glass carriers)

    I would like someone to give a practical explanation to this so called DOF experience using an enlarger . To be honest I have never seen it and I have printed a few negatives in my time.

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