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Thread: Depth-of-focus problem, enlarging lenses

  1. #1

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    Depth-of-focus problem, enlarging lenses

    I need some assistance from an optical guru in the audience. This is a specific problem requiring an optical calculation for depth-of-focus of a specific enlarging lens.

    Background
    In the course of getting an LED enlarger head built for my Omega D2, the latest issue -- and (fingers crossed) the last one -- is getting the light even across the full 4x5 field with my 135mm lens (my only lens for 4x5). It's very close with 120 (80mm) and essentially perfect with 35mm (50mm). It's not quite there yet with 4x5, and I won't go into all the specifics here.

    The (almost) working model of this handsome LED head already has a piece of white acrylic permanently (i.e., irretrievably) inset for diffusion, inset a bit, so that the distance from the bottom acrylic surface to the negative in the carrier is 3/16 inch. I just measured it.

    In order to improve evenness, we are going to add a second diffuser above this one, spaced between it and the LED panel. Rather than cutting the light down as much as white would do, I have ordered a sheet of a frosted acrylic material from TAP Plastics called Satinice, of which I have a tiny sample. It is 1/8th-inch thick (the minimum made) and frosted on both surfaces. The texture will make no difference in this application above the white diffuser, but I would like to know if I can use it also, in a second iteration of the head, in place of the white acrylic. The value of this, if it evens the light sufficiently, is simply more light getting through, to help keep exposure times in a good range.

    Problem at hand
    Therefore, I need to determine whether, given the 3/16 of an inch from the lowest acrylic surface to my negative, my 50mm lens at f16 (please see note at bottom) will bring the texture of the Satinice into sufficient focus to render the texture in a print. I recognize that there are variables in here that I can't specify, principally the fineness of frost texture and size of the print, assuming my lens is pretty good (1970s Nikon). I really don't want to get into minutiae about how close the viewer is. For the sake of common sense, let's assume a 9x enlargement of the negative showing nice clear sky areas.

    I am using the 50mm lens for this problem, because I am assuming that the shorter focal length (than my 80 and 135) will naturally have more depth-of-focus. I have also stipulated f/16, even though I rarely use an aperture smaller than f/8, and more typically f/5.6, simply because I would like to know. Again, someone could start bringing up diffraction limitations, etc., but I'd like to keep this within the realm of slightly broader considerations.

    Thank you in advance to anyone taking the time to run the calculation. I am very grateful. It may save considerable testing time and expense
    Last edited by Ulophot; 20-May-2022 at 14:39. Reason: incomplete
    Philip Ulanowsky

    Sine scientia ars nihil est. (Without science/knowledge, art is nothing.)
    https://www.flickr.com/photos/156933346@N07/

  2. #2

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    Re: Depth-of-focus problem, enlarging lenses

    frosted plastic won't diffuse the light as evenly as milky white plastic, so i'd be more worried about a hotspot than seeing texture.

  3. #3
    Drew Wiley
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    Re: Depth-of-focus problem, enlarging lenses

    A better strategy is to make sure your neg is totally flat in a full glass carrier, that everything is properly leveled and aligned, and that you standardize on larger f-stops which have shallower depth of field. Most enlarging lenses are best about two stops down from wide open. The best diffusion material you'll find at Tap Plastics is called Sign White Acrylite. You can get it as thin as 1/16th inch. It transmits significantly more light than standard white, and diffuses way way better than textured acrylic.

  4. #4
    Tin Can's Avatar
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    Re: Depth-of-focus problem, enlarging lenses

    maybe 7 years ago I got a free oversize 4X8' sample shipped to me in a huge box

    Made for LED diffusion, I have used it on enlargers

    https://www.ebay.com/itm/233392991519
    Tin Can

  5. #5

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    Re: Depth-of-focus problem, enlarging lenses

    Quote Originally Posted by Tin Can View Post
    maybe 7 years ago I got a free oversize 4X8' sample shipped to me in a huge box

    Made for LED diffusion, I have used it on enlargers

    https://www.ebay.com/itm/233392991519
    broken lcd tvs are a great source of high quality diffusion materials.

  6. #6

    Re: Depth-of-focus problem, enlarging lenses

    You want to use a standard 50mm enlarging lens for a 4x5 negative???

  7. #7

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    Re: Depth-of-focus problem, enlarging lenses

    Chauncey, I'm sorry if I was unclear. I did say that my 135 is my only lens for 4x5. I chose the 50, because the shorter focal length, unless I'm mistaken, will have the greatest depth-of-focus (and depth-of-field) of any of my lenses. I use the 50, and sometimes the 80, for 35mm.

    I guess I may have to look elsewhere for getting the calculations; no one seems to be volunteering. I'm sure it involves math that is out of my league.
    Philip Ulanowsky

    Sine scientia ars nihil est. (Without science/knowledge, art is nothing.)
    https://www.flickr.com/photos/156933346@N07/

  8. #8
    Eric Woodbury
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    Re: Depth-of-focus problem, enlarging lenses

    I use Satinice; specifically the 1/8" clear type. This gives about 1/3 stop loss per layer.

    The first layer is 1.5" below the LEDs, but that is for widely spaced, 1 amp LEDs. The second is either just above, about 3/4", the negative for my small negative enlarger and it's the top slice of the diffuser/negative/coated glass sandwich for the large format enlarger. In the latter configuration, I had to remove the texture from the surface of the satinice because in a couple of circumstances, I could see the texture. [I sanded it smooth with wet/dry 600 grit.]

    Since both of my LED heads have adjustable brightness, I operate the lenses at f/11 or maybe f/16.

    If you need even more diffusion, you can use the white satinice, but it takes away a little more than a stop of light.

  9. #9

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    Re: Depth-of-focus problem, enlarging lenses

    Quote Originally Posted by Ulophot View Post
    Chauncey, I'm sorry if I was unclear. I did say that my 135 is my only lens for 4x5. I chose the 50, because the shorter focal length, unless I'm mistaken, will have the greatest depth-of-focus (and depth-of-field) of any of my lenses. I use the 50, and sometimes the 80, for 35mm.

    I guess I may have to look elsewhere for getting the calculations; no one seems to be volunteering. I'm sure it involves math that is out of my league.
    The longer the focal length the greater the depth of focus.

  10. #10

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    Re: Depth-of-focus problem, enlarging lenses

    Bob, that's interesting! So, I should be getting calculations for the 135 instead -- I am, of course, assuming that the depth-of-focus in the enlarging lens is at the back, as it is in a taking lens, the difference being that the light is going the other direction.

    At the same time, Eric's solution of sanding down the Satinice (bottom surface, I presume) sounds like a very viable solution -- something even I can do! as I indicated, I am hoping that this will be the last hoop through which my builder friend and I will have to jump. It has been a long process over several months now, on various weekends, and for my purposes at least, the other problems have been solved in what is resulting as a very handsome design, conceptually based (mostly) on the initial proposal I made here long ago: a affordable, B&W only, non-VC LED head for the D2, which could potentially be adapted for other enlargers, by designing a separate functional part of the head in a lower housing, attached to an upper module, the configuration of which would allow the bottom part to be connected to the enlarger's relevant hardware.

    This one has its own controller/timer, and works with my foot switch, so, once it's been in reliable service for a while, my Zone VI compensating timer will find a new home.

    Anyway, I'm just eager to solve this last problem.

    Thank you both for your helpful responses!
    Philip Ulanowsky

    Sine scientia ars nihil est. (Without science/knowledge, art is nothing.)
    https://www.flickr.com/photos/156933346@N07/

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