Factor for enlarger head height adjustment?

Discussion in 'Enlarging' started by declark, Dec 11, 2008.

  1. declark

    declark Subscriber

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    I was printing from one negative last night and had the 8x10 landscape print down pretty well using split filtration, then decided I wanted to crop in on it a bit more and orient it in portrait mode. I ended up raising the enlarger head probably from 10" to 15" or so. It was getting late and I didn't want to bother with another set of test strips, so I just ballparked it a bit and increased the time about 50%. The print came out OK, but definitely too light. I probably should have taken better notes before posting, but I do recall from a failed experiment in the past where I doubled the height of the head and I increased the time by 4x figuring the inverse-square law would work, but it just toasted the print. Has anyone done this experiment before and have a good factor for this sort of thing?
     
  2. johnnywalker

    johnnywalker Subscriber

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    I use the area of the print: double the area, double the light (twice as long an exposure, or 1 f-stop); quadruple the area = 4 X the exposure or two f-stops.
     
  3. Bruce Osgood

    Bruce Osgood Membership Council Council

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    You're dealing with the Inverse Square Law. Moving a light source farther from the subject it diminishes proportionately to the distance.

    I've worked out a simple Excel program that I simply plug in three variables and come up with New Elevated Exposure


    New elevated exposure equals:

    ((New elevation / Old Elevation) ^2) X Original Exposure

    Supposing your 8x10 elevation from lens to paper was 24 inches @ 16 seconds and your New elevation is 30
    inches, then: ((24/30)^2) = 2.44
    2.44 X Original Exposure (16 seconds) = 39.4 New Elevated Exposure.
     
  4. mmcclellan

    mmcclellan Subscriber

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    Why not just do a new test strip and see how much of a change it is? Better to do that and get the same visual results -- then do all the dodging and burning proportionally. One test strip should tell you everything you need to know.
     
  5. uwphotoer

    uwphotoer Member

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    I have always used an Ilford EM-10 for things like this, it's a nice handy baseboard light meter..... just remember to use it with your safe light off for greater accuracy.
     
  6. declark

    declark Subscriber

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    Thank you all for your insight.

    Bruce,
    Does that formula work out pretty well for you? That is what I thought I did once and it just ended up being way overexposed, so I figured it might be a more linear function.

    Michael,
    The test strip is probably best, but I can punch in a formula a lot quicker than running more test strips every time I move the head a bit. That is if the formula can be relied upon to give equal resuts. I am certainly not in a hurry to make shoddy prints.
     
  7. Konical

    Konical Subscriber

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    Good Afternoon, DE,

    I use the formula Bruce cites. It works very reliably. JUst be sure to hit the right calculator buttons in the dark!

    Konical
     
  8. Bruce Osgood

    Bruce Osgood Membership Council Council

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    It works pretty well but I must admit I do some tweaking before I get what I really want. I think it may be due to the original exposure not being as true as we might think. Any error, even 1 second, when magnified will distort proportionately. In the example above, if the 16 second exposure could have been done in 15 seconds the elevated exposure would be almost two seconds different, thus some tweaking is required.
     
  9. DWThomas

    DWThomas Subscriber

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    Off the top of my head, I think what may be missing here is a bellows correction factor. At least in my experience, there is a fairly significant change in bellows extension at typical enlarging set-ups. (I'm sure I could add that to a spread sheet, but for my limited printing activities, an EM-10 and/or test strips suffice.)

    DaveT
     
  10. uwphotoer

    uwphotoer Member

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    I had a friend introduce me to the EM-10 who was using in a commercial application with sports photos. Having to make multiple prints from the same negative at different sizes.... the EM-10 saved him lots of time and paper.
     
  11. Nicholas Lindan

    Nicholas Lindan Advertiser Advertiser

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    Try the enlarging correction ruler at the Darkroom Automation support files web site.

    You measure the size of the original image and the size of the new image. The difference is the exposure correction in stops.

    Example:

    Original image measures 5.2
    New image measures 3.7
    Difference is 1.5 stops: open the lens aperture by 1 1/2 stops or multiply the time by 2.8

    A table of stops<->seconds and a stops dial for GraLab and Time-O-Lite timers are available on the DA web site support section.

    Or you can use an enlarging meter. The already mentioned Ilford EM-10 requires you adjust the lens aperture to null the meter at the new magnification. The Darkroom Automation Precision Enlarging Meter lets you adjust the time, the aperture or both to correct for the new print size. An f-Stop Timer is handy for correcting exposure in stops.
     
  12. uwphotoer

    uwphotoer Member

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    I use the EM-10 and never change the time, mostly because I print cibachromes and with a time change comes reciprocity failure..... and a color shift with the change in time.
     
  13. dancqu

    dancqu Member

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    The inverse square law applies to systems without optics.
    Once a lens is introduced the speed of the lens must be taken
    into account. Just as the extension of a camera lens will effect
    the amount of exposure so will it with an enlarger. As with a
    camera the more extended the lens the slower is the lens.
    An enlarging lens extended for 1:1 ratio enlargements
    has 1/4 the speed; is two full stops slower. Dan
     
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  15. Nicholas Lindan

    Nicholas Lindan Advertiser Advertiser

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    The two systems are equivalent for practical purposes. Either the effective f-stop is two stops slower (the f-number is twice as big) because the lens-film distance has doubled -or- the amount of light is 1/4 because the area covered is 4x area / 2x linear larger.

    Don't use them together ...
     
  16. declark

    declark Subscriber

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    OK it looks like I may need to do a little science project as a sanity check and to see if I didn't goof up on my earlier inverse square calculation. Thanks for all the guidance. The "enlarging correction ruler" looks like it might be just the thing I am looking for, and I was not aware of the Ilford meter which seems to be very affordable.
     
  17. jeroldharter

    jeroldharter Member

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    I do this all the time. I use 8x10 paper to make a good print. Split contrast printing sequence etc.

    Then, I often make an 11x14 or sometimes a 16x20. I have a Jobo Colorline 5000 analyzer. I use the Integrated mode with a diffuser over the light meter cell and I hand hold the diffuser over the enlarger lens and take an integrated density reading over the center of the print. I write down the density on my greaseboard. Then I raise the enlarger, compose the print with the same proportions as the 8x10, place the meter in the center of the print, and then open the aperture of the lens until I have the same density reading on the analyzer. That way, I use the same exposure times and sequences programmed into the RH Designs Stop clock timer. (the Darkroom Automation timer and meter would work similarly I think and be less expensive.)

    It is remarkably accurate but not perfect. Sometimes the larger print degrades a little and needs a bit more contrast e.g. 1/6 stop more magenta exposure when going to 16x20 but I think the process saves me some time and paper.
     
  18. Nicholas Lindan

    Nicholas Lindan Advertiser Advertiser

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    The Darkroom Automation meter indicates the number of stops of exposure change required. You take a reference reading, which the meter remembers, go to the new magnification and the meter will read the difference in light intensity. If, as an example, it says the light is 2.20 stops less then you can add 2.2 stops to the time or you can open the lens aperture 2 and an ooch stops so the meter reads 0.0 or you can split the difference - open up the lens a stop or two and add the residual difference shown on the meter to the time. In all cases all the dodges, burns and split-filter exposures stay the same as they are all in stops +/- the base exposure.

    It is normal for contrast to decrease slightly as prints get bigger. There seem two principle reasons:

    1) Stray light: The intensity of stray light leaking from the enlarger and the intensity of light reflected from the paper to the surroundings and back to the paper is constant and doesnÂ’t change with print size. The increased exposure time gives this stray light more time to act and causes highlight fogging - decreasing the exposure to compensate for the fog causes the shadow density to fall - in either case the contrast is reduced. Of course, if you open the lens to compensate then the stray light/image light ratio stays the same, but you then end up with 2).

    2) Increased flare: If the lens is opened up a stop or two to keep the exposure time constant then the lens flare will increase.

    The standard progression of print sizes are one stop apart. If you go from 8x10 to 11x14, keeping the crop on the long dimension, then the exposure change is exactly one stop. Ditto 4x5 to 5x7 and 5x7 to 8x10. In each case the linear ratio is 1:1.4, or 1:square root of 2. Obviously 4x5 -> 8x10, 8x10 -> 16x20, and 5x7 -> 11x14 are all two stop changes, and 4x5 -> 11x14 and 5x7 -> 16x20 are three stops.
     
  19. michaelbsc

    michaelbsc Member

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    I don't have the Ilford EM-10, but I've got an old Beseler PM??? color analyzer. The white channel on it works the same as the EM-10. But the trick is finding one that still works properly. Taking your chances on Fleabay might cost more than the new EM-10 that will come with a warranty.
     
  20. George Collier

    George Collier Member

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    Following the lens to paper measurement ratio method (for those of us without metering devices), wouldn't it be more accurate to measure the difference in the lamp to paper difference, which is really the material issue, and would be a different ratio, given the re-focusing necessary?
     
  21. DaveOttawa

    DaveOttawa Member

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    Wow, there are some really complex ways of dealing with this out there!
    The method above is probably the easiest way, just remember you need to measure the illuminated area on the baseboard not just the area cropped by the easel blades. The ratio of the two areas gives the factor to change the exposure time by (assuming you leave the lens aperture the same). As others have stated you will likely need to increase contrast (1/2 to 1 grade typically) if it is a large increase in enlargement ratio.
     
  22. Crayguns

    Crayguns Member

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    Hello,
    I use the enlarging dial in my Kodak Darkroom Data Guide. There is no math calculations involved. A ruler is the only other tool involved. Very simple. There is also a exp correction for using polycontrast filters. Since Kodak seems to be turing it's back on us, they probably don't publish them now(would be nice to be wrong). If you can at least look at a copy, you could make a chart for future reference. If I didn't have a Kodak Data Guide, I'd do the math and make a chart.
    Cecil
     
  23. David A. Goldfarb

    David A. Goldfarb Moderator Staff Member Moderator

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    This being one of the more thorough discussions of this topic, let's make it a sticky thread.

    My personal take on it:

    Recognize these numbers--4, 5, 8, 11, 16? Both the standard sizes (4x5, 5x7, 8x10, 11x14, 16x20) and approximately the f:stop series. One stop more exposure for every standard increase in size.
     
  24. Nicholas Lindan

    Nicholas Lindan Advertiser Advertiser

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    CORRECTION:

    It's been so long since I have written about the ruler, I forgot how to use it.

    You use it to measure lens-to-image distance, not the size of the image on the baseboard.

    The following are experimental results, Beseler 45 series enlarger, condenser lamphouse, 150mm lens.

    Exposure adjustments as measured with a Darkroom Automation meter, the DA exposure compensation ruler and by applying the "inverse square law" on the image size.

    11 mm base image size
    New size.Metered.......DA ruler......Image size square law
    21 mm - 1.11 stops - 1.30 stops - 1.8 stops
    31 mm - 1.97 stops - 2.00 stops - 3.0 stops
    45 mm - 2.75 stops - 2.81 stops - 3.9 stops

    45 mm base image size
    New size.Metered.......DA ruler......Image size square law
    31 mm - 0.78 stops - 0.81 stops - 0.95 stops
    21 mm - 1.64 stops - 1.51 stops - 2.15 stops
    11 mm - 2.75 stops - 2.81 stops - 3.9 stops

    Obviously, the most accurate method is a meter. The stops ruler will get you within better than 0.1 stop for magnifications of 2x or more and within 0.2 stops down to 1:1.

    Gadzooks, just looked at the ruler pdf, and the instructions on it are wrong - cross out 'image size' - it only works for lens height.
     
    Last edited by a moderator: Dec 12, 2008
  25. ic-racer

    ic-racer Member

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    There was a thread about this a while back. Now that this new thread is a 'sticky' it needs to come to the correct conclusion.

    The equation I use is:

    new_time = old_time x (new_M +1)^2 / (old_M+1)^2

    where M = new magnification (print/neg) and m = old magnification (print/neg)

    The exposure time factor would be:

    Factor = (M + 1)^2 / (m + 1)^2

    I wonder if Nicholas could supply the magnification in the above examples. I would like to see how this equation (converted from 'factor' to stops) compares to the metered values measured by Nicholas.
     
  26. Nicholas Lindan

    Nicholas Lindan Advertiser Advertiser

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    The negative carrier size is 9.2mm. The projection of the carrier outline was measured, this contributes somewhat to the error as the carrier is 3 dimensional and the lens was focused on the negative in the middle of the carrier.

    Going from a negative size of 9.2mm to the following sizes, the exposure factor correction in stops is:

    Size......Magnification...Metered...Formula...DA Ruler
    11mm ...1.2
    21mm ...2.3...............1.1..........1.1.........1.3
    32mm ...3.4...............2.0..........2.0.........2.0
    45mm ...4.9...............2.8..........2.9.........2.8

    The ruler and the formula should provide identical results as the ruler is based on the same optical formula - I am not sure where the discrepancy arrises at the 21mm image size. Apart from that they all seem to agree within experimental and rounding error.