Enlarger ----> Sensitometer

Discussion in 'B&W: Film, Paper, Chemistry' started by michael_r, Jul 12, 2012.

  1. michael_r

    michael_r Subscriber

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    This is rather embarassing because after so many years of film testing I should know this already, but I never tested film by contacting and I'd like to try it for some comparisons.

    So, suppose you set your enlarger to some arbitrary height, f-stop etc, and use it to contact a wedge onto 100 ASA film. How do you know how long to expose for?
     
  2. Bill Burk

    Bill Burk Subscriber

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    Here's the "no calculation required" method:

    Give it a whole second. Worst case you make the whole film black and have to try shorter time.

    Best case you will get about 10 steps clearly readable.

    Then you will want to adjust the time (you know the drill - two steps is one stop) until your negative test strip just barely fades to clear for 400 film (fades to clear about 4 steps earlier for 100).
     
  3. michael_r

    michael_r Subscriber

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    Thanks Bill - but how do I know what speed I'm getting?

    By the way I don't mind the calculations - but if it is going to be way too much to write I don't want you to waste too much time.

    By the way if it helps I have Nicholas Lindan's Pyro Enlarging Meter (I've never used it though).
     
  4. Photo Engineer

    Photo Engineer Subscriber

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    I set my enlarger up to make an 8x10 from 35mm and then adjust the starting exposure to 1/2" at f22 which is the shortest cycle for my timer/enlarger and lens. This usually does the trick. Using longer than about 1/2" can cause reciprocity failure problems. In fact, the 1/2" is probably too long but it will work. I have then gone on to dupe transparencies onto negative film (B&W and color).

    PE
     
  5. Bill Burk

    Bill Burk Subscriber

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    PE's advice to keep time short like a half second makes sense...

    For my purposes I take the speed as the rated speed when the curve meets the spec's.

    I only question it when something doesn't make sense. Like... why am I getting EI smack between 50 and 64 when I hit the ASA gradient with Panatomic-X and D-76? Oh well I guess you can't trust the speed of expired film.
     
  6. ic-racer

    ic-racer Member

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    I used a spare enlarger as a sensitometer for a few years. I found it almost impossible to account for all the variables even when I wrote everything down.

    Exact same lens
    Exact same aperture
    Exact same head height
    Exact same lens/light source (focus) distance
    Exact same mixing box
    Exact same filtration (all 'white' or did I give it some blue to make it more daylight ???)
    Exact same bulb (did I use the EKG or the DED bulb??)
    Exact same time
    Exact same method of making the contact (ie exact same glass thickness, which contact frame did I use??)
    Exact same step wedge (I have multiple, which one did I use ??)
    Exact same location on the baseboard (did I put it the step wedge in the center of the carrier image projection, the center of the lightbox projection or the optical center of the lens???)

    Using an enlarger might be just fine for you but it kind of depends on what you want to do. If you want to post some speed info on Shanghi film compared to Tri-x or just get your slope to help with determining negative development time or monitor the activity of your 5 year old replenished developer.

    A few years ago sensitometers were inexpensive and I posted many times that everyone with a densitometer should have one, though, I think the inexpensive ones are getting harder to come by. 8 years ago I got an EG&G off ebay for $13 and I was the only bidder. My white-light Wejex was $50.

    When someone posts about fantastic shadow density with their super concoction fine grained home-made developer I don't give it much credence unless I see a comparison curve with D76 or some other popular developer. A sensitometer is the best way to make curves quickly and easily.
     
  7. Leigh B

    Leigh B Member

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    Perhaps a stupid question, but... Why would you want to do that?

    The accuracy of exposure time with an enlarger is at least ten times worse than
    the accuracy of a camera shutter, and much too long for most films.

    So even with the best possible quality control, your results are just wild guesses.

    You'll get far better (more accurate and repeatable) results shooting a Macbeth chart or step tablet with a camera.

    - Leigh
     
    Last edited by a moderator: Jul 12, 2012
  8. Photo Engineer

    Photo Engineer Subscriber

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    IC;

    I have used an enlarger, a contact frame setup, a professional 1B at EK and an EG&G unit. So, I've covered a lot of territory. I can just say that all of those work for me and have given useful and repeatable results that have been compared over several years. All I can say is use what works for you, and I am happy with the enlarger, contact frame and GG&G that I have. The 1B is probably in the dumpster at Ek.

    PE
     
  9. polyglot

    polyglot Member

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    According to the BTZS book, Delta 100 in D76 or ID11 1+1 with Ilford's recommended times is pretty much smack on the nose of ISO 100. You can therefore use that as a reference for the amount of light coming from your enlarger if you run some tests on Delta 100 at the same time (films exposed concurrently, preferably) as your other films.

    Or, apparently (I have not verified this) if you get EV1 on a light meter, that implies a 1s direct exposure onto ISO 100 film to hit Zone V. Adjust accordingly depending on your film sensitivity and how far up the step-wedge you want to go.

    You want to keep exposure to no more than 0.5s in order to prevent reciprocity failure from destroying your results, unless of course you're trying to make reciprocity-failure measurements. The other issue is that with many enlargers, the bulb takes a good fraction of a second to warm up / cool down, which means that a large part of a short exposure will be at a lower intensity, which means you're going to have reciprocity failure anyway; you can cure that if you put a good mechanical large-format shutter on your enlarger and use that to control the testing exposures (make them square-waves instead of trapezoidal).

    Have you considered bouncing a known-power flash off the ceiling of your darkroom from a fixed location and using a flash-meter to get the power level at the tested film? Again, you can calibrate the exposure from your flash/room combination using Delta 100 as a reference-point. And a flash will be much whiter than an enlarger bulb, which is good when you're testing films (like Acros and CHS-25) that don't have a lot of red sensitivity and for which you would under-estimate speed using a tungsten light-source.
     
  10. ic-racer

    ic-racer Member

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    PE- How is your EG&G holding up? Mine required a new bulb and one of the capacitor bleed resistors to ground fried. Other than that, seems like it should keep going a long time.

    Michael R 1974-- The EG&G manual actually is very good reading for using and understanding any sensitometer, even an enlarger. If you want a PDF let me know. Even if you don't have a 'sensitometery laboratory' or calibrated sensitometer it is interesting to read about how to do an ASA PH2.5-1960 speed test in a step-by-step how-to manner.
     
  11. ic-racer

    ic-racer Member

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    When I did my personal sensitometer calibration I averaged my results from fresh D100, D400 and TMY, but the thought of an APUG community standard of "D100/D76 defined as equal to 100" is interesting. Much better than passing around a standard candle :smile:
     
    Last edited by a moderator: Jul 12, 2012
  12. Photo Engineer

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    I actually had 3 EG&G Sensitometers as gifts from friends over the last few years. Two from one friend, and one from Grant Haist. They are all doing well. Thank you.

    There are 3 different versions of the EG&G units out there. One has 3 selectable times, one has 4, and one has 4 and a curved object platen to account for edge falloff. The AEC (I think) found that the flat platen was not accurate enough and so EG&G designed one with a curved platen with constant distance from the light source.

    PE
     
  13. ic-racer

    ic-racer Member

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    I wondered about that, with there being no diffuser and a somewhat short distance to the bulb. By comparison the Wejex is mostly a hollow box, the size of the box and internal construction (including a diffuser) looking to be designed get the bulb away from the film stage and minimize the cosine falloff.
     
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  15. Stephen Benskin

    Stephen Benskin Member

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    Michael,

    Most people don’t have a meter accurate enough to measure the light from and enlarger for sensitometric purposes. Also, no timer used for printing is consistent and repeatable enough. But you should be able to get an approximate value or at least a relative value.

    How I determine the exposure for sensitometric testing is to first determine which step on the step tablet I want to produce a density that falls around the speed point. We know that the speed equation is 0.8 / Hm. For a 125 speed film, the exposure would need to be 0.0064 lxs.

    Say I want to use the step that equals 2.60. Now all that is necessary is to use these three equations:

    Transmittance = Transmitted light / Incident light
    Opacity = 1 / Transmittance
    Density = log Opacity

    Opacity of a density of 2.60 = 0.0025
    Required exposure at speed point for 125 speed film = 0.0064
    Incident light = 0.0064 / 0.0025 = 2.56 mcs


    Exposure meters want to produce and exposure at the film plane of 8 / ISO. For a 125 speed film that would be 0.064 mcs.

    The difference between 0.064 and 2.56 is 40x or 1.60 logs or 5 1/3 stops. So, meter the enlarger light and open up 5 1/3 stops for this set of conditions.

    I can’t tell you how well you will do working with an exposure meter and an enlarger because I use an EG&G Mark VI Sensitometer that was calibrated by EG&G (many years ago). But I can tell you from personal experience that the above approach calculating the exposure required to produce 0.10 density on a specific step on the step tablet works perfectly well.
     
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  16. Nicholas Lindan

    Nicholas Lindan Advertiser Advertiser

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    If you do all your testing at the same time with an electronic timer there shouldn't be any problem. Any motor driven analog timer won't be adequate.

    With a good timer that pays attention to the niceties of timing AC power you can use any time you like - within the reciprocity limits of the film. If you use Kodak, Ilford or Fuji film then reciprocity isn't much of a problem. With FSU film you can run into reciprocity at 1 second, and testing should be done with a shutter controlling the exposure.

    Step tablet testing with an enlarger does need an enlarger with very even light output and a Sola AC line regulating transformer.

    Such an easily corrected problem ...
     
  17. Photo Engineer

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    IC;

    The box inside the EG&G is silvered and very reflective. I would guess, after using one for over 10 years, that it is pretty good and since it took the AEC to find a tiny problem, I would have no worries about the units.

    In any sensitometric comparison, even with the 1B, I found that averaging was useful.

    PE
     
  18. Stephen Benskin

    Stephen Benskin Member

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    I once did some research into a meter accurate enough for me to calibrate the EG&G myself. It was somewhere between 4 to 5 thousand. And a new EG&G sensitometer around that time was also in the neighborhood of 4 thousand.

    I had this screaming argument with someone from Kodak back in the 90s. He was negating my findings because I had used an intermittent sensitometer instead of an non-intermittent sensitometer. I'm sure for Kodak, who use densitometers that read to 4 decimal places, who wrap their dip and dunk machines in thermal blankets, who use carbon step tablets and revolving step wedges, my set-up was lacking. To me, it produced pretty reliable and very repeatable results.
     
  19. ParkerSmithPhoto

    ParkerSmithPhoto Subscriber

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  20. Stephen Benskin

    Stephen Benskin Member

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  21. Kirk Keyes

    Kirk Keyes Member

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    If you have a Minolta Flashmeter and a flat diffuser, check the owners manual as you can convert the EX readings to Lx and then multiply that value with your exposure time.

    I use 1 second exposures with sheet film as I tend to use exposures around 1 second with large format.
     
  22. Kirk Keyes

    Kirk Keyes Member

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    Also use a 80A filter to convert the enlarger light to daylight.
     
  23. Bill Burk

    Bill Burk Subscriber

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    Wow! So that explains why my Panatomic-X seems faster.

    Because my platen is flat... Step 16 (approximately the speed point for Panatomic-X) is 5% closer to the light than Step 21 (typical 400 TMY-2 speed point). I can sketch the layout on a piece of paper and measure it with a Stanley tape measure. Doesn't take special equipment to prove the point.

    While 5% only causes me mild amusement, I bet this really bugged the serious scientists.
     
  24. Stephen Benskin

    Stephen Benskin Member

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    Kirk, thanks for bring up the daylight conversion because while the exposure problem is solved with a $95 meter, another important aspect of the test exposure that is frequently missed is the color temperature of the light source and how it factors into the determination of film speed as this excerpt from Nelson's Safety Factors in Camera Exposure illustrates.

    "A new formula for speed can be derived which will make use of the 0.1 fixed-density speed criterion and will also provide the desired safety factor of approximately 1.2. If a specification is adopted requiring development to a ΔD of 0.80 or an average gradient of 0.62, for example, the log E difference (ΔX) between the two types of speed becomes 0.29, and the exposure, Ed, at a density of 0.1 above fog becomes 1.9 times greater than the exposure, Es, at the fractional-gradient speed point.

    A revised form of Eq. (12), giving a new kind of film rating or speed that would provide a safety factor of 1.2, may be expressed as follows:

    Speed = 1/2Es. (13)

    Since Ed = 1.9 Es (Ed is the for the assumed development condition, the equation may be rewritten as

    Speed =1.9/2Ed, (14)

    or Speed = 0.95 /Ed. (15)


    A change in the spectral quality of the light to be used in the sensitometer, from simulated sunlight to simulated daylight (sunlight plus skylight), is also contemplated which will have the effect of requiring a constant of slightly more than 0.8 in place of 0.95 in Eq. (15) in order to keep the safety factor at 1.2. If this change in light quality is adopted, the formula for the new photographic speed will be

    Speed = 0.8/Ed (16)"
    (or in today's terminology Speed = 0.8 / Hm)
     
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  25. Mr Bill

    Mr Bill Member

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    I presumed that the silver-colored interior and the sample-holding chamber were somehow designed to make the light distribution more even. (And possibly the striped attenuator, laying in the bottom, was a factor; all of my testing used one of these.)

    When first working with a new EG&G sensitometer (in my job, around 1980?), I was skeptical about eveness of lighting, due to the flat sample holder. I had previous experience with a Kodak "process control sensitometer," where the sample holder was curved, following a radius around the filament of a lamp. (New lamps were calibrated with respect to both operating amperage and location of the filament; the sensitometer allowed such adjustments.) So the simple appearing EG&G unit, with a flat sample holder and fixed-position flashtube seemed a bit shaky.

    We did shake-out tests in all sorts of configurations to test for uneven lighting and consistency. We ran wedges in both directions to test for processing problems, and made special wedges, sandwiched between fixed density strips to detect light falloff. Our conclusion was that the EG&G was perfectly adequate for our purposes. It turned out to be a real work horse.
     
  26. Mr Bill

    Mr Bill Member

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    Stephen, are you saying that the EG&G units are intermittent?

    I don't believe this is true, unless one used multiple flash "pops." Granted, the flash may not be constant intensity (quickly up to peak power, then falling off for the duration of the flash), but I'm not even sure this happens in an EG&G unit. Even if power does fall off substantially, this still doesn't seem to meet the definition of intermittent (I mean a general definition, I don't believe that any of the film speed standards elaborate on this.)