Exposure time for step wedge?

Discussion in 'B&W: Film, Paper, Chemistry' started by Usagi, Jan 4, 2009.

  1. Usagi

    Usagi Member

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    Hi,

    I would like to use my step wedge (trasparent) for getting characteristic curves of the films easily.

    If I have figured out everything properly, then best way to expose step wedge to the negative is contact printing.

    That's okay so far... But how do I determine exposure time?

    If I do contact printing under the enlarger - where do I measure time? And in case of exposure meter, what aperture setting I should give to it?

    Or if I do contact printing at the dark room with white ceilings using flashes, how do I measure proper flash power for the print?

    So far I have used step wedge by putting it to the film holder, top of the film like a some kind of sandwich. Then I have exposed 'evenly' lighted surface. This way I get some results, but evenly lighted surface. It seems to be next to impossible.

    Any tips where to start?
     
  2. trexx

    trexx Member

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    I am just going through Phil Davis's BTZS. It says set the f-Stop of the lens to f11 and roughly in focus. Raise the enlarger head to the point where a incident meter set for ISO 100 reads E.V. 4. At this point the exposure time should be 0.4 for 100 speed film.
     
  3. stevew

    stevew Member

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    Step wedge exposure

    I tape a stouffer step wedge to 5x7 film in the holder. I cut a piece of white translucent plex to fit in filter holder adapter for front of lens. I aim camera at a white wall, read exposure with meter through white plastic, this would give a zone 5 exposure, I increase 5 stops for zone 10. Slip plastic in front of lens for exposure, shoot exposure at the +5 stop.

    This would give a worst case situation for camera flare, but I use it to get one exposure for the full 10 stop range.
     
  4. CPorter

    CPorter Member

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    I run all my film tests by exposing the wedge, with the lens set at infinity for a completely out of focus image of a white or gray mat board as my target. The mat is placed always on the shady side of my house on a sunny day with a clear blue sky (you want no chance of a cloud, even a very thin cloud, moving in front of the sun as the EV will change in the shade). Or, on an overcast day with even cloud cover, as long as the EV remains the same.
     
  5. Uhner

    Uhner Member

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    Jukka, I recommend reading “Beyond the zone system” by Phil Davies. It is a good investment and like trexx writes it covers this aspect of film testing on page 70 and 71.

    I might add that the reading should be done in reflective mode if you point the meter towards the enlarger lens. Another way is to place a white card in the middle of the illuminated area made by the enlarger and measure the light with a spot meter.

    I find that Davies’ exposure recommendation is a good starting point, but I would only make one exposure and check if it is correct before exposing all the sheets required to do a film test.
     
  6. Usagi

    Usagi Member

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    Thank you! The matboard would work, but now it is quite dark here in Finland.. So I will test Davis's method.

    I have BTZS, but have to say that I haven't looked it for couple of years... Embrassing :/
     
  7. Usagi

    Usagi Member

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    After reading BTZS, thinking, etc... I ended to using Stevew's method. I must be really dumb, before that I always but stouffer with film sheet to the holder. I was really a pain in the ... Because there is not much room inside of the film holder.

    After reading Stevew's post I realized that I can tape the wedge outside of holder, so that film can be quickly changed :smile:

    Now I have done series of test, results are really promising.

    I could have used BTZS approach as well, but the final reason that pushed me towards to camera approach was the current situation with my darkroom. New darkroom is under work and currently only way to use enlarger is to carry it to the bathroom in couple of pieces and assemble it there.
     
  8. nworth

    nworth Subscriber

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    There are a couple of things to remember about using contact printing methods with a step wedge. The first is reciprocity. Some films have significant reciprocity failure at exposure times as short as a tenth of a second; others hold their speed even at 10 seconds. Check the manufacturer's recommendations, and make appropriate adjustments. The second is that it takes a significant time for the enlarger lamp to warm up and to cool down. Light is produced during these times, but it is not as bright as the steady state values. Generally the lamp warms up to full brightness in about a tenth of a second, but it may produce significant light for one-half second after it is turned off. Using the enlarger timer for exposures of less than two seconds will not be anywhere near accurate, although results will be repeatable for any given exposure time setting. That means that the method works for producing curves and for relative measurements, but it will not give accurate film speeds. I rigged up an old 4X5 film pack adapter so that I could make a contact exposure in the camera. That means I can use the camera shutter to time the exposure, and I don't have to worry about this stuff.

    As for the exposure time, measure the light at the exposure plane, and give five or six stops more exposure than indicated. Most step tablets have a density range of about 3 (10 stops), and this will put middle gray at about the middle of the step tablet.
     
  9. Photo Engineer

    Photo Engineer Subscriber

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    Well, it seems to me that you have missed the fact that if you are using an enlarger or doing it in-camera, the exposure and light temperature should be that for which the film is optimized.

    Therefore, using an enlarger, you should have a daylight filter set in the beam to make it less like tungsten! For me this is 100 C and 30 M. The exposure should be in the range for the film and if you cannot do this it should be in the range in which reciprocity does not rear its ugly head. I use 1/2" at f22 which is within the low to no reciprocity range of the film I use.

    Just some thoughts.

    PE
     
  10. Kirk Keyes

    Kirk Keyes Member

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    I use an 80A filter over the lens of my enlarger.
     
  11. Martin Aislabie

    Martin Aislabie Subscriber

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    Thank you PE, you have confirmed a concern I have with the exposure system described by Phil Davis - the colour temperature of the light.

    If you test under using an enlarger bulb the colour temp is 2500 ~ 2800K

    If you test using an open shadow under a clear blue sky the colour temp is aprox 12000K

    If you test with in a sunny position under noon day sun then the colour temp is aprox 5600K

    These will all give slightly different results (whether they are significant or not will depend upon a particular films colour response characteristics)

    Surely, if you are going to go to all the effort of testing your own film speed, you should test it under the lighting conditions which you most frequently photograph ?

    Martin
     
  12. Usagi

    Usagi Member

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    Actually one thing that made me favor 5400K flashes for exposing step wedge thru camera was the color temperature.

    To be honest, I don't know how relevant this issue is. On some books I have read, is stated that in case of tungsten light (around 3400K), should give additional ½EV over exposure so that result would match with normal daylight conditions.

    However, Davis's book Beyond The Zone System suggest that color temperature is not an issue. In reality, most of the photographs are taken during dusk or dawn. Or otherwise at conditions where color temperature is not near normal daylight's 5400K.

    About reciprocity; For 120 films I have used times between ½sec - 1/250s and 3400K photographical lights. I guess that these times doesn't have noticeable reciprocity.

    Now I use flash with 1/500sec duration. It should not be a problem.
     
  13. Usagi

    Usagi Member

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    Actually this made me thinking about color temperature issue. The shadows are usually the places which are put to the lower zones (II-IV) and color temperature of the shadows is always towards colder tones.

    So would the best test for speed point be done with light around 10000K?
    That would ensure optimal shadow renderition - unless yellow, orange or other warm colored filter is used.
    For characteristic curve, it does not matter much. I think.
     
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  15. dancqu

    dancqu Member

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    Shooting or not I always have a UV filter installed.

    The color of shadows is the combined result of the available
    light's color and the absorption characteristics of the shadowed
    areas of the scene. Is that not the case within any area of the
    scene? Why are shadows cool color wise? Dan
     
  16. Kirk Keyes

    Kirk Keyes Member

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    Color temp also affects your meter's response, as well as the film's response.
     
  17. Usagi

    Usagi Member

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    Too much accurancy is really a can of worms :smile:

    Perhaps it is enough for good negatives if process is calibrated, regardless light type that has been used.
     
  18. Kirk Keyes

    Kirk Keyes Member

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    Perhaps, but little things can be done to improve accuracy.
     
  19. ic-racer

    ic-racer Member

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    I set my exposure so that the fastest film type I use would give the 0.1 density near the dense end of the wedge. Then, if you standardize your exposure, you can do 'relative' speed comparisons, in addition to getting the slope of the curves.

    You will have to do some test exposures. The easiest way to keep things consistent is to use a dedicated light source. Like an extra enlarger. A flash, in a reproducible, fixed position would also work. But, really a sensitometer takes up less space than a dedicated enlarger, and, now days, they can be picked up for cheap. I got a Wejex (tungsten source) for $50 and an EG&G (xenon source) for $35.
     
    Last edited by a moderator: Jan 12, 2009
  20. Usagi

    Usagi Member

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    Thanks for the tips.
    The sensitometer really would be ultimate solution. Have to look if I found cheap one some day :smile:

    The use of camera for exposing test strip has one weakness: I have found that even if lightning is even, there is slight vignetting when using almost full aperture. In my case I have to use f/8 (lens has maximum aperture of f/5.6).
    The vignetting is small, but noticeable with densitometer. However it can be almost 0.1D at the corner.
    Perhaps I should use center of the negative are instead of upper edge.
     
  21. Martin Aislabie

    Martin Aislabie Subscriber

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    Shadows are not necessarily colder - but they can often be so

    If the sky is perfectly uniform cloud cover - then the shadows will be lit with the same colour light as the rest of the photograph

    If the picture is taken in sunny conditions - then by definition the shadows are lit by indirect light from either the blue sky or clouds.
    Noon day sun is ~ 5600k
    Cloud is ~ 9000K
    Blue Sky is ~ 12000K

    Martin
     
  22. Usagi

    Usagi Member

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    After having tested a lot.
    One thing that I have found is that after all it is better to put step wedge inside the film back, tightly against the sheet film.
    Otherwise there is too often reflections (probably light reflects from the surface of the film, then to the stouffer and then again back to film) which messes all up.

    That made me thinking. What if I should build really DIY version of cheap sensitometer. For a short, there would be step wedge sized hole at the cardboard. The top of hole is opaque plastic glued or taped and then wedge above it (taped).

    Now it's like a light table, but only opaque in the area of step wedge.

    Then put this cardboard thing in the front of the camera, focus to wedge so that it is large enough at ground glass and make precise calculations for bellow extension correction.
    Now, back of the back of the cardboard is for eg. white wall, white cardboard or something else that can be easily lit evenly. With a flashes, with a photo lamps, ...

    And finally, the space between camera and cardboard must be protected from any light. It can be done by using more cardboard or perhaps some dark cloth..

    And there it is. Simple sensitometer implementation which consist of view camera, cardboard, step wedge and lights :smile:

    I have thought it a lot, but found no critical caveats. This aproach would be also good for roll film. Use 6x9 or 6x12 roll film back and you get several step wedges in the one roll. Then cut roll and give different development to each part. It can't be easier.

    Or what do you think? :rolleyes:
     
  23. ic-racer

    ic-racer Member

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    You want an honest opinion? That sounds like the most complicated way to do it :smile:

    I think it is a lot easier with an enlarger (assuming you have a darkroom). You can even get a 'free' spare 35mm enlarger just to set up as a sensitometer. I did this for a while. I taped the step wedge on a glass proof-printer frame under the enlarger. However, sensitometers are not that expensive anyway. Another way is to make a box with an old manual flash in it. Or mount a flash on an old enlarger column. An old manual flash is good because every time you fire it the 'exposure' will be (relatively) constant. You don't need a shutter or a timer. Plus, the color temp will be good.
     
    Last edited by a moderator: Feb 8, 2009
  24. Kirk Keyes

    Kirk Keyes Member

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    Yeah, that sounds like a mess. And IC racer is right - try using your enlarger instead of the camera. Also, with the enlarger, you can either lay the step wedge/film with the light meter sensor dead center on the enlarger baseboard, or place the film in the enlarger and meter it from the baseboard. That way you get the measurements without and with lens/enlarger flare.
     
  25. Stephen Benskin

    Stephen Benskin Member

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    Evenness is definitely a problem when attempting to expose a step tablet in a camera. All lenses have light fall off. In fact, the constant "q" in the exposure formula factors in value for off axis to compensate for the brighter center.

    I would suggest using an enlarger, and what follows is a way to calculate how to exposure using an enlarger. It's been awhile since I've done this. I hope my math is accurate.

    You will want at least the first three or four steps to fall below the speed point. To calculate the exposure for the film speed you are testing, you will need the following equations:

    Transmittance (T) = Transmitted Light / Incident Light

    Opacity (O) = 1 / T

    Density (D) = log(O)

    Hm = .8/film speed (this is the ISO equation for calculating B&W film speed) Hm is exposure in meter candle seconds (mcs)

    So, if you know the density of the step you want to target for the speed point exposure, take that density and find the Opacity which is the antilog of that density then convert it to Transmittance.

    Example,

    Step Tablet aim Density - 4 steps down from 3.0 = 2.6
    2.6 is around where I want the speed point exposure for fall.
    Opacity = Antilog(2.6) = 10^2.6 = 398.107
    Transmittance = 1 / 398.107 = 0.0025
    Finding aim transmitted light for 125 speed film = .8 / 125 = 0.0064 mcs
    Necessary incident light is Transmitted light / Transmittance = .0064 / .0025 = 2.56 mcs

    That would equal 27.5 footcandles for one second - 2.56 * 10.76 = 27.5

    So, meter the enlarger for 27.5 footcandles, set the exposure to 1 second and you should get the desired results.
     
  26. Usagi

    Usagi Member

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    Thank you for the information.
    I'll test the enlarger. After all it is easier, so that my wife don't need to look at my monorail camera setup with studio flashes on the livingroom all days :D

    Enlarger and flash, that sounds good. But how even is the light of the flash? If I put flash on the column of the enlarger, it gives a lot of light. Would it also work if I put flash on the head of my diffuser enlarger?

    I prefer use of flashes because the don't cause any resiproce (unless flash is really short).

    Got a lot of food for thought :D