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  1. #61
    ic-racer's Avatar
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    Skipping a little back and forth between the Wejex and EG&G, here is a generic flash circuit of Edgerton's. The EG&G sensitometer follows it very closely.

  2. #62

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    Quote Originally Posted by Stephen Benskin View Post
    No, I was thinking reciprocity failure on the other end of the scale - ie short. Either way, reciprocity failure doesn't fall within the parameters of the ISO standard. But it must be nice having the ability to test for long exposure reciprocity.

    I just looked up Wejex. Sounds like a nice little device. They mention the choice of a blue or green filter. That sounds like it was designed for X-Ray film testing. Do you know the color temperature of the light source?

    Steve
    Not the exact temperature, but it is a tungsten bulb so I think using an 80A filter would balance it pretty well to daylight.

    Sandy

  3. #63
    ic-racer's Avatar
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    Here is a historic Wejex ad from the American Journal of Roentgenology:

  4. #64
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    I find it interesting how the ad says that for X-ray film "exposure made just before processing eliminates latent image fade problem." That's because X-ray films are usually processed shortly after they are exposed so it's important to have conditions similar to those in actual use. Pictorial films are a different matter. They require a "hold time" which is easy to skip when using a sensitometer. You have the processing equipment all set-up. You then expose the wedge and immediately process the film. This can lead to inaccurate results because most people don't process their film immediately after exposing it. Latent image keeping is similar in nature to reciprocity failure - electrons are given off over time and silver atoms revert back to silver halide. Most of the effect is within seconds of exposure and it tends to plateau after a few hours, but it never stops. In other words, the effective film speed will continue to drop in relation to the length of time between exposure and processing. It is therefore important for the determination of accurate film speed to have a hold time that reflects usage. If you develop the film too recently after exposure, the speed will be unrealistically high.

    ANSI / ISO used to stipulate a minimum two hour hold time but the revised ISO standard actually has two different times, one for amateur films and one for professional. As professionals tend to process their films soon after exposing, professional hold time is not less than four hours or more than 7 days. For general-amateur films, the hold times need to be not less than 5 days or more than 10 days.

  5. #65
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    Today, the LIK is not much of a problem. Most all Kodak films contain the chemistry which inhibits Latent Image Failure.

    So, professional and amateur films are more alike.

    In any event, all Kodak control strips for film and paper are held for a set period of time before freezing and shipping to customers.

    PE

  6. #66

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    Quote Originally Posted by ic-racer View Post

    Filter drawer (not pulled out far enough to see the filter) I put an 80A in there for daylight correction:

    Damn, my Wejex looks just like the one you show, but it does not have that neat little filter box? Is this original or did you add it?

    Sandy

  7. #67

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    [QUOTE=sanking;970188]
    Quote Originally Posted by ic-racer View Post
    Stephen, how did you make the mcs readings with your EG&G?
    This is where having an incident flashmeter like the Minolta Flashmeter IV or VI would be useful. Put the flat diffuser on, and lay the meter on the film exposure bed, and then take a reading. Minolta has a lookup table (and a formula) so that you can make a conversion from the meter reading in EV.
    Kirk

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  8. #68
    ic-racer's Avatar
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    Quote Originally Posted by sanking View Post
    Damn, my Wejex looks just like the one you show, but it does not have that neat little filter box? Is this original or did you add it?

    Sandy
    Yes, mine came with that handy little filter slot.

    You also mentioned a 2 second exposure with yours.

    In mine, the lamp is indeed on for about 2 seconds, and it buzzes for about 2 seconds but the rotating shutter covers the lamp for half that time, resulting in a one second exposure. Are you sure your shutter is working OK?

    You also mentioned you use ND filters. Does yours look like mine on the bottom? If it does, you can pop off that little cover plug an get access to the rheostat for adjusting lamp intensity.

  9. #69
    Greg Davis's Avatar
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    This thread has me interested in trying to build one of these machines. If I were to build a box with a flash unit on the bottom with a diffuser on the top, then a stepwedge and a lid, I would have a simple but working unit, correct? The flash duration is set by the flash unit, but how would I calculate the necessary "aperture"? If I used slits like ic-racer, would it just be trial and error until I got a minimum density on my film?
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  10. #70
    ic-racer's Avatar
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    Quote Originally Posted by Greg Davis View Post
    This thread has me interested in trying to build one of these machines. If I were to build a box with a flash unit on the bottom with a diffuser on the top, then a stepwedge and a lid, I would have a simple but working unit, correct? The flash duration is set by the flash unit, but how would I calculate the necessary "aperture"? If I used slits like ic-racer, would it just be trial and error until I got a minimum density on my film?
    I made those 'slit' filters just to keep my EG&G historically acurate. In reality just use various neutral density filters for your homemade unit. I'd just use some Rosco ND sheets. If they fade in your lifetime of using the senstometer, I'd pat yourself on the back for doing a heck of a lot of testing and buy a new Rosco sheet for seven bucks.

    Before I got my two units I was going to build one with a spare flash just like you mention. I searched for a good box and located non-working shell of a sensitometer but missed out on fleabay. Then a working Wejex came up and I won that, so I didn't have to make one.

    Most important thing will be to get even illumination and good contact between the wedge and the film. If not pressed flat, light bleeds around the step wedge. It should be pretty easy.

    Other somewhat easy things are to get a free, small enlarger and dedicate it as a sensitometer.
    Also, those 4x5in and 5x7in contact printing boxes with a bulb inside (which frequently are thrown away as junk) might also make a good start for a home-made sensitometer.
    Also, a cheap X-ray green-blue sensitometer should also work fine for most all 'home darkroom' testing purposes.

    For Large Format users (that want to test in the 1/2 to 2 second range), a Wejex style sensitometer can also be constructed with common parts. A 30 rpm stepper motor and a 120v 5W bulb are the major components.
    Last edited by ic-racer; 04-01-2010 at 09:32 AM. Click to view previous post history.



 

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