----- Original Message ----- From: "Oliver Raymond" Subject: 67 Negative copying: eek!
> QUESTIONs:... > How in HECKS name can I calculate "5 footcandles (54 lux)"... what f-stop is > this? I can measure light using my Sekonic 501... > Looking at the curves, I'm going for density of 1 with 20 second exposure > (20 lux seconds)... Is this correct, or have I got it wrong? Don't worry, you can test it this way. The duplicating film is very slow, more like contact printing speed if I remember. It is very exposure sensitve, and also pretty development sensitive, so you might want to do a full lab style test on it. Here is what I would do. Set the lens to the best aperture. Then make a test sheet of a grey card (you can use the white side, but you have to subtract 3 stops of speed from the end calculation. Use a really wide exposure range, say about 10 stops. Remember to start with the dark slide all the way out, and progressively cover the film. Use the camera at infinity focus Base your beginning exposure on 5 stops less than iso 12, or whatever your meters lowect iso setting is. Process the film as per the instruction sheet. You will hopefully end up with a sheet that goes from clear to black. Make another test sheet, starting with the exposure time of the last clear strip on the first test. You will have a pretty good idea of the exposure increment from the first test. Process the film according to the instruction sheet. This test should give you your speed point, though I have had to make a third test on some occassions. Your speed point is where the film density is .15 over BDF on a transmission densitomer that is set up for negatives (most come that way). If you don't have a desitometer, a close up lens adapted to a spot meter can work, put the neg on a light table and measure stops. The speed point is one stop over BDF. If you can refine the test down to 1/3 stop increments, you don't need a measuring tool. The speed point would be the third visible density patch from BDF. Now to find the proper development time. Take a sheet of unexposed but normally processed film and put it in your neg carrier. Make sure it is in focus. You can gently scratch the emulsion to have something to focus on. Set your lens to its best aperture. Make a test print (if you use VC papers, use grade 2) to see where the first visible density increase is. This is the tentatively correct exposure time for this film/ paper combination at that particular magnification. Use this time and elevation for the remainder of the test. Now, back to the camera. Expose a few film sheets of a gray scale using the correct exposure from the first test set, and process one of them according to the instruction sheet. Make a print of it using the correct exposure time. Inspect the print to see if you need more, or less negative contrast. Process a second sheet, but adjust the development time, or temperature or dilution (whatever strategy you normally use to affect the amount of development will work) in the direction you need to go. I try to make sure I over correct at this stage, as that way I have a parameter limit that I can work back from. Make a print of this negative at the correct exposure time. Decide now if the correct exposure time is actually correct. If you have had to make a big change to the amount of film development, you may have to look at if the BDF has shifted, which will throw off the correct print exposure. Don't be surprised if you need to increase the exposure by a bit to see light tone deliniation, but your exposure time should be within a half stop, more likely a third stop. Now we can really start to fine tune things. Process the third sheet of film, hopefully using a development time somewhere in between the first two development times. Print this sheet at the adjusted exposure time. Inspect to see what adjustments you need. You may decide you need a bit more film exposure, or perhaps a bit less, or that you may still need some refinement of the development time. Change only one parameter at a time when running additional tests. Have fun William Robb - This message is from the Pentax-Discuss Mail List. To unsubscribe, go to http://www.pdml.net and follow the directions. Don't forget to visit the Pentax Users' Gallery at http://pug.komkon.org .
