Interessante discussione da un topic su apug.org , questo e' quello che scrive David S. Allen:
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The real key to testing a film/developer combination is to use a consistent and repeatable system. I recently answered a PM with the following system that I have taught for many years:
Now the key to achieving consistently good negatives is the correct placement of your shadows when exposing the film and ascertaining the correct development time for achieving good separation without losing the highlights. A simple and relatively quick way to way to pin all this down for the future is to do the following (WARNING: reading these instructions is more time consuming and a lot more laborious than actually doing it!!):
1. Find a scene with with a good range of tones
2. Using the box speed, meter the darkest area in which you wish to retain shadow detail
3. Move the camera so that you are only photographing this shadow area
4. From the meter's reading close down the aperture by 2 stops or increase the shutter speed by two stops and then expose 6 frames at: the given exposure then +1 stop, +2 stops, -1 stop, -2 stops and -3 stops less than the meter has indicated
5. Process the film
6. Using the frame that was exposed at -3 stops less than the meter indicated (which should be practically clear but will have received lens flair and fogging - i.e a real world maximum black rather than an exposed piece of film that has processing fog) and do a test strip to find out what is the minimum exposure to achieve maximum black - Print must be fully dry before assessing this
7. Do another test strip with the first exposure being what you have selected for achieving maximum black minus your dry-down compensation then plus 1 second, 2 seconds, etc
8. The time that achieves full black inclusive of compensation for dry-down is you minimum exposure to achieve maximum black for all future printing sessions - print must be fully dry before assessing
9 You now know the minimum time to achieve full black inclusive of exposure reduction to accommodate dry-down
10. Using this minimum exposure to achieve maximum black exposure time, expose all of the other test frames.
11. The test print that has good shadow detail indicates which exposure will render good shadow detail and achieve maximum black and provides you with your personal EI for the tested film/developer combination
12 If the negative exposed at the meter reading gives good shadows, your EI is (when metering shadows where you wish to retain good detail) the box speed (i.e. for 400 film you need to set your meter at 400)
13. If the negative exposed at +1 stop more than the meter reading gives good shadows, your EI is (when metering shadows where you wish to retain good detail) 1/2 the box speed (i.e. for 400 film you need to set your meter at 200)
14. If the negative exposed at +2 stops more than the meter reading gives good shadows, you EI is (when metering shadows where you wish to retain good detail) 1/4 box speed (i.e. for 400 film you need to set your meter at 100)
15. If the negative exposed at -1 stop less than the meter reading gives good shadows, you EI is (when metering shadows where you wish to retain good detail) double the box speed (i.e. for 400 film you need to set your meter at 800)
16. If the negative exposed at -2 stop less than the meter reading gives good shadows, you EI is (when metering shadows where you wish to retain good detail) 4x the box speed (i.e. for 400 film you need to set your meter at 1600)
You have now fixed your personal EI but there is one more testing stage to go.
1. Find a scene with with a good range of tones
2. Using your EI, meter the brightest area in which you wish to retain highlight detail
3. Move the camera so that you are only photographing this highlight area
4. From the meter's reading open up the aperture by 3 stops or decrease the shutter speed by three stops
5. Expose the whole roll at this setting
6. In the darkroom, process one third of the film for recommended development time
7. When dry put negative in the enlarger and make a three section test strip exposing for half the minimum black time established earlier, for the established minimum black time and for double the minimum black time.
8. Process print and dry it.
9. If the section of the test strip exposed for 1/2 the minimum black time gives bright highlights with a trace of detail then the film requires 20% more development
10. If the section of the test strip exposed for the minimum black time gives bright highlights with a trace of detail then the film is correctly developed
11. If the section of the test strip exposed for double the minimum black time gives bright highlights with a trace of detail then the film requires 20% less development
12. You can use the rest of the exposed highlight test film to fine tune the development time.
YES - it is VERY boring but . . .for the investment of minimal materials and a few of hours you will have pinned down so many variables that it is really worth doing.
Back in the real world, all you need to do in future is meter the shadows that you wish to retain good detail with meter set at your EI and then stop down the aperture 2 stops or increase the shutter speed by 2 stops. In the darkroom start your first test print with the minimum exposure to achieve maximum black (inclusive of dry-down compensation) and go from there.
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