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> F-Stop Printing Table, a poor man's diffuser enlarger filter calibration system |
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F-Stop Printing Table, a poor man's diffuser enlarger filter calibration system
Many of us know the trouble of finding the ideal exposure and contrast when printing. And the problem is exacerbated when exposure differences occur due to changing filtration on diffuser enlargers. This phenomenon is particularly true at the filter extremes and especially with the higher contrast filters. (Tim Rudman explains why in his master printing book).
So I made the first test strip with filter grades 0-5 including white light all at the exact same exposure, so no compensation. I believe the 3 bottom and 3 top filter grades are not optimal. Then without the aid of densitometers, step wedges, plotting charts or "A" level mathmatics, I made test strips at each grade in 1/6th steps using the boxes from the f-stop table from "Way beyond monochrome" to calculate the compensation, either -/0/+. The result is shown in the second set. So in the future, all I'll need to do when changing filter, is count the boxes from my base exposure on the f-stop printing table. Naturally there will be variations with papers but it should give a good starting point. Please forgive the pic quality as they're just from my phone.
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MartyNL “Reaching a creative state of mind thru positive action is considered preferable to waiting for inspiration.” - Minor White, 1950 Last edited by MartyNL; 17th October 2019 at 01:00 AM. |
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exposure increments
No need for 'A' level mathematics. One-sixth of a stop is a good increment - each is just over 10% more exposure than the previous one. Years ago I made a series of prints on grade 2 at one-sixth-stop increments and showed them, not in sequence, to a small group of sixth-formers that I was teaching darkroom technique. Assessing whether they could detect differences in density/print darkness seemed like a good exercise. Some of the students could just detect the differences and it was not until I presented them in sequence that everyone could work it out.
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Looks interesting and useful Marty. My assumption is that this applies to single filter such as the Ilford MG ones and that you apply the stops cumulatively so that by the time you move from 0 to whatever higher grade you need you add all the compensations together on the bottom set?
So the Ilford instruction that you get the right exposure for grade 0 and then make no changes in moving to grade 3.5 is slightly wrong? When you move from say the compensated grade 3 to grade 5 then instead of doubling exposure you double and then reduce the exposure by the amount listed? Have I got this right? The bottom line certainly look to be better exposures Thanks Mike |
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Ralph Lambrecht, from way beyond monochrome has written an extensive piece on Contrast Control with Color Enlargers and specifically for the Durst CLS 501 colour head. all very interesting and very technical and I wanted something more practical. In reality, I tend to start with a somewhat lower contrast filter to get my base exposure before mucking about with filter grades. However in theory you're absolutely right moving from grade 0-5 would be 12 boxes in 1/12th stops on the f-stop printing table so the equivalent of 1 whole stop. In fact, my times were 18sec for grade 0, 9sec for grade 5 and a base exposure of 12.7 so half a stop or 6 boxes from either end! I think the f-stop printing table is simply a primitive form of the RH designs stop clock, which I don't own.
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MartyNL “Reaching a creative state of mind thru positive action is considered preferable to waiting for inspiration.” - Minor White, 1950 |
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Thanks Marty. When you say a variable contrast head, how does this work? Are there two filters built into the head and connected to one dial so in theory you ought to have a constant exposure for all grades?
I ask because this is a bit of a new thing to me. From what I can find I think these are called VCCE heads. Is that right Mike Mike |
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__________________
MartyNL “Reaching a creative state of mind thru positive action is considered preferable to waiting for inspiration.” - Minor White, 1950 |
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Thanks Marty. So on your experience and compensation tables based on actual pictures there would appear to be a defect in the VCCE head in that it does not provide the correct compensation for all grades, hence the need to find in the form of 1/6th stops what this compensation needs to be.
I have yet to try the same kind of experiment with the Ilford MG filters. Has anyone done this and of so does it confirm that Ilford have got it right when it claims constant exposure from 00 -3.5 and then double from 4-5 or are there similar kinds of slight exposure problems requiring compensation Thanks Mike |
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Thanks . |
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Most papers come with tables showing the filter settings for single or dual filter usage for different brands. With colour heads, changes in filtration to get different grades will normally necessitate in new test strips being made and hence this post about calibration/compensation to make this more predictable. Also, most colour heads won’t deliver a full grade 5 but to be honest I’ve never made a print at grade 5 that I’ve been satisfied with and if I truly need more contrast or split grade, then I prefer to use my condenser enlarger.
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MartyNL “Reaching a creative state of mind thru positive action is considered preferable to waiting for inspiration.” - Minor White, 1950 |
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Then there is the question of interpretation. Is it about the purely scientific calibration of the enlarger head based on a technical laboratory standard or the practical application in a darkroom environment to the vagaries of a printers taste.
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MartyNL “Reaching a creative state of mind thru positive action is considered preferable to waiting for inspiration.” - Minor White, 1950 |
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