For test strips, I normally use 8 seconds, 11, 16, 23, and 32, mimicking the f stops on the lens. That means if I have a 23 second exposure and want to make it 32 seconds for more time for dodging and burning, I just close down the lens one stop. I also have a chart on the wall with full stops, half stops, third stops and quarter stops, in case I want to dial it in. If I want to make a larger print, I can map the exposure in stops.
How do you know that is the correct method to use to deduce the correct time? What about contrast filters being used..Originally Posted by Tim Meisburger
That is what test strips are for, usually made with a #2 filter. And if you make the leap to split-grade printing, you don't need to determine the grade, either.
F-stop timing has saved me lots of time over the years. I have an RH Designs Vario f-stop timer which can be used for split printing because it has two channels. I have been able to set up the basic and burning exposures on a small print in both channels, then make a change in the basic exposure for a larger or smaller print and the timer corrects the burning exposures automatically. The resulting print is usually very close. Saves time and money and works great with cold light heads that vary in brightness over a longer exposure since it has a sensor which can detect the light source changes and correct the time. Good stuff, but not easy to get because it comes from the UK.
Let's see... opening the aperture one stop doubles the amount of light hitting the paper. Decreasing it on stop halves the light hitting the paper.
Therefore, closing one stop would require twice the exposure time to compensate for the halving of the intensity. So, your 23-second exposure should become 46 seconds if you close the aperture one stop.
Due to the inverse square law, the amount you have to open the aperture to double the light is subject to a factor of the square root of 2. Time, on the other hand, follows simple doublings and halvings to get a stop difference.
So, you can think of your time sequence, 8, 11, 16, 22, 32, 45... as half-stop intervals, not full-stop intervals. Apertures on the lens, 8, 11, 16, ..., however, are full stops.
Best,
Doremus
Still doesnt make much sense.. what about my wee little omegaron 50mm lense thats 3.5 4 5.6 8 11 16 on the aperture dial
A stop is a stop, no matter the lens. Just remember 3.5 is a half-stop faster than 4. But the ensuing stops are a full stop apart.
Except for f/3.5, the aperture numbers on your (and almost every other) lens increase or decrease by the square root of 2. In the past, several other systems have been used, although our present system seems the most logical in practical use once we become familiar with it. While f/2.8 would have been a logical stop instead of f/3.5, perhaps opening up that extra fraction of a stop would have resulted in images that someone decided was not sharp enough with that particular lens. Since that sequence of numbers is familiar to most photographers, f-stop timing is logical for us. It's a personal matter. It may seem desirable to vary the aperture to bracket the exposure, but many lenses have a narrow range of apertures that render adequate sharp images. Your modestly priced Omegaron should be stopped down a stop or two for best sharpness, but not much, if any, more because diffraction will then degrade the image. Even with a better enlarging lens, I like to stop down at least one stop after focusing to help cover sloppy focusing, enlarger misalignment, and film curvature.
Apertures nowadays are marked in f-numbers. The "f" stands for "focal length." These numbers are always given as fractions of the focal length of the lens. f/8, therefore, means that the optical diameter of the aperture is 1/8 the focal length of the lens. The beauty of the system is that the same amount of light gets through a 200mm lens at f/8 as a 100mm or a 50mm lens at f/8 (assuming the same exposure time, of course).
The aperture sequence is based on a doubling of the light intensity for each stop. Because of a quirk of physics, the pesky inverse square law, doubling the aperture diameter gives 4x the light intensity. To get just twice the intensity, the aperture needs to be changed by a factor of the square root of 2 (approx. 1.414). So, multiply, say 8 by 1.414 and you get 11.3, etc. (I did say approximate), on up the scale.
The f-stop sequence in their common full stops: 1.4 - 2 - 2.8 - 4 - 5.6 - 8 - 11 - 16 - 22 - 32 - 45 - 64 - 90 - 128 (that should cover just about any LF lens.) Numbers in between are fractional stops. You can find a ton more info at: https://en.wikipedia.org/wiki/F-number
Shutter speeds are a lot simpler: double the time, double the light intensity. Want one stop more than 10 seconds? That'd be 20 seconds.
Working these two things together: Close the aperture one stop, double the time to keep the same exposure. Open one stop: halve the time to keep the same exposure.
Many lenses that open a bit wider than full-stop f-numbers have maximum apertures that don't agree with the full-stop sequence. Hence your f/3.5 on the Omegaron and the f/7.7 on my 203mm Ektar, etc., etc.
F-stop timing for enlarging simply uses the unit of a "stop," i.e., the doubling or halving of the light intensity as a basic unit. The increments are then divided further into halves, thirds, sixths or even 12ths of a stop. The problem is, calculating the intermediate values requires the use of fractional exponents (see here: https://www.largeformatphotography.i...op-calculation posts #6 and #7).
I prefer percentages, which has the same advantage, but is easier to figure in my head and without a chart. Even Ralph Lambrecht, an exponent of f-stop timing, gives the percentages for fractions of a stop in his chapter on f-stop timing in his book, "Way Beyond Monochrome":
"For normal paper grades, between grade 2 and 3, enlarging time differences of a 1/3 stop (~20%) are significant in tonal value, 1/6 stop (~10%) can easily be seen and differences of a 1/12 stop (~5%) are minute, but still clearly visible, if viewed next to each other. Smaller increments may be of use for paper grades 4 and 5..."
Check out the whole chapter here: https://www.tmax100.com/photo/pdf/fstoptiming.pdf
Clearer now?
Doremus
Buy an F stop primer, you'll never look back
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