 Mike Johansen |
Need help changing F-Stops & Shutter Speeds About 10 years ago I had a little 'chart' that told me how to change for time exposures. So, if my medium format camera gives me a night reading of 5.6 at 1 sec, and I want to do 'time' and get to stop 32, what is the progression? (or is it DIgression?)Are there 1/2 stops for both F-stops and shutter speeds? Have been looking for a chart, or will make my own, but with the correct numbers! I feel so numerically challenged! thanx, mike
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Jon Close |
Print in a column down a strip of paper the aperture values in full stop increments in ascending order: 1 1.4 2 2.8 4 5.6 8 11 16 22 32. On another strip of paper print the full stop shutter speeds in descending order: 1/4000 1/2000 1/1000 1/500 1/250 1/125 1/60 1/30 1/15 1/8 1/4 1/2 1 2 4 8 16 30 1min. 2min. ... Hold them side by side so that f/5.6 and 1 sec. are on the same row. You can then read all the equivalent exposure combinations and you'll get f/32 and 30 seconds. This is a linear equivalence and does not take into account reciprocity failure/error - the fact that film's chemical process reacts more slowly as the exposure time increases. For very long exposures you may need to add +1 or more stops of exposure to that equivalent exposure. Reciprocity failure varies by specific film, some need correction for exposures longer than 10 seconds, others may not need it unless exposures exceed 1 minute. Fuji, Kodak and others publish data sheets that indicate the correction times, or you can simply bracket - start with the linear exposure from the chart, then do another at 2x, 3x etc.
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Jon Close |
oops. I typed the order of f-stops and shutter speeds correctly, but technically that aperture series is in DESCENDING order (progressively smaller apertures), and the shutter speed series is in ASCENDING order (progressively longer times).
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Michael H. Cothran |
For each full f-stop you close down, double your shutter speed time, and for each full f-stop you open up, halve your shutter speed time. Examples - 1 sec @ f5.6 = 2 sec @ f8 = 4 sec @ f11 = 8 sec @ f16 = 16 sec @ f22 = 32 sec @ f32. Any one of these times will equal the same exposure, BUT!...if you are shooting film you will run into a little gremlin known as "reciprocity failure," which means that the film's sensitivity to light will slow down with shutter speeds slower than a few seconds. So while 32 sec @ f 32 is the technically correct exposure, your film may still be underexposed due to RF. Michael H. Cothran www.mhcphoto.net
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