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- Feb 8, 2009
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- Large Format
We sometimes make a good print at a modest size, say 4” x 5”, 5” x 7”, or 8” x 10” and then want to duplicate that at a larger size on the same paper stock with as little wasted material as possible. Or, having made a satisfactory large print, you might want to make an identical smaller print.
There are formulas we can use to estimate the new exposure time based on the previous exposure, magnification, projection distance, area differences, and so forth. But these models often fail to predict the behavior of your enlarger’s light system at different magnifications. In general, the inverse square law doesn’t accurately model the light output of an enlarger at different projection sizes. Likewise for ratio of areas formulas, and other schemes.
This has been discussed in the following thread and others:
https://www.photrio.com/forum/threa...nlarger-head-height.98470/page-2#post-2742033
One useful tool is to develop an exposure table for your enlarger at various magnifications using a light meter. This is accurate and easy. Put a negative into the carrier and install the lens. Compose and focus for the print size you generally use for work prints. Use white light for the brightest, easiest-to-meter projection.
Remove the negative and replace the carrier. Take a meter reading on the easel directly under the lens and record it with the aperture wide open or closed no more than one stop for a bright, easily metered projection. Also record the intended print size. You might need to choose a film speed setting, such as ASA 1000, and a shutter time of several seconds to get a useful reading.
Now put the negative back into the carrier and resize the projection for another size you might want to make. Focus, compose, remove the negative, and replace the carrier as before, take the meter reading, and record it and the other data for this setup.
The reason for using a negative and focusing it is to place the negative at the correct distance from the lens and light source before taking the meter reading. Then the negative is removed from the carrier to make the projection bright enough for metering and without the varying density of parts of the negative from interfering with the accuracy of the meter reading. This is especially important for large projections due the relative dimness when the negative is in the carrier.
Repeat for each print size you might want to make. Place all these notes into a notebook or into a computer file for later reference. Now you can easily see the difference in f-stops between any two common print sizes at the same aperture and for any print size you’re likely to want. The DIFFERENCES in light intensity at the image plane at different magnifications will be the same when the negative is present.
This should work for any two prints made on paper from the same package.
Here’s an example using a 4/80 mm Rodagon on a Saunders/LPL 7700 with the M670 dichroic diffusion color head using a 6 x 7 cm carrier. I got the following meter readings at f/4 at ASA 1000 and 8 seconds. The lines are print size and the meter reading on a Sekonic L-508 with the meter’s diffuser pointed upward and centered under the lens.
4” x 5”, f/32 + 0.4
5” x 7”, f/22 + 0.7
8” x 10”, f/22
11” x 14”, f/16 + 0.2
16” x 20”, f/11 + 0.3
The time factor = 2^Δf (where Δf = the difference in stops)
I use this data to construct a table of the exposure differences in f-stops and time factors.
4” x 5” to:
5” x 7” = 0.7 stops, 1.62X
8” x 10” = 1.4 stops, 2.64X
11” x 14” = 2.2 stops, 4.59X
16” x 20” = 3.1 stops, 8.57X
5” x 7” to:
8” x 10” = 0.7 stops, 1.62X
11” x 14” = 1.5 stops, 2.83X
16” x 20” = 2.4 stops, 5.28X
8” x 10” to:
11” x 14” = 0.8 stops, 1.74X
16” x 20” = 1.7 stops, 3.24X
11” x 14” to:
16” x 20” = 0.9 stops, 1.87X
new time = t0*factor (smaller to larger print)
If I’d made a good 4” x 5” print at 5 seconds, and want to duplicate it at 16” x 20” on the same paper stock, I can accurately calculate the required exposure as
new time = 5 seconds*8.57 = 43 seconds
When making a smaller print based on the exposure of a larger one, I simply divide by the factor.
New time = t0/factor (larger to smaller print)
For example, if I first made a 16” x 20” print at 43 seconds and now want a matching 4” x 5” print on the same stock, I calculate the time as
New time = 43 seconds/8.57 = 5 seconds.
I also did an 8” x 10” to 16” x 20” comparison for my Beseler 23CII using an 80 mm f.5.6A EL Nikkor set to f/5.6 and with the condenser unit focused for proper 6 x 7 cm coverage. My readings at ASA 1000 and 8 seconds were
8” x 10”, f/32 + 0.2
16” x 20”, f/16 + 0.5
This gives a difference of 1.7 stops, factor 3.24X, which are the same as those for the Saunders/LPL 7700 between these two print sizes.
Once you’ve generated the table of differences in stops and their factors for your enlarger, you don’t have to do it again. Other enlargers will produce a different set of readings and possibly different differentials and factors.
This process is inherently accurate because it’s based on actual light meter readings of the projections made with your enlarger and its unique light distribution characteristics. It should be accurate for the range of magnifications used in home darkrooms, as the longest printing times used won’t entail significant reciprocity error.
For making very large prints, reciprocity error will require making exposure tests on small pieces of paper cut from the intended stock.
There are formulas we can use to estimate the new exposure time based on the previous exposure, magnification, projection distance, area differences, and so forth. But these models often fail to predict the behavior of your enlarger’s light system at different magnifications. In general, the inverse square law doesn’t accurately model the light output of an enlarger at different projection sizes. Likewise for ratio of areas formulas, and other schemes.
This has been discussed in the following thread and others:
https://www.photrio.com/forum/threa...nlarger-head-height.98470/page-2#post-2742033
One useful tool is to develop an exposure table for your enlarger at various magnifications using a light meter. This is accurate and easy. Put a negative into the carrier and install the lens. Compose and focus for the print size you generally use for work prints. Use white light for the brightest, easiest-to-meter projection.
Remove the negative and replace the carrier. Take a meter reading on the easel directly under the lens and record it with the aperture wide open or closed no more than one stop for a bright, easily metered projection. Also record the intended print size. You might need to choose a film speed setting, such as ASA 1000, and a shutter time of several seconds to get a useful reading.
Now put the negative back into the carrier and resize the projection for another size you might want to make. Focus, compose, remove the negative, and replace the carrier as before, take the meter reading, and record it and the other data for this setup.
The reason for using a negative and focusing it is to place the negative at the correct distance from the lens and light source before taking the meter reading. Then the negative is removed from the carrier to make the projection bright enough for metering and without the varying density of parts of the negative from interfering with the accuracy of the meter reading. This is especially important for large projections due the relative dimness when the negative is in the carrier.
Repeat for each print size you might want to make. Place all these notes into a notebook or into a computer file for later reference. Now you can easily see the difference in f-stops between any two common print sizes at the same aperture and for any print size you’re likely to want. The DIFFERENCES in light intensity at the image plane at different magnifications will be the same when the negative is present.
This should work for any two prints made on paper from the same package.
Here’s an example using a 4/80 mm Rodagon on a Saunders/LPL 7700 with the M670 dichroic diffusion color head using a 6 x 7 cm carrier. I got the following meter readings at f/4 at ASA 1000 and 8 seconds. The lines are print size and the meter reading on a Sekonic L-508 with the meter’s diffuser pointed upward and centered under the lens.
4” x 5”, f/32 + 0.4
5” x 7”, f/22 + 0.7
8” x 10”, f/22
11” x 14”, f/16 + 0.2
16” x 20”, f/11 + 0.3
The time factor = 2^Δf (where Δf = the difference in stops)
I use this data to construct a table of the exposure differences in f-stops and time factors.
4” x 5” to:
5” x 7” = 0.7 stops, 1.62X
8” x 10” = 1.4 stops, 2.64X
11” x 14” = 2.2 stops, 4.59X
16” x 20” = 3.1 stops, 8.57X
5” x 7” to:
8” x 10” = 0.7 stops, 1.62X
11” x 14” = 1.5 stops, 2.83X
16” x 20” = 2.4 stops, 5.28X
8” x 10” to:
11” x 14” = 0.8 stops, 1.74X
16” x 20” = 1.7 stops, 3.24X
11” x 14” to:
16” x 20” = 0.9 stops, 1.87X
new time = t0*factor (smaller to larger print)
If I’d made a good 4” x 5” print at 5 seconds, and want to duplicate it at 16” x 20” on the same paper stock, I can accurately calculate the required exposure as
new time = 5 seconds*8.57 = 43 seconds
When making a smaller print based on the exposure of a larger one, I simply divide by the factor.
New time = t0/factor (larger to smaller print)
For example, if I first made a 16” x 20” print at 43 seconds and now want a matching 4” x 5” print on the same stock, I calculate the time as
New time = 43 seconds/8.57 = 5 seconds.
I also did an 8” x 10” to 16” x 20” comparison for my Beseler 23CII using an 80 mm f.5.6A EL Nikkor set to f/5.6 and with the condenser unit focused for proper 6 x 7 cm coverage. My readings at ASA 1000 and 8 seconds were
8” x 10”, f/32 + 0.2
16” x 20”, f/16 + 0.5
This gives a difference of 1.7 stops, factor 3.24X, which are the same as those for the Saunders/LPL 7700 between these two print sizes.
Once you’ve generated the table of differences in stops and their factors for your enlarger, you don’t have to do it again. Other enlargers will produce a different set of readings and possibly different differentials and factors.
This process is inherently accurate because it’s based on actual light meter readings of the projections made with your enlarger and its unique light distribution characteristics. It should be accurate for the range of magnifications used in home darkrooms, as the longest printing times used won’t entail significant reciprocity error.
For making very large prints, reciprocity error will require making exposure tests on small pieces of paper cut from the intended stock.
