Calibrating digital negatives without making calibration target prints

[john-isner-2]

After learning Keith Schreiber’s method for making “pseudo stouffers” (posted elsewhere in this forum) it occurred to me that a similar approach can be used to do digital negative calibrations without printing linearization targets. This could reduce the time for a full calibration from several days to less than a day. I haven’t tried it since my P400 PiezoDN printer is currently mothballed, but if the idea has any merit, I may try do so.

Make a process profile (a.k.a. characteristic curve) by printing one or more Stouffer step wedges at the standard exposure (this is your one trip to the darkroom). Measure Luminances with a spectro. Make a smoothed curve that maps logH → Lum.

Mirror the process profile and do some interpolation to get 51 evenly-spaced transmission densities from zero to a maximum that matches the process exposure scale (labeled QTR Printer Profile in the diagram). Paste these into the PPE Target Measurements tab.

Starting with the Master quad and the 51-step PiezoDN target, make a negative using Print Tool. But instead of making a darkroom print, measure the negative with a UV transmission densitometer. Paste these measurements into the PPE Measurements tab.

Make a new quad (Q1) by linearizing with the Master quad, the 361T measurements, and the target measurements. The box labeled “M-PPE-Q1” in the diagram is my shorthand for the PPE workbook used for iteration zero (I don’t like reusing workbooks since it destroys the calibration history).

Iterate until the Measurements curve is linear.

I’m probably missing something about the choice of starting quad and the possible need for limiting.

This method, assuming it even works, will not be as accurate as one based on measuring target prints, since it depends entirely on the accuracy of the process profile and has no feedback loop to act as a correction mechanism. It’s a tradeoff between calibration speed and accuracy.

[PiezographySupport]

As long as you match some arbitrary dMin and dMax between your target densities and measured densities, PPEv2 is already set to do this type of calibration (has been for several years actually since the very beginning of PPEv2).

Simply put the target film densities (however many you have 21->) in the target measurements starting with the top density at A1 going down.

eg:

2.91
2.44
2.32
2.13
…

Then put your measured film densities in the measurements part. Ideally you have something like the 21x16 or similar target that you print for densities as this will give you a good averaging and more accurate density reads (a Barbiere or i1Prov3 may be useful here).

Just make the top density and bottom density =. So if the target density ramp says 2.9 for spectral white and you measure 3.2, just set it to 2.9 as long as the next down is less.

Anyone with a good 361T UV densitometer can paste their densities (that are validated through the normal precise PiezoDN process) here and others with the same densitometer can get quickly there (almost).

warm regards
-Walker

[john-isner-2]

Thanks, Walker. This obviously needs more thought on my part. Again, the point of using densities rather than spectro measurements to linearize is avoiding trips to the darkroom. I would rather spend an hour with a 361T than an hour in the darkroom followed by more hours waiting for the print to dry.