Scanner as densitometer.....
My densitometer died. I've got enough experience to make do with a "proper proof" but.......Has anyone ever determined an accurate, repeatable method of using a scanner as a densitometer for B&W? I feel like it could be done but haven't figured out a way to create a standard and so never got past the "I wonder" stage.
I suppose you could start with an ICC target? To calibrate your scanner and then use measurements off the grayscale part of the target to check against the densities on your b&w negative? Except of course the target is a positive with relatively short Dynamic Range and the neg is a negative.
Hmmmm. any thoughts?
Re: Scanner as densitometer.....
I had it roughly working with negatives. I scanned a step wedge along with the negative and set the range based on the step wedge. Then I plotted the pixel values in a spread sheet vs the step wedge values. Plotted a graph and used the eye dropper tool and the graph to read the values. It worked, but you needed to make sure you didn't change anything on the scanner. It was also painfully slow since I didn't automate any part of the lookup. It wasn't long after that that I bought a densitometer.
I never tried it for prints, but with a calibrated step wedge it should work as well.
Re: Scanner as densitometer.....
Thanks, food for thought.
Another thought related to this-not as precise. Is there something like a "proper proof" workflow for a scanner-doing a raw scan (on a calibrated scanner negative as positive) then just setting the black clip based on film edge-minimum clip required to render film edge as black.......letting the highlights fall where they may?
Re: Scanner as densitometer.....
Vuescan gives you a reading for what is under the cursor, for bw or color. But the problem is that in a normal scan, the software adapts the readings to the negative, so the readings will be relative rather than absolute. So you have to standardize how you do the scan so that it doesn't do that. I'm afraid I don't remember how to do it, but I did do it some time in the past. The help files for Vuescan may tell you how to do it. Or, if you send email the Ed Hamrick, he will probably tell you. Once you have done that, it would simply be a matter of taking readings from some standard source such as a step wedge for the purpose of calibration.
Re: Scanner as densitometer.....
Kirk, I frequently do almost exactly as Larry described. Set the scanner conditions exactly the same each time for the step wedge (calibrated Stouffer) and the negative; they need not be scanned at the same time. V750 is very consistent. Read off the density number with the eyedropper in PS (0 to 256) for both the step wedge and negative before doing any corrections. Make a nomograph of step density vs eyedropper value. Check calibration periodically. Quite a sensitive procedure but a PITA to do. You don't have to go to the pixel level though.
Nate Potter, Austin TX.
Re: Scanner as densitometer.....
Quote:
Originally Posted by
Kirk Gittings
Thanks, food for thought.
Another thought related to this-not as precise. Is there something like a "proper proof" workflow for a scanner-doing a raw scan (on a calibrated scanner negative as positive) then just setting the black clip based on film edge-minimum clip required to render film edge as black.......letting the highlights fall where they may?
Yes, but you can adjust both ends and capture a bit past black and a bit past white to adjust after scan. No point in throwing away before scan.
Re: Scanner as densitometer.....
Quote:
Originally Posted by
Kirk Gittings
My densitometer died. I've got enough experience to make do with a "proper proof" but.......Has anyone ever determined an accurate, repeatable method of using a scanner as a densitometer for B&W? I feel like it could be done but haven't figured out a way to create a standard and so never got past the "I wonder" stage.
I suppose you could start with an ICC target? To calibrate your scanner and then use measurements off the grayscale part of the target to check against the densities on your b&w negative? Except of course the target is a positive with relatively short Dynamic Range and the neg is a negative.
Hmmmm. any thoughts?
You're headed in the right direction but you should use a B&W negative step wedge as your standard to compare to your negatives. There is no real calibration of a scanner in B&W, but you can use a reference negative for comparison. It'd be pretty simple to lay the reference and the test neg on the glass side by side and scan them together. If not that then save a scanner setting that you use for this purpose every time.
Re: Scanner as densitometer.....
I would like to do this for BTZS purposes but just haven't bought a step wedge yet. Certainly you can do it in VueScan by using known settings (warmup time on lamp, fixed RGB gain, fixed everything in the colour tab, etc) and then interpreting the 8-bit values that are displayed under the eyedropper. There's a lot of mapping from raw values to 8-bit values though and if you're in negative mode, there is a gamma involved.
However, I suspect a better approach would be to do the fixed-process scan thing (as positive) and save the 16-bit raw scan data as TIF. You can then open the TIF in the editor of your choice and determine the mean value over an area to high precision - the averaging is important because of grain and noise. Of course, you then map that to a density using a reference scan of a step-wedge.
I also suspect that if you can include some raw scan-platen (i.e. with NO film in the optical path), that might get you a reasonable 0D reference point and allow you to directly measure density (to within the linearity of the scanner CCD) without requiring a reference step-wedge for calibrating it. Say the 0D calibration value is M, then at some other region with mean value N, D = log (M/N). Clearly it would be beneficial to verify this at least for each scanner model against a step-wedge before relying on it, but if it works it means you could do density measurements that are immune to fluctuations in the backlight level or variations in the scan exposure time. The primary limitation is that you need a white-point that is not clipped, either numerically or electrically, and that would limit your Dmax unless you also calibrate the linearity of your scanner's response with respect to its exposure time.
Re: Scanner as densitometer.....
In VueScan, under Prefs... Enable density display.
Then in the Preview pane... Hold down the control key.
Re: Scanner as densitometer.....
OTOH for a down and dirty estimation of your processes, it is pretty easy on your histogram to just set your black point on the clear film edge, bring in your highlight slider in till it just touches. This will tell you roughly a couple of things real quick (if you remember your shadow highlight exposure for the negative) about your ISO setting and development. This is not tooooo different than the old "proper proof".