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Scanner densitometry; TMX

Michael Young , Mar 24, 2004; 03:05 a.m.

I sandwiched an IT8 to a cut down sheet of 100TMX, and shot the attached. The intention was to plot some development curves using the scanner as a densitometer. Visually, though, the shot has some problems. The shadows are very cleanly separated, but the shoulder is very broad and flat. Is it simply overexposed?

A larger, unmanipulated negative image is at:

http://www.photo.net/photodb/photo?photo_id=2229782.

Would appreciate your comments.


Inverted grayscale image.

Responses


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D Poinsett , Mar 24, 2004; 07:31 a.m.

Using a scanner as a densitometer is theoretically possible but two related problems must be overcome. I suspect these problems are at work in the example you have submitted.

1) The scanner itself must be calibrated against known density values before using it to measure unknown denisities. You can scan a Kodak or Stouffer step wedge, but...

2) In order to optimize scan quality, software for many consumer-type scanners cannot disable automatic exposure adjustments. These will undo any calibration.

These unknows must be controlled or you cannot reliably use the results from ordinary scanners to evaluate your film. I have experimented with using a scanner as you propose but have not been to produce results that match those from my densitometer. I've gotten close enough that it seems with reach but have not spent the additional time required to solve the problem. Perhaps others will suggest a solution.

Michael Young , Mar 24, 2004; 12:01 p.m.

The scan is as accurate a reproduction of the negative as my equipment will allow. Auto-exposure and other distortions are turned off. The thumbnail above was converted to grayscale in Photoshop, and inverted to produce a "normal" view. The larger scan, link above, is the unadulterated scan.

I exposed the frame five stops above what the light source metered, putting it on Zone X. That seemed to be the right thing to do. Does the rest of the frame look OK? The film-base, visible at the left, isn't blown out or too dark. The shadow densities, though, look a bit suspect.

I would suspect the meter calibration, but I use it expose color trannies regularly with good results.

Michael Young , Mar 24, 2004; 08:02 p.m.

Maybe it's not *too* far off

Maybe it wasn't too far off the mark. Here's a rough plot.


Attachment: TMX-000-Curve.jpg

Michael Young , Mar 24, 2004; 08:07 p.m.

Smaller plot

Oops. Meant to resize that smaller... Here's a 400 px wide plot.


Attachment: TMX-000-Curve.jpg

D Poinsett , Mar 24, 2004; 11:15 p.m.

Try scanning the IT8 film directly as a reference using the same settings and procedures you used for the TMX film. Since you know the density values in the monochrome steps, if your measurement and calulation methods are correct they should yield the values of the steps. If you get different values (and I suspect you will), you may possibly use them as a rough calibration reference to convert the TMX scan results to actual density values.

You may also need to repeat the test with the IT8 film at different times to be sure that variation in lamp brightness and other factors are not in the mix. (Fluorescent scanning lamps vary in brightness with temperature among other things. With auto-calibration turned off, the compensation is likely to be lost.)

How did you calculate the film density values in your graph?

Stephen Benskin , Mar 25, 2004; 12:46 a.m.

Mike, your curve doesn't look right. I think you are fighting an uphill battle here. The reasons densitometers exist is to obtain reliable results and they are only so accurate at that. With a scanner, I don't think you can ever be sure.

Michael Young , Mar 25, 2004; 10:15 a.m.

Both the original IT8 and negative scan are in a folder. Development and scanning details are in the comments.

The chart values came from comparing and interpolating the test negative against the presumed 1/2-stop steps in the scanned IT8 gray step wedge. The IT8 calibration data contains unambiguous CIE- color values for each patch. This would be a more direct translation in Photoshop, and might try this next. (I'm giving up on manual measuring and processing. See below.)

Hi, Stephen. Aye! and verily! What can be simpler than measuring 21 spots on a negative, and plotting the values? I have a scanner and software development tools, but no densitometer. And so, I asked myself: How hard can that be? Maybe the next guy to come along will mumble to hisself: "I have a scanner and this cool software. Who needs a densitometer?" It needs doing; I'm going to do it.

Stephen Benskin , Mar 25, 2004; 02:47 p.m.

Mike, how about having the negative read by a lab with a densitometer and then use that data to confirm your scanner's response?

Michael Young , Mar 25, 2004; 03:55 p.m.

Stephen: Yup. The local community college has one in their darkroom. I enrolled in a course so they'll let me use it.


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