Working with a densitometer

[Michael Graves]

I have an X-Rite densitomenter that I acquired a while back and have never really got around to using. Recently I got the bug to see if I could calibrate it and so some film speed testing. So I ordered a step wedge. Two of them shipped, but there were no instructions. One has the number 0054 handwritten in pen on the clearest step and the other 0002. When I zero out the densitometer and take readings from each wedge, they both give identical readings of .08. As I take reading from the wedges up the scale, they generally give identical readings, except for a couple of steps that are off from one another by .01.

Is there a good book on this subject. I have no idea how to use those step wedges to calibrate the machine.

[Brian Ellis]

It sounds like you maybe got a plain step wedge rather than a calibrated step wedge? A calibrated step wedge includes the manufacturer's readings. You make your readings of the calibrated wedge and compare them to the manufacturer's. If they're different, adjust your densitometer up or down until your readings match those on the calibrated wedge as closely as possible. At least that's how I remember doing it, someone who's done it more recently than I have can correct me if I'm wrong.

I don't know that you'd want a whole book on sensitometry just to do film speed tests but IIRC Phil Davis' book "Beyond The Zone System" includes a good introduction to the subject for photographers.

[Keith Pitman]

You used to be able to get operations manuals in PDF from Xrite's website. Give it a try. Otherwise, post what model you have someone may have a manual. I have a couple of those PDFs myself.

[Michael Graves]

I have the X-320 and they don't list that on on their website. That's the first place I checked. But if anyone has a copy, I would be deeply indebted.

[ic-racer]

Is there a good book on this subject. I have no idea how to use those step wedges to calibrate the machine.

There certainly is a good book on the subject, and it's free to download:

http://motion.kodak.com/motion/uploa...y_workbook.pdf

That won't tell you how to calibrate you densitometer, you will the instruction manual for the machine to explain where the calibration trimmers are.

[Michael Graves]

There certainly is a good book on the subject, and it's free to download:

http://motion.kodak.com/motion/uploa...y_workbook.pdf

That won't tell you how to calibrate you densitometer, you will the instruction manual for the machine to explain where the calibration trimmers are.

Thanks! This is perfect. Now I have a little light reading for when I go to bed tonight.

[Keith Pitman]

I have a pdf manual for the XRite 310-42 (color transmission densitometer). It may be similar. If you give me an email address, I'll send to you.

[neil poulsen]

Which model X-Rite do you have? Is it a black and white version, or color? A lot of black and white models that I've used have two adjustments, one for zero and one for gain. One adjusts the zero so that the gauge reads zero with no wedge or density under the sensor. (The sensor needs to be against the light, in proper measurement position.) Then, one takes a known standard and sets the gain until it reads correctly. I think that my standard is around the 2.1 density units.

If yours works this way, set the zero, then pick a wedge at about 2.5 (or 3?) density units and adjust the gain until it reads the same as the value provided with the wedge. Then interate by going through this two-step process again. Afterwards, check different wedges and see how well the densitometer does on all the wedges. They should all be within about 0.02 or 0.03 density units of the known value.

I've always used a Stouffer's non-calibrated wedge. The difference is that calibrated wedges target specific densities to within some delta, whereas non-calibrated give you a reasonable range of densities from clear to dark. As long as one knows the actual densities of each wedge on a non-calibrated step-wedge, it'll work fine.

X-Rite is notorious for keeping people at bay who do not have current models. Gretag was good about helping people with older models. But, X-Rite gobbled Gretag up. (That was a sad day.)

[Michael Graves]

The X-320 is a color densitometer. It has four tiny set screws for each of the primary colors plus black.

[patfahey]

"Is there a good book on this subject. I have no idea how to use those step wedges to calibrate the machine."

Kodak has a pretty good paper on sensitometry. Here's the link:

http://www.motion.kodak.com/motion/u...y_workbook.pdf

I modified an old 4x5 holder (pried up the film rails just a bit) so that I can insert a Stouffer 4x5 21-step transmission wedge in front of a sheet of film. Exposing for zone ten, I get a full range of zones in one shot. That makes measurement, plotting, and comparison pretty easy.

-Pat