Densitometer reading upside-down?

[Helen Bach]

...
Also, it is possible to zero a reflection densitometer with the correct calibration tablet.

I stand corrected. I've never seen a zero density reflection target. Where do you get them from? I have one that is close to zero (L* = 99.5, density = 0.0056) but not zero. Is this simply a difference in what we mean by zeroing?

Thanks,
Helen

[Helen Bach]

I should have written it another way, shouldn't I: you don't zero a reflection head in the same way you zero a transmission head - you don't 'zero' to zero, and you don't do it with nothingness. This is why I had difficulty connecting the evidence with the readings: if you were using a reflection head for transmission readings, zeroing with nothing there would not make sense.

Hopefully I've cleared up the confusion I created, but maybe not.

Oh well.
Best,
Helen

PS I've just tried to use the reflection head of my TR924 to measure transmission densities, and it worked reasonably well. It didn't produce the strange behaviour that is being described here.

[Jim Noel]

My reflection calibration tablet came from the densitometer manufacturer.

[Kirk Keyes]

When I zero it on the .04 strip, then reading nothing returns -1.96

It sounds like something was knocked out of alignment when you hit it with dust-off.

If you can find something that is really thin, like some very thin acetate or perhaps you could find a "standard" for your zero point calibration that was closer to 0.00 than 0.04.

Anyway, if you can live with all your readings being 0.04 low, which really shouldn't be a big bother, you could take your zero reading through the 0.04 cal standard and then for the high cal standard point, use your 2.0 or whatever you are going to use to calibrate the other end of the calibration curve, and then just put a value into the instrument that is 0.04 less then what it really it.

That is, zero on 0.04. The 0.04 standard should now read 0.00.
Then, set the high point, say it has a density of 2.00, adjust the instrument so that the high std now reads 1.96.

So now your instrument will read the differences between all points correctly, but all readings will be 0.04 lower than they really are - you will have a slight negative bias to all your readings. This should not be a big deal.

[blevblev]

I figured to do something like that, and then I realized that I can use my "Precision Enlarging Meter" from Darkroom Automation as a densitometer. I'm going to do that and compare it to the (corrected) densitometer readings. You multiply exposure meter's f-stop reading by .3 and it gives you optical density units. Or you can flip it over and divide the desired reading by .3.

So, if I want to read a negative and am looking for .1 over film base + fog, I put the fb+f negative in the enlarger, zero the meter, put the Zone I negative in and look for .33 (.1/.3) which will equal .1 OD.

[Kirk Keyes]

For doing film curves, it's better to measure the film directly without putting it into the enlarger. You have to worry about flare from the lens and the enlarger system if you measure it by projection.

You could just lay the neg onto the enlarging meter and use the enlarger as a light source if you wanted.

But it sounds like the densitometer would work too with a couple, simple work-arounds.

[blevblev]

Well, like I said, I'm going to compare the readings against the densitometer which seems to read correctly with the offset takent into account. I just don't know how much longer the densitometer is going to last, and I'm not particularly ready to buy another used densitometer.

I'm going to put my personal ASA/EI test negatives in the enlarger and measure both (film base + fog and Zone I negatives.) Will these negatives using that lens, the same focus, and enlarger height have the same amount of flare? If they do, I suspect that the flare won't affect the relative readings. If I'm right, the Zone I negative will still read as .1 above fb+f.

I'll post the results.

[blevblev]

So, here are the results on a roll of TXP-320

Densitometer readings
1) zero densitometer on film base + fog negative
2) ASA 100 negative - .10 OD
3) ASA 125 negative - .08 OD

Darkroom automation meter readings
1) zero the meter with fb+f negative in enlarger (press delta key)
2) ASA 100 negative - .33 Stops
3) Asa 125 negative - .26 Stops

to convert from stops to OD multiply by .3

.33 x .3 = .099 or .10
.26 x .3 = .078 or .08

Pretty close, huh?

[Seymour]

I have an Xrite 404 densitometer and the technical people of Xrite guided me thru a recalibration of my unit. I suggest you call them.