Question about film density measurement

Hi all,

I think it's probably a stupid question to ask, since I guess there has been international standardization on this one, but I'll ask it anyway:

Recently, I have been experimenting a bit with my development due to thin negatives and shooting controlled test exposures (zones), and now today I have measured the film density on a dedicated measurement instrument (densitometer in Dutch, I don't know the English word for it). Now I have done this before, on another instrument. For that instrument, I had a reference chart showing the desired reference curve for film density. I don't have a similar graph for this other instrument, but values were in a similar range.

Now my question:

Can I safely assume, taking into account the fact that I have set the 0-reference of the measurement instruments in a similar manner using an unexposed clear piece of film, and assuming well calibrated instruments, that a density value of X means a similar film density on instrument A as compared to B?

E.g. Does a film density value of 0.82 measured on machine A and B represent the same actual film density?

Thanks for responses.

If the two machines are setup right then yes.

Marco

If you have ONE lightmeter, you know exactly what the exposure needs to be. If you have TWO lightmeters, you can never be sure.

Replace 'lightmeter' with densitometer, watch, compass, etc... The above statement remains to be true.

The short answer is that both densitometers use the same units. So if they are properly calibrated the values should be very close.

The long answer:

Density is defined as Log(1/Transmittance) or Log(1/Reflectance)

So if film transmits half the light the density is Log(1/.5) = Log(2) = .301

if film blocks 75% of the light the density is Log(1/.25) = .602

if film blocks 90% of the light the density is Log(1/.1) = 1

As you can see for each stop (halving of light) the density increases by about .3

Marco B said:

(snip)

Can I safely assume, taking into account the fact that I have set the 0-reference of the measurement instruments in a similar manner using an unexposed clear piece of film, and assuming well calibrated instruments, that a density value of X means a similar film density on instrument A as compared to B?

E.g. Does a film density value of 0.82 measured on machine A and B represent the same actual film density?

Thanks for responses.

Click to expand...

Marco, it depends upon how exacting you wish to be; to how many decimal points.

Each densitometer, within a brand as well as with brand/component combinations, can vary in linearity and accuracy depending upon user calibration and on baseline calibration of the system to traceable standards.

If you are hoping to have two densitometers agree 100% on all measured film densities, I don't think you are going to be happy at all unless these units are of the same brand, model, very close to each other in date of manufacture AND recently calibrated to traceable, verifiable standards.

You can, however, determine a fairly repeatable offset between the two with extensive measurements over time between the two, provided all variables per unit remain the same.

If your use is casual and not critical, as long as you use a repeatable user calibration scheme on each densitometry, you should be fine.

BTW, I would purchase a nice new 21 step wedge to plot response curves for each machine and to use as a calibration tool. Base plus fog or just clear leader is NOT a good calibration target; you need a verifiable density you can share between the machines. Besides, you don't subtract out the density of the base as a general rule when measuring density of a film strip; the base IS part of the overall density.

Frank

You can zero a transmission densitometer by taking a reading of nothing - that's the normal way of doing it. As Frank says, the important thing is the calibration - taking a reading of a piece of film of known density (preferably one close to the maximum density you will be measuring) and calibrating the instrument to it. Then you have zero and span both set.

Both densitometers would have to be using the same filtration if they were to stand a chance of reading the same value for anything that wasn't neutral.

Best,
Helen

Thanks all for your answers,

As Kino already assumed, my case is not critical, and I am not looking for 0.01 precision, it's just a guideline to proper development and exposure to me, based on a reference curve that already has a tolerance or range with upper and lower limits that differ by far more than a magical 0.01 digit measurement... So again thanks to you all, my question has been answered.