Is there a not too expensive densitomter that one can buy through mail order (considering I live in Japan). I'd like to read some negatives and the only thing that I've foud that wasn't terribly expensive was a Quantum Calcu-Lite XP light meter with a FOX-1 fiber optic probe, the whole thing for under $200 (you get the light meter as an extra) from B&H. However, it only measure to .1 density units... not precise enough. Of course, I will try looking for something at my used dealer here, but I'm not very hopeful. Has anyone proven more lucky?

-- Raja A. Adal (d60w0635@ip.media.kyoto-u.ac.jp), March 24, 2000

Answers

Any sufficiently accurate lab meter is fine. The density is the logarithm of the ratio

time measured for unexposed film

over

time measured for area in question

Particularly when measuring high densities, make sure there is no stray light.

A second, though probably less accurate option might be using a gray step wedge printed at the same exposure as the negative in question. The density of the area in question is approximately that of the gray step having the same gray in the print.

-- Thomas Wollstein (thomas_wollstein@web.de), March 24, 2000.

Thomas, I have two questions:

First, you recommend the equation . However, zone I, absolutely no exposure, should be measuring film base + fog. So in the above equation are you not measuring only the fog. It would seem theoretically correct to measure the and then divide that by the . This is based on NO practical experience, just on theory.

Two, would a Minolta Spotmeter f, which is supposed to measure to one- tenth of an EV, fit the bill? What do you use?

Raja

-- Raja Adal (d60w0635@ip.media.kyoto-u.ac.jp), March 24, 2000.

Sorry, in the above reply, everything which I had put in brackets vanished from the screen. What I wanted to ask was whether using a spotmeter, such as a Minolta spotmeter f (supposed to be accurate to one tenth of an EV, and measuring

The EV for the light table with no film

divided by

The EV for part of the negative whose density I am looking for

And then taking the logarithm for that, would give me the density for that part of the negative. Would that work? Thanks a lot for all of your suggestions, its great to have such a forum. Raja

-- Raja (d60w0635@ip.media.kyoto-u.ac.jp), March 26, 2000.

Raja

I use my lab timer, a Hauck Trialux. This instrument has a small sensor. You place it on the baseboard of your enlarger in an area to have it indicate the exposure (in seconds) that area would take to become completely black. When you use it as a densitometer, it is good advice to take all readings for one film off the same place in the negatives. This serves to elminate the effect of any uneven lighting of the image area. (Another way would be to take readings from a couple of spots and average.)

When I calibrate a film, I start by leaving one negative truly unexposed, for example by shooting at the smallest aperture and fastest speed with the lens covered. This is my base plus fog reference, T0. Then I calculate my way backward from the exposure for zone V and bracket from 5 or 6 stops below to 7 stops above that. The exposures at 5 or 6 stops below should be practically identical with that indicated for the unexposed shot. Suppose the timer suggests a time T for one spot. The density is then log T/T0. (For the exposures at -5 and -6, this will be about log 1, thus zero, or film base plus fog.)

I have read that you can use spot meters and a light table to measure negative densities, but I haven't tried this (yet). The most important requirement is then that the area to be measured in the negative is sufficiently large to fully cover the area scanned by the meter. I think it would be a good idea to fill the complete frames with uniform density.

In principle, I would think that the same rules hold as for lab timers, i.e. you can do the maths with the exposure times at the same aperture. Without thinking about this a little longer I would say, however, that you must not take the log of the EVs as density, as the EVs are log units already. The EV scale of spot meters should facilitate their use as densitometers. Remember that one EV increment equals one f-stop, thus TWICE the amount of light. Let me see: Let us assume you measure an EV, X, for the unexposed negative. One EV increment should then equal 0,3 density units (d.u.), so that a negative where you measure an EV of say X+4 should have a density of 4 x 0,3 = 1,2. (To the others on this forum: Please correct me if I take a wrong turn somewhere!)

The tenth EV scaling of your meter should be precise enough for all practical purposes.

-- Thomas Wollstein (thomas_wollstein@web.de), March 27, 2000.

The enlarger baseboard method is a good idea......but it's only going to give a very rough and ready result compared to a true densitometer. A densitometer should have a collimated optical system that will look at a precise area of the film with a known measuring angle. Projecting an image onto a sensor of wide reception angle will lead to all sorts of errors. The sensitvity will change, depending where on the baseboard the measurement is taken, for two reasons. The first is the Cosine to the fourth power law which will diminish the brightness away from the optical axis and secondly the sensor will be looking obliquely at the light rays. All this is assuming that the enlarger has perfectly even illumination to start with. So unless you fix the position of the sensor on the optical axis fairly accurately, and in some way move the negative around, then the method has quite limited application. Even then, stray light from other areas of the negative is probably going to upset the result, unless you restrict the sensor angle, say with a long blackened tube. Even then one tenth of a stop is a very optimistic estimate of accuracy.

-- Pete Andrews (p.l.andrews@bham.ac.uk), March 27, 2000.

I wouldn't worry overly about this. True, a dedicated densitometer is ideal. True, ideally something that measures to a tenth of a stop is a little more accurate. However, I have tested films with the Calculite and fibreoptic probe, with a Pentax spotmeter improvised with a 50mm lens to focus close as well as a regular densitometer. The tests are really close. The Calculite reads to only a third of a stop i.e., about 0.1 density units. This means that you need to use a little theory in drawing your film curves i.e., the density jumps are discrete and the accuracy is only good to a +/- 0.05 density units. Therefore do not treat the density reading as gospel - it probably is being rounded off to the nearest third of a stop - in other words a density reading of 1.1 could be anything from 1.05 to 1.15. As long as you draw a theoretical curve of best fit (which can be done using a program like Excel or a statistical software package like SPSS or SAS), you will get remarkably accurate film tests (the results of which will not vary frightfully if you use a dedicated densitometer). The spotmeter adapted for use as a densitometer has some appeal since the EV scale is analog and allows you to estimate densities with a little more accuracy. However, it is quite horribly prone to flare. Believe me, if you do not mask off the entire area outside of the spot, flare WILL affect your readings quite noticeably. Not that it can't be done. Use tape to mask off the area carefully (see Phil Davies 'Beyond the Zone System' for details on adapting the spotmeter for use as a densitometer). If you aren't going to take the care to mask off the outside areas well enough, this will actually give you greater measurement error, especially since the densities of adjoining areas will vary as you move over the test strips. A regular densitometer is nice because it measures a precise, preset area. However, note that they are quite expensive and the added accuracy of their readings will only, in essence, mean a smoother curve (which is what you would get if you realized that your readings with another method basically incorporated a greater amount of measurement error). So my suggestion is that you stick to the Calculite probe. After you have your density readings, plot them in Excel (or any other program, freehand is quite doable too) and draw a curve of best fit through these points (the points which fall outside the curve probably represent measurement error, rounding error etc). The advantages to the Calculite method are - no flare, easy to do, you already have the necessary stuff. If you're after more sophisticated measurements, you probably do need a densitometer but if its for getting your family of film curves and development times and speeds, I would humbly suggest that its not worth it (the Calculite is plenty precise enough for that task). DJ

-- N Dhananjay (ndhanu@umich.edu), March 27, 2000.

After all this talk about doing it using various hardware: If you are not interested in numerical densities, why not calibrate by printing? This does not involve any maths, and it has the additional advantage of accounting for paper characteristics, too (which becomes a disadvantage if you switch to a different paper.)

BTW: Pete: Remember reading in my description that I recommend measuring all negatives at the same spot of the baseboard? This eliminates uneven lighting and any angular sensitivity. As to stray light: Assuming that the stray light has a constant intensity (which is reasonable, as it usually comes - via walls and ceiling - from the enlarger head which is on during all measurements) its importance grows as the measured intensity decreases, i.e. at higher densities. So the uncertainty in the higher densities is greater. I think it might even be reasonable to infer that this is not an uncertainty but rather a systematic error, as falsely high intensities, whence falsely low densities are determined. So the method tends to underestimate the high densities, and consequently the gradient, but probably not too much if you take some care to minimise stray light. The critical low densities, however, are not affected to the same degree, because there, the signal to be measured (intensity transmitted by the negative) is high, stray light being constant.

-- Thomas Wollstein (thomas_wollstein@web.de), March 28, 2000.