Response to Densitometer

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

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