Raja(posted 8859 days ago)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.