briand@tekig1.UUCP (Brian Diehm) (07/18/84)
{} Yes, indeedy, a light meter can be used as a transmission densitometer, and it's not at all difficult. While the mathematics takes about 10 minutes to derive, the result is apallingly simple: Each 1/3 f-stop change represents 0.10 density. For example, you have a spot meter and a set of 4x5 negatives, each exposed for zone I at various EIs. 1) Set up light source and meter pointing to read it, 2) read the light source without the negative, and 3) read the light source through each negative. The negative that gives a reading of 1/3 stop difference from the reading without any negative has a density of 0.1. This is a happy accident and is "only" precise to 3 or 4 decimal places. You should be careful with the lab setup, however. Make sure that the light source is an evenly illuminated source (flourescent behind drafting tissue or perhaps a slide viewer light table), make sure the field of view of the meter is completely filled by the light source, and make sure that the light entering the meter is ALL coming through the negative being measured - hold it right up to the meter cell entrance, and finally, make sure that the negative density is even across what the meter will measure. Specular transmission density, which is measured by a spot light source, is probably too difficult to do accurately with this method. A spotmeter makes this a little easier, and will allow you to measure the negative held at the light source. With a spotmeter, 35mm can easily be meas- ured in this way for zone system calibration. This has been a fairly quiet mathematical/photographic "secret of the trade" until Parry Yob described (with much more effort than needed) how to use a spotmeter as a densitometer in Petersen's Photographic several years ago. I notice the last Ansel Adams series mentions the technique in Book 2, The Nega- tive. Densitometer manufacturers haven't been known to spread the word. Note that the results of all this are plenty good for "users" (i.e. Zone System photographers), but would hardly fall into the ballpark of the accuracy of real densitometers, which are used primarily by labs for process control. I don't think working photographers really have much need of a $4,000 Macbeth. For a good sensitometry reference which describes the reasons for all this see "Photographic Sensitometry" by Zakia & Todd, the seminal work in the field. I believe it is published by Morgan & Morgan around 1968. -Brian Diehm Tektronix, Inc. Pedantic P.S. - Since N.D. filters are notated in density, this relationship is an easy way to tell the number of f-stops reduction of such a filter.