One does not need to know anything about sensitometry to make good photographs, but it can be a useful tool that will provide greater understanding of the relationship between film density range and process exposure scale, an understanding that is not only satifying but that can also save a lot of time and money in testing.

A very good work on the subject is Phil Davis' Beyond the Zone System. I particulary recommend his film testing method, which is based on giving five step wedges equal exposure, developing each for different times, reading the densities with a densitometer and plotting the curves. With this simple test, which can be done in an evening, you can figure out the time of development with a specific film/develper combination for a variety of SBR conditions. It would take days of field tests to provide you with as much information.

Now, there is a problem with densitometers and staining developers. Many densitometers have only one reading, a Visual reading based on a combination of Red, Green and Blue light. Unfortunately a Visual reading does not read the color of the stain and indicates a density range that is much less than the effective printing density range. Color densitomers have a blue reading channel and this is what you should use for graded silver gelatin papers since the density range as read will be very close to effective printing density range of these papers. For alternative processes that use UV light a densitometer capable of reading in UV mode is required since the blue channel is not capable of reading the UV stain.

So, for silver gelating printing, including AZO, a color densitoimeter with blue reading mode is needed for stained negatives. These are available commonly on ebay at a fraction of original cost since many labs that until recently wet processed have gone digital and are now discarding their densitometers and related types of equipment.

For printing stained negatives with any of the alternative processes (pt/pd, kallitype, carbon, albumen, salted paper, etc.) that use UV light sources you will need a densitometer capable of reading in UV mode since a blue channel reading will indicate a density range that is quite a bit below effective printing density range. This type of densitometer is much less common. Two that will work are the X-Rite 361T and the Gretag D-200 II (with accessory reading tube). UV reading densitometers tend to be quite a bit more expensive than regular color densitometers. On ebay you will typically find them in the printing/graphic arts area rather than in the photograhy area.

Sandy King