How To Understand Densitometer Readings

[IanBarber]

I have been lent a densitometer to experiment with.

Has anyone got any good online resources which I can read to better understand how to use them.

For example, With the emulsion side up. I have done measured a deep shadow on the negative and its reading 0.13 but at the moment I am not quite sure what that is telling me

Any information would be helpful

[Michael R]

Ian - you cannot do much better than this:

https://www.kodak.com/uploadedfiles/...y_workbook.pdf

It is an excellent, easy to follow introduction to sensitometry/densitometry, which is what you're doing.

To make a long story short, for transmission densitometry (reading densities of negatives), the densitometer reading is a measure of the opacity of the film. The higher the number, the more dense the film, and the more light it blocks. Low numbers would be relatively clear (shadow areas) and high numbers would be relatively dense (highlight areas).

Whether the negative should be read emulsion up or down depends on the densitometer type.

The densitometer may or may not need to be calibrated.

0.13 would be a very thin deep shadow. Assuming the densitometer is calibrated, you should first take a reading of an unexposed region of the film. This will give you the lowest density (film base + fog). Then you subtract that number from all your subsequent readings to give you the effective "net" density. The net density can be thought of as image density. For example suppose you read an unexposed part of the negative and the densitometer indicates 0.05, that would be your base+fog density. Then you would subtract 0.05 from all your other readings. 0.13-0.05=0.08, meaning the area you read has a net (image) density of 0.08. In Zone System terms that would be a very deep shadow on the low side of Zone I.

[IanBarber]

Thanks Michael, I shall investigate that link you sent.

At this stage, I guess I am more interested to make sure I have enough density in the shadow areas to reveal some texture. I have read that Zone 1 is 0.1 above film base and fog so I am guessing that Zone 3 should be about 0.9 above film base and fog but I could and are probably wrong there

[Michael R]

Actually Zone III would be much lower than 0.9, closer to 0.4 depending on EI and contrast index.

Zone I is typically targeted at ~0.1 above film base + fog.

Keep in mind a densitometer reading can only be a guide, since "detail" is a function of contrast, not density. Most films will have good local contrast at Zone III densities if you are calibrating to a Zone I density of 0.1. Ultimately you have to judge shadow detail by looking at your prints (or digital output), not densitometer readings.

[IanBarber]

Ian - you cannot do much better than this:

https://www.kodak.com/uploadedfiles/...y_workbook.pdf

This has to be one of the best documents I have read to date

[IanBarber]

The densitometer may or may not need to be calibrated.

0.13 would be a very thin deep shadow. Assuming the densitometer is calibrated, you should first take a reading of an unexposed region of the film. This will give you the lowest density (film base + fog). Then you subtract that number from all your subsequent readings to give you the effective "net" density. The net density can be thought of as image density. For example suppose you read an unexposed part of the negative and the densitometer indicates 0.05, that would be your base+fog density. Then you would subtract 0.05 from all your other readings. 0.13-0.05=0.08, meaning the area you read has a net (image) density of 0.08. In Zone System terms that would be a very deep shadow on the low side of Zone I.

Are there any charts that map densities to zones ?

[Leigh]

Whether the negative should be read emulsion up or down depends on the densitometer type.

If a transmission densitometer gives different readings for emulsion-up v. emulsion-down, the meter is broken.
Or else you have extremely bright ambient light sneaking into the measurement aperture.

This would be analogous to producing different enlargements depending on emulsion orientation.
It doesn't happen (except for the image being reversed).

- Leigh

[Leigh]

To answer the original question...

Density is one way of expressing the amount of ambient light transmitted through the emulsion.
This is of significance during printing (or viewing of a transparency).

The units is logarithmic. A factor of 2 equates to a value of 0.3, 4 = 0.6, 8 = 0.9, 10 = 1.0.
Each step of 0.3 equates to one aperture stop.

A density of 0.6 would equate to closing an aperture by 2 stops, or speeding the shutter by 4x.

- Leigh

[Drew Wiley]

A helpful book from the heyday of densitometers is Sensitometry for Photographers by Jack Eggleston.

[Greg]

Highly recommend the following book:

Photographic Sensitometry: The Study of Tone Reproduction
by Hollis N. Todd, Richard D. Zakia | Hardcover

You can easily find a copy up for auction for $5-10.00. When I was at RIT, became good friends with Hollis Todd. The book at first seems overwhelming at first but once you skim its contents, it becomes easy reading.