Zone System Question - Help Me Out

[D Poinsett]

The log 0.04 value above paper white was chosen because it is close to the limit at which we can see tonal difference on typical papers at the bright end of the scale. The brighter the tone, the more sensitive we are to density differences. Darker tones require much greater density difference to be noticed.

I just did a quick test with a calibrated reflection densitometer. I have a contact print made from a Stouffer 0.1 step wedge using a a #00 Ilford filter on Ilford MG Warmtone paper. The base reflection density of white is log 0.07. In rather poor illumination (2.5m from a 40W frosted bare bulb), the first noticeable tone that I can clearly distiguish from white measures log 0.08 on the densitometer. The next band, which looks dramatically darker (relatively speaking), measures log 0.13. This implies that the log-0.04-above-white figure in the ANSI/ISO standard is reasonable in the conservative direction.

Doing the same quick test in the same illumination at the dark end of the scale requires a reflected density change of log 0.10 (from log 1.69 to 1.59) in order to see any distiguishable difference in tone.

[Michael Kadillak]

Just got back from a marvelous day shooting and was elated at finding all the thought provoking contributions. My sincere thanks for sharing your knowledge with all of us. Cheers!

[neil poulsen]

You take your reflection density value and switch the two numbers, multiply by 100%, and you'll find that you are right on target! (Just kidding.)

To speak only to the relationship between percent reflection and "RD", the reflection density reading from your reflection densitometer, you can use the following formula:

Percent = 100% x 10^(-RD)

where ^ means "taken to the power of". For your reflection density value of 0.81, I come up with a percent reflection of 15.5%. I would say that you are pretty close.

[D Poinsett]

For what it's worth, I just dug a gray card out of the drawer and measured its reflection density. It was, coincidentally, log 0.81.

Mr. Poulsen insightfully provided the commonly used formula for converting density change in log units to percent. This may be exactly what Mr. Kadillak was asking for.

Whether 18% really represents "middle gray" is an interesting consideration. In a perfect world, log 0.81 is the middle of a log 1.62 density range. If you fiddle with the effects of an imperfect world, a log 0.81 measurement is the middle of something around a 1.4 to 1.5 density range. In printed material, that's not a very deep black. A toned silver gelatin print could easily hit 2.40 if allowed to go full black. Black construction paper measured 1.38. If 18% gray is used for a refrence when taking a meter reading for a photograph, it's the middle only when the brightness range is about 5 stops.