Why is Zone System EI often about half rated ISO/ASA?

[Bill Burk]

Zone System film speed testers often conclude that a given film's EI is approximately half (or maybe 2/3 stop below) rated ISO/ASA speed.

Is there a simple reason? Does the fact tests are usually done in-camera, make it difficult to separate the variables and identify the discrepancy?

[ic-racer]

Zone System film speed testers often conclude that a given film's EI is approximately half (or maybe 2/3 stop below) rated ISO/ASA speed.

Is there a simple reason? Does the fact tests are usually done in-camera, make it difficult to separate the variables and identify the discrepancy?

Steve had a thread on safety factors awhile back.

[Michael R 1974]

Bill, I think there are a few reasons. See my latest post in the ISO thread. This is what I ultimately wanted to get at in that thread.

[Ian Grant]

Why did Kodak recommend that their 50 EI film Tmax 100 be used at 50 EI if you wanted good shadow detail and fuller tonal scale, it's there in Kodak's first datasheets. Oh and Kodak had the ASA part of the ISO test ammended before they could release Tmax 100 with a 100 ISO.

It's about getting the best tonal range from a film, Tmax at half the box speed and suprisinglt EFKE 25 was twice the box speed - which was the Tungsten speed not daylight.

Reality it's about what you want in your prints (or scans) if the shadow detail isn't there you can't tweak it out later. Developers make a big difference using Pyrocat HD I get almost full box speed and a long tonal scale and plenty of shadow detail.

Ian

[Bill Burk]

Bill, I think there are a few reasons. See me latest post in the ISO thread. This is what I ultimately wanted to get at in that thread.

Ah, but you touched on one of about 30 topics that I am anxious to figure out... I'll keep following your thread and add where I can...

For cross-reference...

Post 72 of this thread...

ISO Speed Determination Constants

[Bill Burk]

I like to keep in mind Ansel Adams developed the Zone System before the print studies were done. He had no knowledge of the 0.3 Gradient.

He also aimed to correlate prints to the Weston Meter. Thus he arbitrarily labeled 10 Zones (8 if you only count the "Textured" Zones) instead of using scientifically more accurate 7 2/3 Stops.

[Stephen Benskin]

Two reasons. When the Zone System was developed, the ASA was one stop slower that it is today. The Zone System's EIs and ASA was the same. Although the reasoning was very different. When the ASA film speed changed in 1960, the ZS didn't. Now the difference in reasoning comes into play. The fractional gradient speed was lower before 1960 because of a safety factor. The elimination of the safety factor increase film speed. The Zone System has a lower EI because it uses a different exposure meter reading to film speed ratio than the ISO standard. It is larger. The difference between the metered exposure point and the ISO speed point is 1.0 log-H (3 1/3 stops). The Zone System uses 1.2 log-H or 4 stops. While the average shadow exposure is considered to fall 1.30 log-H (4 1/3) stops below the metered exposure point, flare and the exposure constant move the shadow exposure slightly to the right of 0.10 over Fb+f fixed density.

[pentaxuser]

If half box speed held good for all or nearly all films when in users' hands as opposed to the strict test under lab conditions for "true box" or is that "foot speed" then wouldn't it be in film makers interests to say that while true speed is say 400 they advise users to try 320/200 as that is the speed at which film X gives proper/full shadow detail.

Sounds like a bit of "Devil's Advocate" on my part but in reality I'd rather have instructions that rendered sufficient shadow detail at a lower speed than a "faster" film that sacrificed it.

Of course this gums up the works for all the automatic cameras that can only read the DX code and can't be overridden and maybe for the vast majority of users, speed is better than good shadow detail.

I wonder how many here rate their films at 2/3rds to 1/2 of box speed? If enough of we 45,000 APUGers responded under a poll of the say the three top films from each manufacturer this might give newcomers to analogue useful information.

If 44,900 rate say TMax at 400 but only 1000 rate D400 at 400 or vice versa for balance then that might say a lot about the respective film speeds

pentaxuser

[Stephen Benskin]

Personally, I don't care how anyone rates their film. That is a question of personal taste. What the question should be is about the accuracy of a testing method and the precision of the resulting information. Technically, the Zone System can't be considered a film speed methodology. And what Bill is asking about is why it produces different test results than the ISO speed standard.

[pentaxuser]

Just a thought on my part which depending on the volume of pollers might produce useful information - for some. It might be useful to some newcomers to film. You are not in that category and nor is Bill Burk but just a bit of thinking "out of the box" on my part

However I'll get back into my box now and try not to darken this thread's door again

pentaxuser