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

1. Originally Posted by Michael R 1974
The caveat is you kind of have to assume a normal luminance range (as discussed in the other thread).
Where does it say that? I made a post last night on the other thread on this topic.

Drew, the various toe shapes is one reason why fixed density film speeds like the Zone System method are inferior speed methods.

2. It doesn't say that. That was my own "kind of" caveat. I'll edit that now.

I'm a little behind in the other thread so I haven't read your most recent post. Apologies.

3. Back to why Zone System and ISO "speeds" are different. With the Zone System, you meter a target and stop down four stops. In the example below, the ISO speed of the film is 125. That means the exposure at 0.10 over Fb+f is 0.0064 lxs making 0.8 / 0.0064 = 125. The metered exposure is 8 / 125 = 0.064. Four stops down from the metered exposure is the exposure 0.0041 or too low for the 0.10 density speed point. In order for the four stops down exposure to fall at 0.10 over Fb+f it requires 2/3 stop more exposure. One way to do this is to rate the 125 speed film at 80.

Yes, I am repeating myself, but hopefully in a slightly different way. So Bill, except for experimental error, for general purpose developers this is the primary reason why Zone System EIs and ISO speeds are different.

4. Originally Posted by Stephen Benskin
With the Zone System, you meter a target and stop down four stops. In the example below, the ISO speed of the film is 125. That means the exposure at 0.10 over Fb+f is 0.0064 lxs making 0.8 / 0.0064 = 125. The metered exposure is 8 / 125 = 0.064. Four stops down from the metered exposure is the exposure 0.0041 or too low for the 0.10 density speed point. In order for the four stops down exposure to fall at 0.10 over Fb+f it requires 2/3 stop more exposure. One way to do this is to rate the 125 speed film at 80.
Well, now THAT is one heck of a good, simple explanation.

And it explains why an Incident Meter reading, won't agree with a Spot Meter reading, where the shadow is Placed on Zone (shadow Zone of your choice).

5. Would this mean if I translated the ISO speed point into a zone would it lie at zone 0 1/3 from the view point of the ZS?

6. Hi Andreas,

Part of why I asked the question. I think the practical implication is that when you Place Shadows on Zone II, you fundamentally make the "2/3 stop" adjustment (even though you make a whole stop).

7. I use exactly none of above systems, with the exception of simply labeling sheet film shots that need
other than "normal" development, and need to be otherwise lumped into N+1, N-1, etc batch sessions.
What I do is almost instinctively or instantly visualize the actual developed film curve in my head, and
where on it I want to place specific spotmeter values. It's a helluva lot more accurate than any kind of
math forumla or generic zone model. I can readily switch between different films, lighting ratios, developers and timing, filters, the whole nine yards, and nearly always nail it spot on. Interpretation of
a neg onto print all transpires in the darkroom anyway. After you've done enough densitometer plotting
and have enough field and darkroom experience, it all seems to become intuitive. I've even worked without a light meter just from memory of analogous settings. .. and I never rely on the "latitude" of a
film. I want to know exactly how the shadows, midtones, and highlight are going to differentiate. Might
initially sound tricky, but having first been schooled (school of hard knocks) by shooting and printing
chromes, black and white exposure is relatively easy to cope with.

8. Originally Posted by AndreasT
Would this mean if I translated the ISO speed point into a zone would it lie at zone 0 1/3 from the view point of the ZS?
I think trying to approach it this way can cause a lot of needless confusion.

9. When I started to read about personal film speed, I found lots of websites showing detailed tests to do with a range of exposures and developing times.

Almost all of them came to the same conclusion - Halve the speed and decrease the development time by about 25%.

That was enough for me. Rather than do the tests myself, I just tried HP5+ at EI 200 and developed it for 75% of the ISO 400 time.

I liked the results so kept doing it.

Steve.

10. Everything in the real world. Does your subject land anywhere on something other than a totally predictable straight-line part of the curve. Of course it does. You've got gradation curves inherent to
shadows and highlights, and what you do to midtone expansion affects your choice of how this works.
ASA is really arbitrary in this respect, even from one shot to another with the same film. What counts is how you actually distribute specific parts of the scene onto the curve geometry. The Zone System is
just a generalized way to classify how you go about this, which you can make as simple or as complicated as you with. Walking around with a calculator and some other model might work perfectly
well for someone else. But in effect, you are still altering your ASA whenever you change your endpoints
or midpoint. It's all relative. Hypothetically, I might even want to blank out the shadows ala Brett Weston, but gotta know what's going on at the bottom of the curve, regardless of ASA. One film will
do it, another will just leave you with mud, even though under ordinary circumstances you would assign
them exactly the same ASA. I guess you could go around with a pocket computer programmed with
calculus and fiddle with all kinds of silly integers or micro-hedgehogs in it or whatever. If that's the kind
of thing you enjoy, fine. I do plenty of densitometer work in the lab when it's actually relevant. But in
the field I want to instinctively know what's going to happen under a wide and often suddenly-changing
scope of lighting. The way I conceive of ASA might shift just as fast.

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