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

1. Originally Posted by Stephen Benskin
I think trying to approach it this way can cause a lot of needless confusion.
I think getting a yes, or no answer would make it easier to understand.

2. Originally Posted by AndreasT
I think getting a yes, or no answer would make it easier to understand.
Well, no it really wouldn't. I'll try to put something together. In the mean time, what part of my explanation so far don't you understand?

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?
What makes you think it would?

3. Originally Posted by Stephen Benskin
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.
Finding the ZS EI in Schaefer is described this way as well. Not really informing you of anything here Stephen (It's merits can be argued against some other perceived better way, but it's how I do it), but just for others who care------if, after plotting the curve, an ISO 400 film does not attain a 0.1D until Zone II, for example, then all the curve data points would then be shifted 0.3 log exposure units to the left and the curve redrawn, and an EI of 200 is then assigned. The redrawn curve then represents a characteritic as if the original curve was produced at an EI of 200 (no need to produce another curve at EI 200). The ISO 125 curve reaches 0.1D at Zone I 2/3, therefore would simply be redrawn after each data point of the curve was shifted 0.2 log exp units to the left to represent the curve at an EI of 80.

Only, the horizontal axis in Schaefer is not divided in 0.2 log exp units, as is presented by your graph, but is shown with major divisions at 0.3 log exp units (as in The Negative), the attachment shows the differences; also, there would be a Zone 0 indicated without an exposure unit applied. The log value of each exposure unit, starting at Zone I, jives nicely with zone divisions at 0.3 units of log exposure, the most important log value, of course, being log10 of 2 = 0.3. I just think it makes looking at curves easier, probably a trivial thing to mention to most, but that's just my opinion.

4. Originally Posted by CPorter
Only, the horizontal axis in Schaefer is not divided in 0.2 log exp units, as is presented by your graph, but is shown with major divisions at 0.3 log exp units (as in The Negative), the attachment shows the differences; also, there would be a Zone 0 indicated without an exposure unit applied. The log value of each exposure unit, starting at Zone I, jives nicely with zone divisions at 0.3 units of log exposure, the most important log value, of course, being log10 of 2 = 0.3. I just think it makes looking at curves easier, probably a trivial thing to mention to most, but that's just my opinion.
I based the design off a graph from Kodak's R&D department or at least from the couple of sheets Dick Dickerson gave me. Each dot is Δ0.02 logs apart. Each square is Δ0.1 log. The solid lines represent Δ1.0 logs. The coordinates are labeled every 0.2 logs. This design is very good for easy to read precision plotting, especially by hand. For my four quadrant curves which are meant to incorporate a larger view of the process has the coordinates labeled in Δ0.30 log intervals.

A quick look through Schaefer's book shows that while he does have coordinates at Δ0.30 log intervals (actually labeled in Zones), he also used graphs with the coordinates labeled every Δ0.20 logs.

5. 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?
No.

I think you went the wrong direction. More like Zone I 2/3 (that's why I said place shadows on Zone II - close enough).

But, come to think about it. Now if I did use a lower EI... And I still place my Shadow on Zone II - It's like I really placed my shadows on Zone III.

Gets messy real fast.

Fortunately that isn't a "bad" exposure.

6. Andreas, what you’re wanting to do is compare two different lengths of measurement that start at the same point and want to end at the same point. If we lay both lengths down with one end of each flush at the same point, one of the lengths will be 1.20 – 1.00 = 0.20 Δ log-H longer than the other. If one end of each measurement begins at the metered exposure point on the film’s characteristic curve, an exposure of 8 / film speed, the end of the shorter, 1.0 Δ log-H, length measurement will fall at 0.8 / film speed where the film density is 0.10 over film base plus fog. The end of the other length of measurement, 1.20 Δ log-H, will be 0.20 Δ log-H further down.

Under these conditions, the 1.0 length measurement will fall at a relative Zone I 2/3, but the problem is Zone I needs to fall at 0.10 over Fb+f. So exposure needs to be increased by 2/3 of a stop. The 1.20 length is moved to the right so that the ends of both measurement lengths are at the same point, but now the starting ends fall at different points. A variable has changed with the 1.20 Δ log-H length of measurement, but not with the 1.0 Δ log-H length of measurement. It’s not possible to make a direct comparison under these conditions.

I don't know if this is going to help or not. It's a breakdown of exposure for an average scene luminance range for a film speed of 125 without flare. The Zone indications are on the left. The metered exposure is on step 10 at 0.064 lxs. The speed point is Δ1.0 log-H further down on step 20 at 0.0064 lxs. Zone I falls at step 22 or 2/3rd of a stop below speed point.

7. Originally Posted by Stephen Benskin
A quick look through Schaefer's book shows that while he does have coordinates at Δ0.30 log intervals (actually labeled in Zones), he also used graphs with the coordinates labeled every Δ0.20 logs.

Yeah, that's in a part(s) of the book that I never have gotten into---Specialty Black and White Films, Alternative Processes, etc.....but within the context of the ZS and testing discussions, it'll be found the other way----I should've been more specific than just referrencing Shaefer alone.

8. Aaaargh...!

9. Originally Posted by Stephen Benskin
Well, no it really wouldn't. I'll try to put something together. In the mean time, what part of my explanation so far don't you understand?

What makes you think it would?
I came to this conclusion because of the lower ISO rating with the ZS. Well I just going to have to reread all this and think. It is starting to hurt.

10. Maybe this will help.

What is basically happening is the Zone System test changes the ratio between the speed point and the metered exposure from the ISO's Δ1.00 (10x) to the Zone Systems Δ1.20 (16x). It isn't so much about determining film speed.

The examples deal with exposure in testing and not exposure placement in use. Flare still has to be added to the mix.

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