BTZS + Modern Incident Meter w/ Digital Readout

[david.jade]

The way I'm interpreting what you are saying is that the ExpoDev assumes the speed data is from a no flare model and any flare value input into ExpoDev will then affect the EFS. If this is correct, then ExpoDev is adjusting the EFS for flare, up to average flare, which has already been compensated for.

So I'm not sure I am following "which has already been compensated for."

Plotter's simulation model of Flare is only enabled when the user specifies a flare density value during analysis, which is applied to the curve's data points as I previously outlined. When exporting data, that flare simulation is always turned off so it is the raw EFS data either from 0.1 above FB+F or the Approximate CI method of speed point determination. So yes, ExpoDev always assumes that no flare model is in effect when it applies its flare compensation, because it knows that Plotter has exported the data without using its model of flare.

Neither Plotter nor ExpoDev present flare as stops, they both deal with flare as an average density that gets added, typically 0.02, which when translated to EFS adjustments in ExpoDev is a very small adjustment that is much, much less that 1 or even 1/2 a stop. It is the amount of exposure needed to reduce the negative density by that flare density amount. For instance with an EFS of 100, with a flare density factor of 0.02 added the EFS gets adjusted to only about 102. We're talking very small fractions of a stop.

What I'm saying is that the placement of the speed point Hm at a difference of 1.0 log-H from P (metered exposure point 8 / ISO) assumes flare as part of the speed equation

Now if you are saying that the fundamental method of determining the speed point via the ISO triangle has a built-in model of flare, then I guess so - ExpoDev is applying a flare compensation over that built-in amount. But I'm not sure I am following your reference to "the standard exposure model for a 125 speed based exposure" though.

Plotter's method of speed point determination just uses the standard model of speed point determination by (simulated) placing the ISO triangle for a curve with an average gradient of 0.62. Plotter interpolates a curve from the film data with a average gradient of 0.62 and then finds the speed point on that curve and assigns an EFS that the user stated as the ISO of the film (there's a little more to this that involves a calibrated personal reference point since not all films are the ISO they say they are and that has to be compensated for as well). Then Plotter finds the speed points for for each of the other curves and determines their EFS in relation to the found ISO point. It sounds to me that Plotter is using a different way of determining the EFS of the tested films from what you are doing.

However, back to what I think your question is: where does Plotter determine the exposure point, H, to be when it finds the adjustment to use for flare compensation. Plotter simply looks at the curve to find the FB+F density that equals the amount of flare specified, i.e. where the FB+F density is 0.02 typically. From that point it can then determine the amount of exposure needed to double the density from that section of the curve and thus knows what additional exposure amount will produce an increase in density of 0.02 for flare for that particular film and developer combination.

[Bill Burk]

...And also, Minor White invented BTZS. His "Zone System Manual, How to Previsualize Your Pictures" outlines "Short Cut Exposure-Development Calculations"

1. Take a meter reading off the palm in shadow
2. Place on Zone IV to set exposure
3. Take a meter reading off palm in sun
4. Development determined by difference in the two readings.
5-Zone difference N-2
4-Zone difference N-1
3-Zone difference N
2-Zone difference N+1

I exaggerated Minor White's contribution. It's not right to say he invented it.

It's more realistic to say he planted the seed for it.

Minor White clearly outlined a technique for two-incident-readings exposure and development estimation.

And this was carried further into a fully workable system by Phil Davis.

Kind of how John L Davenport's Constant Quality Prints inspired the Zone System in the first place.

[Bill Burk]

Neither Plotter nor ExpoDev present flare as stops, they both deal with flare as an average density that gets added, typically 0.02...

So the end-user who performs a "black box" test... directly inputs the density of the negative that they get reading the image of the box opening (which would be a clear spot without flare)?

[Stephen Benskin]

So I'm not sure I am following "which has already been compensated for...Now if you are saying that the fundamental method of determining the speed point via the ISO triangle has a built-in model of flare, then I guess so"

First, thank you for your time answering my questions. It's been very informative.

Yes, that is what I'm saying. Film speeds would be about a stop slower without flare.

But I'm not sure I am following your reference to "the standard exposure model for a 125 speed based exposure" though.

It's just the statistically average conditions - 2.20 log luminance range, 0.0 RD for highlight, Illuminance = 7680 fc , flare between 1 to 1 1/3 stops, using the standard exposure equation with q = 0.65. But as the ExpoDev doesn't use a camera image / flare curve, it wouldn't be applicable.

Plotter's method of speed point determination just uses the standard model of speed point determination by (simulated) placing the ISO triangle for a curve with an average gradient of 0.62. Plotter interpolates a curve from the film data with a average gradient of 0.62 and then finds the speed point on that curve and assigns an EFS that the user stated as the ISO of the film (there's a little more to this that involves a calibrated personal reference point since not all films are the ISO they say they are and that has to be compensated for as well). Then Plotter finds the speed points for for each of the other curves and determines their EFS in relation to the found ISO point. It sounds to me that Plotter is using a different way of determining the EFS of the tested films from what you are doing.

Slightly, the average gradient of 0.62 is a reference to the contrast parameters defined in the ISO standard, except that the standard actually uses a more limited log-H range,1.30, because it is concerned more with the gradient of the lower end. The log-H range used in determining the gradient can make a difference especially with long toed curves. If the contrast parameters are met, the fixed density film speed will have a good correlation with the fractional gradient method. The ISO standard is basically a simplified version of the fractional gradient method. Although one that only works at that specific contrast. For development less than or greater than the ISO parameter, using the fixed density method introduces a progressive error. In other words, the fixed density method of speed determination only is accurate for that one contrast. C.N. Nelson writes in Safety Factors in Camera Exposures "The fixed-density criterion tends to underrate films that are developed to a lower average gradient and to overrate films that are developed to a higher average gradient." As such he suggests "The fractional-gradient speed criterion (and its approximate equivalent, the simpler ΔX speed criterion described in Ref. 12) will continue to be useful as a supplement to the fixed-density speed criterion when an evaluation is desired of the effective picture-taking speeds of films that have been developed to average gradients higher or lower than the proposed standard average gradient." BTW, the "simpler ΔX speed criterion" reference is what the ISO speed standard actually uses. That's where the contrast parameters come from.



Because of the inverse relation of the difference from the fractional gradient point to the fixed density point with the "average" gradient, film speeds using the fractional gradient / Delta-X method don't change nearly as much as with the fixed density method. This means that a negative processed normally will have a different overall density than one developed to a higher gradient even though their film speeds might be the same. As Jones observes, "From the standpoint of tone reproduction theory there seems to be no justification for the adoption of any value of density as a significant criterion of the speed of a photographic negative material. The primary function of the negative material is to record brightness [luminance] differences existing in the scene. Density, per se, has no significance as an indication of the ability of the photographic material to perform this function."

From a practical shooting standpoint the different speed values derived from the fixed density and the Delta-X speed methods aren't that big of a deal considering the variability of flare from scene to scene and that fact that film speeds were at one time about one stop slower than they are today without any detrimental effects (and ZS speeds are still there). It's more a question of accuracy.

Granted, BTZS is a large step or two above the Zone System, but it's still a system. I wrote my own programs because I didn't want to compromise on either exposure theory or tone reproduction theory. Plus, I didn't want to deal with all the made up BTZS terms.

[david.jade]

So the end-user who performs a "black box" test... directly inputs the density of the negative that they get reading the image of the box opening (which would be a clear spot without flare)?

Yes, if a user really needs to manage flare precisely they could perform a test to determine the amount of flare for their conditions, equipment, etc..

For me with modern lenses, I typically use the suggested value and it works just fine. I suspect that this is what most people do. Even during Phil's 5 day workshops testing flare was not something anyone did.

[david.jade]

First, thank you for your time answering my questions. It's been very informative.

It's been interesting for me too, I haven't really looked at the Plotter code in many years now. ExpoDev on that other hand, well that's been my life for the last 7 months or so getting it out there again. Still, much of the exposure calculation code is 8+ years old now too (as evolved from the Palm version).

Granted, BTZS is a large step or two above the Zone System, but it's still a system.

For me I've always taken a very pragmatic approach to using any system. The BTZS system (and the tools used) allow me to get negatives that are so much better than what I got with traditional Zone System testing, exposure, etc... with very little additional effort (all upfront). Things are just more consistent for me now.

Most would probably also think that as the author of the BTZS tools that I spend a lot of effort with film & paper testing but it's the opposite actually. I test once when materials change. In fact I used Phil's test data for years until Kodak changed some things. I find something that works and I get out and take as many photographs as I can. Tools like ExpoDev (and Plotter) just make me more efficient and free up my mind to concentrate on the image vs. the process.

[blevblev]

If you have an iPhone, don't bother with the Palm Pilot thing. Expodev has be rewritten for the iPhone, and is available on the app store. Search for 'BTZS expodev.' Maybe the author can't say anything, but I can.

[baachitraka]

When taking low and high reading, always set the meter ISO to 100 initially. Once you determine the SBR using the BTZS, consult the EFS charts to set the film speed and take shadow reading and determine the exposure and use SBR to determine development times.

Thanks for the recommendations. I mentioned in my post that I am aware of the ExpoDev software, but I'm trying to grasp the fundamental concept. Meters with dials often behave differently than meters with numeric readouts, and explanations can be... ambiguous.

I actually have a Power Dial, and find that the exposures it recommends are at odds with what Phil Davis recommends on pp. 136 paragraph 2 of the latest edition of his book. In Appendix 5, he gives some instructions about its use, but there are some questions about which ISO to set, which remain unanswered. I get the impression that he was rushed by one reason or another, when adding that section to the book. For example, he tells you to use the box speed of your film - except 250 when shooting TXT, and 80 when shooting FP4. He doesn't explain why those films would be handled that way - and not others. Many people shoot those films at lower-than-box speed of course, but it's surprisingly inconsistent, considering how earnest and meticulous the bulk of his writing are.