A Primer on Incident Metering.

Bill Burk said:

Point meter at the camera while you are standing in the sunlit part of the scene, then walk to the shaded part of the scene and point at the camera. Add 5 to the difference to obtain your SBR. From the SBR you will find development time and EI (which in the BTZS system floats with development time).

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That's a decent summary of how BTZS deals with the difference between taking incident metering and reflective spot readings.

Doubling the EI on a shadow reading (say from 100 to 200) doesn't lead to a 1-stop underexposure though.

We have to remember that an incident meter always tries to suggest a camera setting that will place the subjects in the measured light "normally", like caucasian skin at Zone VI or so. Caucasian skin at Zone VI in the shadow area isn't normal, IV or V more likely.

That means that if the incident meter is in the shadows the meter reading should suggest a setting that would normally result in too much camera exposure. Doubling the EI number in brings the suggested camera setting back closer to where a normal spot metering zoner would place exposure.

What is truly interesting to me to see the effect of the entrenched system (reflective spot metering) on the implementation of incident metering systems. How we get hobbled having to use the old language, metaphors, and maths.

For example "Add 5 to the difference to obtain your SBR" converts the incident readings back into spot metering numbers so that they fit into the old established ZS equations.

Another example is the messing with the meter's EI number in the middle of taking your readings for a shot. A simpler, and IMO more understandable way to bias the exposure the direction the BTZS wants us to go is to duplex a shadow reading and a normal readings. It's essentially Dunn and Wakefield's duplexing for slides, revamped for negatives.

HiHoSilver said:

'Also the first I've heard about deliberately making shadows darker, rather than wringing as much detail as possible.

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Whoa there HiHoSilver.

I want to nip a myth in the bud real quick here. The idea you are referring to here works only on slides, negatives are a whole different beast.

With negatives there is no direct connection between the camera exposure and the placement of subject matter in the print, period.

The placement of the subject matter on the paper is managed during printing not with the camera.

Wait, whoa all. It's confusing I'm sure but for starters keep in mind doubling EI is ONLY for doing BTZS duplex incident metering.

You are underexposing an incident reading taken in the shade, not a shadow spot reading.

In the end you arrive at about the same exposure recommendation that you would get if you used Zone System with a spotmeter and placed shadows on Zone III or Zone IV. I haven't tried to find exactly what it equates to, but you are giving plenty of exposure for the shadows.

Hi Mark.
Certainly there is intervening process from neg to print - wouldn't argue otherwise. But if I get my shadows too dark, burning isn't likely to help much. So direct, or indirect, if I want shadow detail, it needs to be visible in a neg. Is that wrong? 'Gladly jetison a false habit - especially a mental one.

HiHoSilver said:

Hi Mark.
Certainly there is intervening process from neg to print - wouldn't argue otherwise. But if I get my shadows too dark, burning isn't likely to help much. So direct, or indirect, if I want shadow detail, it needs to be visible in a neg. Is that wrong? 'Gladly jetison a false habit - especially a mental one.

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Not wrong.

Once you reach the threshold where that detail exists on the negative though the exact exposure above that level doesn't matter for a while.

Bill Burk said:

Set the EI on your meter at double the usual EI (only for BTZS not for any other system) and use the shadow-based reading to obtain f/stop and shutter speed. This will "underexpose" the shadow region by one stop, which will make it look dark in the print (which is exactly what you want the shadows to look like - dark).

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Bill's statement here is wrong.

Yes, changing the ISO setting changes the placement of the subjects on the film curve, moving them lower, but nowhere near underexposure and that change will not affect the print because of where the original reading was taken.

The placement of the meter in shadow area means that the meter will be asking for a considerable extra exposure, probably 2-4 stops over what I would call normal in relation to the placement we might really want. Changing the ISO setting on the meter one stop is nowhere near enough of a change to off set the camera exposure to a point where it would create a loss of important detail.

In short using the BTZS meter ISO setting does not change the print.

The Edward Weston method: Incident reading + 1 stop

In BTZS, set what ever E.I(film speed + filter factor) to the incident meter and take one shadow reading dome pointing to the camera and add one stop to it.

Without the correction the shadow may render as gray instead of desired black.

It can be more than one stop but most of the times I simulate the shadows so one stop is safer for me.

'Again, Mark - thank You. The complexity & time required make btzs sound like a method for shots that one can take time on. 'Very much appreciate your kindness in taking the time to share what you know. Its already helped more than once.

HiHoSilver said:

'Again, Mark - thank You. The complexity & time required make btzs sound like a method for shots that one can take time on. 'Very much appreciate your kindness in taking the time to share what you know. Its already helped more than once.

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My pleasure.

For someone skilled in their chosen method the ZS and BTZS really don't take that much extra time to use. That's actually true of most all metering methods. What takes time is becoming skilled.

markbarendt said:

Bill's statement here is wrong.

Yes, changing the ISO setting changes the placement of the subjects on the film curve, moving them lower, but nowhere near underexposure and that change will not affect the print because of where the original reading was taken.

The placement of the meter in shadow area means that the meter will be asking for a considerable extra exposure, probably 2-4 stops over what I would call normal in relation to the placement we might really want. Changing the ISO setting on the meter one stop is nowhere near enough of a change to off set the camera exposure to a point where it would create a loss of important detail.

In short using the BTZS meter ISO setting does not change the print.

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I'll concede the words I used are unfortunate. Everything you say here is right markbarendt. It's hard to explain because BTZS does some counterintuitive things that need explaining.

I wanted to try to explain that doubling the EI when calculating exposure based on the shadow reading shifts the recommended full exposure from an inflated reading because it was taken in the shadow one stop in the right direction towards underexposure, making the shadow look like a shadow (otherwise if you exposed at normal EI the shadow would get fully exposed as if it were the main subject).

baachitraka said:

In BTZS, set what ever E.I(film speed + filter factor) to the incident meter and take one shadow reading dome pointing to the camera and add one stop to it.

Without the correction the shadow may render as gray instead of desired black.

It can be more than one stop but most of the times I simulate the shadows so one stop is safer for me.

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This is what I was trying to say. I agree that "add one stop to it" is probably a better practice than setting EI to double your tested speed, because you are less likely to leave the meter in "BTZS mode" by mistake. I always worried about the advice to double EI.

For example with ISO 100 film, Rodinal 1+50 as the developer and an yellow filter with filter factor +1 stop, I arrive E.I 25 and set this value to the meter and take the reading.

If the shaded incident reading is EV 10 and my camera settings will be for EV 11.

This works remarkably well for me

"What takes time is becoming skilled. "
Too true. If you don't love it - its not worth the work. If you do - you have fun while you struggle to gain the skills. I'm paying my dues & having some fun in the process.

Bill Burk said:

This is what I was trying to say. I agree that "add one stop to it" is probably a better practice than setting EI to double your tested speed, because you are less likely to leave the meter in "BTZS mode" by mistake. I always worried about the advice to double EI.

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I agree, and in practice I don't adjust my meter's ISO settings in the middle of metering a scene, as you suggest I think that is a recipe for disaster.

One of the struggles I see people fight with incident meters on is with the expectation that "the meter reading" is "always" what the camera setting should be. The caveats are often ignored or forgotten.

As always a direct reading is only correct when the meter is in the same light as the subject and pointed directly or squarely (parallel to the camera shooting line) at the camera.

As the situations get more specialized other caveats become important. For example if the lighting is artificially controlled (anything indoors, flash, scrims including trees shading the scene...) the incident reading needs to be taken at the subjects nose.

In mixed lighting (say; indoor & outdoor or artificial & ambient) both need to be read to understand the situation.

Incident meters are great tools but they can't fix the real placement differences between differently lit subjects.

When used in lighting that is different than the main lighting, essentially as the BTZS suggests for finding camera settings, then the incident reading becomes very much like a spot meter reading of a fixed target at that point and it needs to be treated as such. Think of it like using a spot meter to read a gray card under the bush where want to place shadows at Zone III.

That is also true when used in a studio, each subject needs to be measured separately: background, main subject, secondary subjects, ...

markbarendt said:

Think of it like using a spot meter to read a gray card under the bush where want to place shadows at Zone III.

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Yup