BTZS + Modern Incident Meter w/ Digital Readout

[sanking]

"The milky plastic dome accepts light from a very wide area - approaching a solid angular field of 180 degrees - and transmits approximately 18% of the incident light to the cell underneath so that the meter provides the equivalent of a three-dimensional gray card reading..."

Ken,

The plastic dome transmits approximately 18% of the incident light to the cell, that is why it gives a similar reading to a reflected light reading with an 18% gray card.

But the internal calibration of meters is not for 18%, but as Mark indicates, 12% or 12.5%. I believe that information is also in BTZS in the pages I mentioned. That is part of the reason why we have to adjust exposure with an incident reading for shadow and highlight readings with B&W film.

Sandy

[Mark Barendt]

Ken,

First, the principles of setting exposure, etcetera, put forth in the BTZS books is not the same as the ISO standard. Two different animals. Manufacturers don't have to use domes that transmit a certain percentage of light.

Second, spot metering the darkest spot we want detail in and incident metering in that same light should normally provide very different readings.

Example, a groom wearing a black tux on a sunny day with the afternoon sun behind him. An incident meter reading taken at His chest is probably going to be about f/4 @ 1/125 for ISO 125 film, a spot reading on the black fabric might read f/4 @ 1/8 for ISO 125.

[Ken Lee]

Yes Mark, you're certainly right.

Part of the problem here is that with technical matters, it can often take a lot of words to describe exactly what we mean. "Taking a reading in the shadows" is a vague statement. "Taking an incident reading in the shadows" is still a vague statement. Of course, if we know what the writer means already, it's clear, and we're puzzled that others don't.

That's why I started this thread, with my question about what Phil meant when he wrote so-and-so about using a meter.

Perhaps I should take the good advice offered, and perform my own tests. To that end I have just purchased a nice meter that will do both Spot and Incident readings. The dome on my analog Sekonic meter (which I bought as a backup for travel because it's so tiny) is so small that unless you hold it just right, the edge of the meter covers the dome. When the real meter arrives, I'll be testing in earnest, and hopefully all will be revealed.

Many thanks for your help.

[Bill Burk]

The second principle is that 18% is a traditional number, but not the correct midpoint for subject brightness. It's actually 12.5%, or even 9% if we do the arithmetic. Therefore, metering the reflection off an 18% gray card will result in 1/2 stop underexposure if we consider 12.5% to be the correct average gray. If we use 9%, then we're underexposing by a full stop. Since incident meters are also based on the (wrong) 18% standard, they also give us readings which result in 1/2 stop underexposure (or 1 stop, if we use the 9% value)... unless we work around the problem.

How we work around the problem, is the part that gets obtuse :-)

The third principle is that we must base our exposure on the shadow (low) reading, because it's critical with b&w negative film. ("Expose for the shadows, develop for the highlights").

The obtuse part is that simply metering for the shadows would normally result in overexposure, since shadows need to be dark. However, we know that the incident meter is going to give us a reading by which we will be under-exposing by 1 stop. So it ends up OK in the end.

I spent the past couple evenings reviewing all the urban legend sites about 18%.

I don't think you should worry about it. The meters aren't calibrated to 18%, not even the incident meters. They are trying to recommend the correct exposure. (I think our poster on a previous thread who did a test and found his specific meter calibrated at 18% may have found a meter that is calibrated to 18%, the folklore supports that can happen).

When you take the incident meter into the shade of your subject for BTZS, and use the EI for the SBR that the Power Dial or Expo-Dev recommends. You are properly exposing and developing your film. That's my take.

The urban legends can be updated at our leisure but I really don't think they enter into it unless you really take an 18% card into the field with you because - then - you have an 18% to adjust to whatever your meter is calibrated to.

By the way, has anybody ever made a "real" Power Dial? The one where you take slivers of masking paper and print paper and actually make a dial with gray wedges? Just curious, bet they are rarer than anything.

[Ken Lee]

When you take the incident meter into the shade of your subject for BTZS, and use the EI for the SBR that the Power Dial or Expo-Dev recommends. You are properly exposing and developing your film.

You are probably right, but inquiring minds want to know how and why, so that they can do the same thing on their own, armed with only a camera and light meter.

That's why my original post reads: "I'm also aware that the ExpoDev software automates much of this, and incorporates additional variables like flare, filter factor, bellows extension, reciprocity, etc - but I'm trying to grasp the core principal here. "

[Bill Burk]

You are probably right, but inquiring minds want to know how and why, so that they can do the same thing on their own, armed with only a camera and light meter.

That's why my original post reads: "I'm also aware that the ExpoDev software automates much of this, and incorporates additional variables like flare, filter factor, bellows extension, reciprocity, etc - but I'm trying to grasp the core principal here. "

Traditional Zone System gave you an EI that didn't vary whether you used N-2 or N or N+2 ignoring the "fact" you get "more" speed when you develop longer and less speed when you develop less.

BTZS took in that fact and gives you a "sliding EI" that tells you to use different meter EI according to the scene.

Did you get the core principle you wanted? I'd say it is to meter in the shady part of your important subject - (and yes, go back later after picking your EI and meter shadow again if the meter dial lost your first reading). This metering determines exposure and it's incident so what the meter says is what you should expose. It will give you good shadow detail even on something dark gray in shade.

Then incident meter in the full brightly lit part of your important subject. The difference in readings plus 5 is the subject brightness range. You don't use the reading here for exposure setting. Just for development.

[Ken Lee]

Yes, thank you

[Stephen Benskin]

An easy way to determine the relative "average" reflectance is to divide the exposure constant of the reflection meter by the exposure constant of the incident meter. The basic standard is K = 1.16 cd/ft^2 / 12.5 cd/m^2 for a reflected exposure meter and C = 30 fc / 322 mc.

K / C = average reflectance

(1.16*pi) / 30 = 0.12

[Ken Lee]

An easy way to determine the relative "average" reflectance is to divide the exposure constant of the reflection meter by the exposure constant of the incident meter. The basic standard is K = 1.16 cd/ft^2 / 12.5 cd/m^2 for a reflected exposure meter and C = 30 fc / 322 mc.

K / C = average reflectance

(1.16*pi) / 30 = 0.12

Let Average Reflectance = K/C
Where K = Exposure Meter = 1.16 candles per square foot (which is the same as 12.5 candles per meter squared) ?
and C = 30 foot candles (which is the same as 332 meter candles) ?

Then K/C = 1.16/30... where does PI come in please ?

What is the exposure constant of a meter ? What do we mean by "relative average reflectance" ? Could you explain these terms in a little more detail please ?

Thank you. It's wonderful to have such knowledgeable people on the forum.

[Stephen Benskin]

Let Average Reflectance = K/C
Where K = Exposure Meter = 1.16 candles per square foot (which is the same as 12.5 candles per meter squared) ?
and C = 30 foot candles (which is the same as 332 meter candles) ?

Then K/C = 1.16/30... where does PI come in please ?

Yes 12.5 is candles per meter squared and 332 is in meter candles if I have my units correct. You can use these values for the equation, but you still need to use pi. I like to use cd/ft^2 for K because it works with the calibration equation and I can invision the factor aspect of the K factor better using it. (see embedded page below)

pi is to convert cd/ft^2 to Footlamberts.

You can, of course, also use the average illuminance and the average luminance. (297 * pi) / 7681 = 0.12
Where do those numbers come from? Take f/16 squared and multiply it by the constants K = 1.16 and C = 30. That's why you can use the constants to determine the relative average reflectance.

What is the exposure constant of a meter ? What do we mean by "relative average reflectance" ? Could you explain these terms in a little more detail please ?

Meters don't see reflectance, but you can determine a reflectance that should balance results from the reflected meter to the incident meter under the same illuminance. It's relative.

What is the exposure constant of a meter?

In a nut shell, the constants can be considered a light loss factor that takes into consideration the characteristics of the exposure meter and the camera's optical system. A hand held meter doesn't see through the camera lens. While on the whole, the constants can be considered a light loss factor, it doesn't necessarily apply to all the variables involved in determining the constants.

You can find a detailed explanation of the constants at http://www.apug.org/forums/forum37/8...ancel-out.html

Here's an sample page from that discussion.