Incident Exposure Questions

[Alex Hawley]

We begin today's discussion with a verse from St. Ansel:

"The incident meter does not, however, give any indication of the difference between a light subject and a dark on, since these qualities can be evaluated only by measuring the light actually reflected from the subject, rather than the light falling on it". Page 166, Chapter 11, "The Camera" .

"Go forth with thine one-degree spot meter, splendidly calibrated, and thou shalt record the truest of exposures". (Satirical paraphrasing)

Now, back to reality. Its well-known that the BTZS methods have debunked one of St. Ansel's Canons. However, I'm perplexed on how this is done. How are the incident readings taken to come up with the bright and dark readings? Can I point my old Gossen Pilot at the dark and bright areas and come up with a Subject Brightness Range (SBR)? Needing some enlightenment here.

[Donald Miller]

Alex,
I am sure that others will chime in on this. From my understanding the method of metering with reflective meters and incident meters differs markedly.

To begin...an incident meter is designed to arrive at an average exposure based upon orienting the incident dome in such a way that the light striking it is akin to the light that is striking the object or scene being photographed. In other words the incident dome is pointed from the object being photographed back toward the camera. In reflective metering the meter is pointed from the camera toward the object being photographed.

To arrive at the scene brightness ration two incident readings are taken. One for a highlight value and one for a shadow value. Then the EV of low reading is subtracted from the EV of the high value. This is how one arrives at a SBR value. A normal SBR would be a value of seven.

[Alex Hawley]

That was the way I understood it too Don. But what happens when you can't walk up close to the subject to take the readings?

[Flotsam]

I haven't looked in to BTZS either and am also curious as to how the brightness range of the scene is determined. Even if you take two incident readings, you are still not taking into account the tonal difference between say a whitewashed door and a charred log in the same light. I'm sure that BTZS deals with this but I just don't have the reasoning power to figger it out.

[Flotsam]

Although in the studio, I often use an incident meter with a flat disk in place of the dome to balance and set the lighting ratios.

[Alex Hawley]

Although in the studio, I often use an incident meter with a flat disk in place of the dome to balance and set the lighting ratios.

That I can understand Neal, but what about outdoors in a landscape situation or where one can't walk up and get in the shadows?

[Helen B]

'That I can understand Neal, but what about outdoors in a landscape situation or where one can't walk up and get in the shadows?'

Then one imitates the shadows, or finds similar lighting conditions, taking great care not to measure in too deep a shadow.

The caveat is that haze cannot really be taken into account, except by operator intervention. Obviously, haze (aerial perspective) lightens the distant shadow values to a degree that can't be measured by an incident meter.

Though the incident system is referred to by many as the BTZS system, Minor White mentioned it in his 1967 Zone System Manual and it may not have been radical then, for all I know.

Best,
Helen

[Flotsam]

Nice avatar Helen. With that hat and beard, I thought I was looking in a mirror for a second.

I only know that BTZS is a popular exposure system that uses incident metering. My understanding is that you need a densitometer in order to make use of it so I never looked into learning more.

[wm blunt]

I have switched to using the incident meter with the BTZS Power Dial, a poor mans computer. The View Camera Store also has what they refer to as BTZS Lite which is kind of a crash course that excludes a lot of the testing of the BTZS and explains the metering tech. and explains why it works. As soon as it soaked in how it worked my negatives started coming out much better than with the traditional zone system and spot meter. I still carry the spot meter and use for a few situations but about 90% of the time I use the incident meter with success. As Helen mentioned, you have to be careful not to meter too deep in the shadows, but that is explained in BTZS Lite. My negatives are exposed and developed for Palladium printing and now my print exposure times are around 4 to 8 minutes instead of some reaching 15 minutes or more.

[Donald Miller]

'That I can understand Neal, but what about outdoors in a landscape situation or where one can't walk up and get in the shadows?'

Then one imitates the shadows, or finds similar lighting conditions, taking great care not to measure in too deep a shadow.

The caveat is that haze cannot really be taken into account, except by operator intervention. Obviously, haze (aerial perspective) lightens the distant shadow values to a degree that can't be measured by an incident meter.

Though the incident system is referred to by many as the BTZS system, Minor White mentioned it in his 1967 Zone System Manual and it may not have been radical then, for all I know.

Best,
Helen

Helen, I agree you have a nice avatar. While what I am about to say doesn't directly respond to Alex's original question, it does relate to BTZS as I understand it.

The major difference in the BTZS as opposed to just taking an incident meter reading of the object or scene is that the BTZS begins by determining the characteristics of the paper. That is something that Ansel Adams failed to adequately address in his formulation of the Zone System.

The way that I have determined the characteristics of the paper is by contact printing a step wedge (such as a Stouffers) onto the paper I am testing. Next I develop the paper as normal. When the paper has dried, I read the reflection densities of the steps on the paper. From that I am able to determine the exposure scale of the paper. When I determine the exposure scale of the paper then I can determine the appropriate exposure and development times of the film to give me the appropriate density range on the negative to match the paper.

I have found that since I started using this system that my negatives print easier and my prints are more in keeping what I want.