Incident Exposure Questions

Discussion in 'Exposure Discussion' started by Alex Hawley, Oct 26, 2004.

  1. Alex Hawley

    Alex Hawley Member

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    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.
     
  2. Donald Miller

    Donald Miller Member

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    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.
     
  3. Alex Hawley

    Alex Hawley Member

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    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?
     
  4. Flotsam

    Flotsam Member

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    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.
     
  5. Flotsam

    Flotsam Member

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    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.
     
  6. Alex Hawley

    Alex Hawley Member

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    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? :confused:
     
  7. Helen B

    Helen B Member

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    '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
     
  8. Flotsam

    Flotsam Member

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

    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.
     
  9. wm blunt

    wm blunt Member

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    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.
     
  10. Donald Miller

    Donald Miller Member

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    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.
     
  11. Alex Hawley

    Alex Hawley Member

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    Bump into Prime Time.
     
  12. Jorge

    Jorge Inactive

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    You cannot point your gossen and read the light and dark places and come up with a SBR. You are measuring different things, the reflective readings are measuring luminance of the subject, IOW the light they reflect. The incident metering system relies in measuring the illuminance or light falling on the objects to be able to determine the difference in light strenght (contrast) and expose and develop the film accordingly. Since the incedent meter averages the light falling on the object, it is a more reliable way to expose for the shadows, in addition if you want increase or decrease the contrast of the negative, all you have to do is give a little less or more light to the meter to determine the exposure. All the light meter reading does is determine the contrast range and the required development times for that specific contrast range.

    I have found this system far more reliable than the ZS or spot metering. But you do have to do the inital testing, after that, it is a piece of cake.
     
  13. photomc

    photomc Member

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    OK, I'll bite..it sounds from reading Donald and Jorge and others comments that after the intial testing that your exposures become easier and the negatives are easier to print. Is this correct? Now, all the software, curves, and desnitometers are they really needed or is there a 'poor' mans version?
     
  14. Jorge

    Jorge Inactive

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    There is a poor man's version, mainly graph paper and french curves, but the ExpoDev is really handy, not only does it do everything for you, it gives you a record at the end with all the information. The plotter allows you to do many more things than just make curves, if you have the data, you can compare different films and how they will print on a certain grade of paper, you can compare the curves for two different films (Phil has done a lot of testing and he includes all his data in the software) hell you can just load his curves in the exposdev, adjust them a little and not even have to do the testing yourself. I recently lost all my files, so I just uploaded his data for TMY and HC110 and got perfect negatives, just for that the price is worth it for me.

    Of course, one needs to know how to do all that the expodev does like bellows compensation, filter factors etc. But hey, the older I get the dummer I get, I am glad to have a gizmo to help me out... :smile:
     
  15. Art Vandalay

    Art Vandalay Member

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    Damn! I thought the thread said Indecent Exposure. I'm outta here.