I am utilizing the BTZS system for exposure and using an incident meter to assess shadow and highlight SBR values. The BTZS book, along with other written sources, advises that when using an incident meter, measurements are done aimed at the camera lens.
I have attached an image to help explain my question ... I am shooting a railroad shed with the sun behind me and rising (AM). Using my meter, I measure the shadow value at EV9 and the highlight value at EV14, for an SBR of 10. I make my exposure and development notes and take the shot. For this image, my meter was aimed at the camera lens (as per established practice) and was pointing in the direction of the light source (sun) for the highlight value (again ... the sun was behind me).
Here is my question:
I now want to take an image at a 45 degree angle to the shed, so I move my camera off to the side. Using the practice of keeping the meter pointed at the camera lens, I take my shadow reading ... still at EV9. In taking the highlight value, the reading is different and lower at EV13. This because the dome of the meter is no longer directly pointing at the sun, but slightly shaded due to the 45 degree angle I am now using it at.
Now, there has been no change in the lighting ... same shadows - same bright morning sunlight hitting the walls of the shed. I have only moved the camera at an angle. In my thinking, there should be no reason to change the exposure since there has been no change in the light. If I do change, I am looking at possible burned out highlights.
Would it not be best to measure ONLY the contrast values (shadow and highlight), considering the image before you and take the shadow reading and then aim the meter toward the sun for the highlight? If I do this, I will have uniformity of contrast measurement. If I keep shifting the meter to face the camera, I will probably be fine with my shadow measurements, but my highlight data will be inconsistent.
Looking forward to your responses ...
Thanks!
Rick
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