How can I explain why my test gives me a satisfying shadow?
I hit 0.07 density 2 1/3 stop below 18%. That's a pretty satisfactory low density.
That corresponds to about 0.0079 MCS, this leaves room for a possible 0.0034 MCS flare.
I must assume that the difference between what you would expect from a 3.6% reflectance comes from effective underexposure due to a variety of possible causes; error in metering, incident meter not calibrated to 18% or maybe mechanical shutter speed problem. But it sure looks like I underexposed the test.
Which effectively gave me a good shadow placement.
I don't know. What I do know is that 0.0080 is 0.8/100 at speed point for a 100 speed film. You rated the film at 50. You've also hit this value 2 1/3 stops below meter exposure which is one stop higher than the 3 1/3 stops it should be and the shadow exposure is still one stop further below that.
Originally Posted by Bill Burk
But you shot the sun facing the target at f/16 at 1/250. Shouldn't that have been at 1/50 sec? So let's say the TMX has a speed of 100. That's a potential 1 1/2 stop underexposure. That's now getting closer to the shadow exposure value except that flare won't have the same effect as if the actual reflectance is lower.
Film tested as 80. Fell short of ASA parameters.
I have the meter set at EI 100.
Sun was late afternoon, fairly low in the sky following a storm (clear skies with minimal dust scattering).
But I just checked and the meter still reads same f/16 at 1/250. BUT, it's in spotmeter mode.
So I must have spotmetered the 18% background of the test chart and not made compensation!
If 3.6% isn't black enough for a flare test, I guess I will have to get the top-hat out for the next test...
These are the difficulties I run into. Designing a good test ain't easy.
Definitely a topic I've been focusing on lately. Good advice to keep in mind with speed and development tests too.
Originally Posted by Michael R 1974
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In a recent thread, there was a beginner who generated a gray scale on his computer and was planning on using it to do speed and development testing. To this person, the black to white scale represented the full black to white range he needed for the test. What he didn't realize was the average scene range is greater than what can generally be reproduced on paper, so his target didn't represent actual shooting conditions. The results of his test would therefore be in question, but since he wasn't aware of this, he could easily have been satisfied with whatever came from the tests.
It seems to me that in order to engage in reliable testing it's important have an understanding of the conditions of the test and an idea of the expected results from the test. Having an example of the exposure with and without flare for a typical scene, I was able to compare the conditions and results of Bill's recent flare test to what is expected. The results Bill experienced didn't appear to match what should be expected based on the model. His shadow density resulted from a patch in which the reflectance should have resulted in a much higher density. Even without knowing all the details of the testing procedures, this type of discrepancy could indicate a problem with the test.
When I was setting up my sensitometer, I had to determine the amount of exposure I needed for a given film speed. I chose a density from the step tablet which I wanted to fall around the speed point. First thing I need to know was what the value of the exposure needed to be for the different film speeds. I wanted the speed point to fall around 4 steps down from the highest density on the step tablet. The instructions with the sensitometer had a chart of the average exposure expected from the different settings. From that I was able to determine the filtration I needed which I included when I had the sensitometer calibrated. Then I ran some tests to determine if the exposures at the different settings created the densities I wanted. So, I knew what I had and what the results should be before I actually did any testing. That way if there were any results were off, I could look for unwanted variables instead of accepting any bad results as representing an accurate test.
I think when testing for an unknown variable, like a flare test, it's important to know what the results should be without the variable and to also know approximately what to expect from the tested variable so if the results fall outside a certain range, the test should then be in question. In other words, know the theory and control the variables.
I agree. The recent grey-scale thread you reference is a rather extreme example in that there was a fundamental misunderstanding of the difference between subject luminance/reflectance and paper reflectance. It is a common error people make when reading Adams etc. Often people look at the page that shows the patches of grey tones from black to white, and they assume that is the range of reflectances they will be metering.
But aside from such glaring errors, even if one knows what the results should be, the actual design of an appropriate test is often difficult. There are limitations to what kinds of situations we can easily create under controlled conditions, counterintuitive as this may seem at first. And one must be careful not to introduce new sources of error when solving a given experimental problem. This is particularly true when attempting in-camera tests. I was on the right track with the test I originally posted to start this thread, but it was still of limited value because of the inherent constraints.
For example, a major constraint in a controlled test for flare is the luminance range we can generate for the target without creating measurement problems.
A possible approach to the flare test would be to mininize the variables in the test. Lock down the camera with the target facing the sun. What ever the camera settings, they shouldn't be touched again during each part of the test. Obviously you should attempt to place the metered exposure at an exposure of 8 / ISO. Meter a gray card and open up 1/2 stop. The target should have have a sufficient range of reflectance to represent an average luminance range.
Shoot the target that has the background completely blacked out. This will not produce zero flare but it will be greatly reduced. Remove the "flag". About 1/3 to 1/2 of the frame should be sky. Shoot the set-up. Include a sensitometric strip in the processing of the test.