The ratio between the speed point and metered camera exposure is what links film speed to camera exposure. As Connelly writes, "It is evident that the relationship between the sensitometric measure of exposure Hm and the photographic exposure requirement Hg must be determined." It being exactly Δ1.0 log-H may have involved some rounding or maybe not. It could just as easily be Δ1.3 log-H from the fractional gradient speed point if that speed method was still the standard. Part of the ISO speed constant is a compensation for a change in the color temperature of the test exposure, from simulated sunlight to simulated daylight. Otherwise the ratio would be 8.4x or Δ0.925 log-H.
Originally Posted by Bill Burk
i have now finished my tests.. finally after determining the EI (ISO320) I adjust die logH min until that the development intersect with the Development-Time...
In my Case ISO320 must be N Development.... Is this approach okay ?
Looking at the Effective Film Speed graph, it looks like to me that the line reaches 0.0 at EI 500 for "N".
my fault. i dont change the logH min to 0,64 in my case.
But I have to fight another problem.
Verifying my Tests:
Yesterday I photographed a white sock and a black shirt; daylight.
I measured the black shirt (V 2.8 1/30) and the white sock (V 11,7 1/30 )
Therefore the contrast of this scene is about 5 stops
I put the shirt on zone III (2.8 1/125); the socks should be shift to Zone VIII (according the contrast of 5 stops)
After Developing the negative with my determined development time of 9,75 (N) I got the black shirt with a density of about 0,17 and the white sock about 1,02▀!
I┤m confused... The shirt should be about 0,37 and the sock about 1,30, or ?
Last edited by gmfotografie; 04-01-2013 at 02:51 AM. Click to view previous post history.
It sounds like the processing is fine. A five stop range with one point at 0.17 would place the upper exposure value around 1.03. If the lower point is placed at 0.37, the higher point should fall close to 1.30. This means you are underexposing for what you want.
Originally Posted by Michael Hoth
The WBM test only calculates relative speed, not actual. Simply adjust the EI until you obtain what you want. My calculations show it to be about 1 2/3 stops. Now, I did a test once with black, gray, and white targets. The results with the gray and white targets worked as expected. The results with the black target were lower than expected. One possibility is that different materials can reflect invisible parts of the spectrum, like infrared, differently which can affect the meter reading. Do another test and try it a 1/2 stop increments. Double check the exposure off a gray card. Remember to open up 1/2 stop from the reading.
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Originally Posted by Michael Hoth
I'm trying to understand better film test based on WBM. If you do not mind I have few questions.
You said that your EI = ISO 320 but in attached file in cell L9 you have 400.
in pdf description to WBM test regarding graph av. gradient vs relative log exposure I found that:
"..In the upper right-hand graph (fig.7c), the average gradient
is plotted against relative log exposure. A black horizontal
line marks the chosen normal average gradient. A red vertical
line marks the corresponding relative log exposure. Its position
is unfortunately not automatic. Estimate the relative
log exposure required for the red and black line to intersect
the curve, and enter that value into cell ĹL10ĺ."..
Like I understand we should modify value in cell L10 until red and black line intersect with each other and the curve. Is that correct?
In attached file they do not intersect with the curve. You wrote later that you corrected log min to 0.64 - I'm assuming to make them intersect?, did you modify your EI as well or yours is 400?
sorry but the attached graphic is incorrect...L9 shloud be corrected to 320...you're right.
having said that i have to retest the effective iso speed :-)
ok I understand.
You need to retest effective iso speed because your N development time was not correct?
so i did the retest on a white and a black shirt.
i setup the iso speed for the 400tx, put the shirt on zone 3 and take photos changing the aperture half steps ( 80mm cfe).
a full stop lower than the recommended film speed i get the black shirt with 0,37 and the white with about 1,27
thank you all for your help
Michael, would you mind posting the image from the shirt test?
Also, could you explain why you had specific densities in mind for your Zone III and Zone VIII?
The shirt should be about 0,37 and the sock about 1,30, or ?"
Last edited by Stephen Benskin; 04-01-2013 at 10:00 PM. Click to view previous post history.