Ilford DELTA 100 reciprocity
I was using Fuji Acros for all my B&W photography, but since moving to LF, I can't find this film in sheets locally. So I'm considering moving to Ilford Delta 100 (a few less coins too).
Now here is my question: I can't find table for reciprocity failure of this film (mathematic formula would be nice). The graph that Ilford is showing on their PDF is a bit on a smallish side, especially on the beginning of the graph (1/2-10 sec). Is there any kind soul out there, that can help me with their experience?
Sandy King published a formula for calculating the increased exposure. Many of us made charts Here is mine http://home.pacbell.net/mkirwan/reciprocity_failure.htm. Click on the workbook link and you can down load the tables. It includes Delta 100
Originally Posted by marko_trebusak
How about Agfa APX?
How does this formula work? I'd like to get better data on Agfa APX 100 and 400 and the Efke films.
I think you might get a better fit if you fit an equation to just the amount you must add to the indicated exposure time.
Thus, (tadj - tmeas) = a * (tmeas ^ b)
You can linearize it by the log transform to:
log(tadj - tmeas) = log a + b log tmeas
Just call log a = c and solve for b and c. Then a = 10^c. On log-log paper, this is a straight line with intercept a at tmeas =1 and it goes through 100^b at tmeas=100.
I have found that for the films tested and reported by Howard Bond in Photo Techniques, the b coefficient is 1.6. The only significant difference among those films is the a coefficient. I found these values to fit the data quite well:
Thank you, folks!
Mike, are you sure your courve is right one? I mean, on Ilford's pdf, the curve is much steeper.
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The chart in the Ilford Delta 100 data PDF data sheet uses a linear scale on both axes. The chart I show on my site uses a logarithmic scale on both axes which will give a linear and less steep curve.
Originally Posted by marko_trebusak
Hope this helps
Simply plotting the Ilford data on log-log paper will not give a straight line. The Ilford curve plots measured exposure plus additional exposure against measured exposure. If you first subtract out the measured time from the total time, you will get a linear plot on log log paper. All the curves for the films I listed above are parallel lines on log-log paper.
Originally Posted by MikeK
You see the problem: the log of a sum is not the sum of the logs. The log of a sum plotted against the log of either of the addends is therefore not a straight line.
By the way, when I plot the Ilford data, subtracting the measured time from the total time, the result is a straight line exactly parallel to the data provided by Howard Bond, but shifted upward. On log-log paper, this means that the data from Ilford differs from that from Bond by a constant multiplier.
Gotcha. This is really strecthing my poor old brain. The sum of logs is multiplying, subtracting is dividing. OK now off to take a well earned nap
Originally Posted by gainer
Mixing Long Exposures and Short Exposures on the same roll
I'm shooting this film in a Hasselblad 501C, and developing it in Kodak XTOL.
My question: As long as I apply the reciprocity conversion to long exposures (e.g., 15 minutes), I can shoot other shots on the same roll at normal shutter speeds (e.g., 1/125th sec.), right?
Basically, what I'm asking is if it's okay to mix long exposures and short exposures on the same roll, and then develop the roll according to my normal procedures.