# Flash as Sensitometer

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• 05-30-2013, 05:18 AM
freemp
Thanks a lot for sharing all your experiences. I'm sure it will help to avoid some of the stumbling on the way to success.
• 05-30-2013, 02:18 PM
Stephen Benskin
The idea of using a flash isn't very different than using a EG&G Sensitometer. The output is alot higher than is needed so ND filters are usually required. If you want to measure the output with an exposure meter, I would measure the flash without the ND filters and then caluclate them in. Not only would it be difficult to measure something like 1.5 lxs with a photographic exposure meter, I would question the meter's accuracy at such a low light level. The meter is probably more accurate at normal pictorial levels. I did some research into finding a light meter that I could use to calibrate my sensitometer and the price was around \$4000.00.

Rafal, I didn't go over everything, but so far it all looks good. There is one small thing though. Luminous exposure is H, and I believe the 99.25 lxs value would still be considered E because it has yet to pass through the step tablet. H being the exposure at the film plane.
• 05-30-2013, 07:22 PM
Rafal Lukawiecki
Stephen, many thanks for letting me know about H vs E, and for your comment about the meter's accuracy—much appreciated.

Freemp, please let us know how you got along, when you get there. Good luck.
• 08-18-2013, 09:29 AM
freemp
It took a while, but here's what I did for my first trial:

* F-Stop calculation for step no.28 (2.62 logD):

k^2 = 0.8/C*10^D

* By means of an exposure meter, I was able to determine the C value for my flash meter: C=218

--> k = 1.237 <-> f-stop 1.0 + 6/10

* Flashed the step tablet several times onto HP5+ 120 roll films.
* Developed in Xtol for 4, 5.5, 8, 11, 16 mins @ 20°C.
* Measured all fields with my densitometer, calculated average values.
* Filled the values into the test evaluation sheet (please find attached to this message).
* The normal EI value is just an estimation from the previous measurements. It needs another film to get the real value.

I'm wondering if the result is more or less reasonable or if I'm fundamentally mistaken about something?
• 08-18-2013, 12:02 PM
Bill Burk
Hi freemp,

At first glance, this all looks very good. I don't think you need more tests for EI, I think you could find an 0.62 curve in the set and take the speed for granted... that it is how I rate EI for myself (considering my lab is not a standard facility - I treat the rated speed of fresh film working back to my evident exposure)
• 08-21-2013, 08:43 AM
freemp
I just realized that the films I was using for 5.5 and 11 mins are from a different batch. That's why the corresponding curves reach the 0.17 mark earlier than expected which has a visible effect on the resulting estimations for development time and effective film speed. I did not really care about the batch numbers since I never would have guessed that impact would be that strong. However, doesn't this mean that as soon as I purchase new films which are most likely from a different batch, I actually would need to redo the whole calibration process?
• 08-21-2013, 08:38 PM
Bill Burk
Hi freemp,

I would take the speed at 16 minutes as 400 because it meets the ASA criterion.

I personally would take the 16 minute time as "N" and then the different N-times are one unit different than what is programmed as N numbers in the spreadsheet.

Experiment with a sheet at 21 minutes, that may be your N+2 (my N+1).

As far as adjustments for batches go... It would be significant to a lab, but I wouldn't consider the variation you see significant for personal photographic use. I just keep my Time-CI chart updated with scatter plot points from occasional sensitometry test sheets that I put in with my everyday film runs. And if the Time-CI chart predicts my results fairly well, I continue to work off the older charts.

p.s. I graphed your data on paper, and my results are similar to the spreadsheet (except as mentioned, I call 16 minutes N)
• 08-22-2013, 06:26 AM
freemp
Hi Bill,

thanks a lot for the manual verification. I'm sure this took some time and I really appreciate. I just tried to follow about the 400 ASA @ 16 mins and I found you are right as long as I strictly follow the specification. However, as soon as I adapt the contrast to my enlarger or scanner, the normal EI needs to be adapted.

Regarding batch variations, I think it's a good idea trying to keep track by occasionally mixing some test sheet into regular film processing and adjust curves accordingly, if required. Next to batch variations in film production, I guess other components like developer and fixer are subject of batch variations as well, not to mention the water as the very basic.