When I did my personal sensitometer calibration I averaged my results from fresh D100, D400 and TMY, but the thought of an APUG community standard of "D100/D76 defined as equal to 100" is interesting. Much better than passing around a standard candle
Originally Posted by polyglot
Last edited by ic-racer; 07-12-2012 at 07:05 PM. Click to view previous post history.
I actually had 3 EG&G Sensitometers as gifts from friends over the last few years. Two from one friend, and one from Grant Haist. They are all doing well. Thank you.
There are 3 different versions of the EG&G units out there. One has 3 selectable times, one has 4, and one has 4 and a curved object platen to account for edge falloff. The AEC (I think) found that the flat platen was not accurate enough and so EG&G designed one with a curved platen with constant distance from the light source.
I wondered about that, with there being no diffuser and a somewhat short distance to the bulb. By comparison the Wejex is mostly a hollow box, the size of the box and internal construction (including a diffuser) looking to be designed get the bulb away from the film stage and minimize the cosine falloff.
Most people don’t have a meter accurate enough to measure the light from and enlarger for sensitometric purposes. Also, no timer used for printing is consistent and repeatable enough. But you should be able to get an approximate value or at least a relative value.
How I determine the exposure for sensitometric testing is to first determine which step on the step tablet I want to produce a density that falls around the speed point. We know that the speed equation is 0.8 / Hm. For a 125 speed film, the exposure would need to be 0.0064 lxs.
Say I want to use the step that equals 2.60. Now all that is necessary is to use these three equations:
Transmittance = Transmitted light / Incident light
Opacity = 1 / Transmittance
Density = log Opacity
Opacity of a density of 2.60 = 0.0025
Required exposure at speed point for 125 speed film = 0.0064
Incident light = 0.0064 / 0.0025 = 2.56 mcs
Exposure meters want to produce and exposure at the film plane of 8 / ISO. For a 125 speed film that would be 0.064 mcs.
The difference between 0.064 and 2.56 is 40x or 1.60 logs or 5 1/3 stops. So, meter the enlarger light and open up 5 1/3 stops for this set of conditions.
I can’t tell you how well you will do working with an exposure meter and an enlarger because I use an EG&G Mark VI Sensitometer that was calibrated by EG&G (many years ago). But I can tell you from personal experience that the above approach calculating the exposure required to produce 0.10 density on a specific step on the step tablet works perfectly well.
Last edited by Stephen Benskin; 07-12-2012 at 09:00 PM. Click to view previous post history.
If you do all your testing at the same time with an electronic timer there shouldn't be any problem. Any motor driven analog timer won't be adequate.
Originally Posted by Stephen Benskin
With a good timer that pays attention to the niceties of timing AC power you can use any time you like - within the reciprocity limits of the film. If you use Kodak, Ilford or Fuji film then reciprocity isn't much of a problem. With FSU film you can run into reciprocity at 1 second, and testing should be done with a shutter controlling the exposure.
Step tablet testing with an enlarger does need an enlarger with very even light output and a Sola AC line regulating transformer.
Such an easily corrected problem ...
Originally Posted by Stephen Benskin
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The box inside the EG&G is silvered and very reflective. I would guess, after using one for over 10 years, that it is pretty good and since it took the AEC to find a tiny problem, I would have no worries about the units.
In any sensitometric comparison, even with the 1B, I found that averaging was useful.
I once did some research into a meter accurate enough for me to calibrate the EG&G myself. It was somewhere between 4 to 5 thousand. And a new EG&G sensitometer around that time was also in the neighborhood of 4 thousand.
I had this screaming argument with someone from Kodak back in the 90s. He was negating my findings because I had used an intermittent sensitometer instead of an non-intermittent sensitometer. I'm sure for Kodak, who use densitometers that read to 4 decimal places, who wrap their dip and dunk machines in thermal blankets, who use carbon step tablets and revolving step wedges, my set-up was lacking. To me, it produced pretty reliable and very repeatable results.
Is there a way to convert the meter's readings into lxs?
Originally Posted by ParkerSmithPhoto
If you have a Minolta Flashmeter and a flat diffuser, check the owners manual as you can convert the EX readings to Lx and then multiply that value with your exposure time.
I use 1 second exposures with sheet film as I tend to use exposures around 1 second with large format.