Enlarger ----> Sensitometer
This is rather embarassing because after so many years of film testing I should know this already, but I never tested film by contacting and I'd like to try it for some comparisons.
So, suppose you set your enlarger to some arbitrary height, f-stop etc, and use it to contact a wedge onto 100 ASA film. How do you know how long to expose for?
Here's the "no calculation required" method:
Give it a whole second. Worst case you make the whole film black and have to try shorter time.
Best case you will get about 10 steps clearly readable.
Then you will want to adjust the time (you know the drill - two steps is one stop) until your negative test strip just barely fades to clear for 400 film (fades to clear about 4 steps earlier for 100).
Thanks Bill - but how do I know what speed I'm getting?
By the way I don't mind the calculations - but if it is going to be way too much to write I don't want you to waste too much time.
By the way if it helps I have Nicholas Lindan's Pyro Enlarging Meter (I've never used it though).
I set my enlarger up to make an 8x10 from 35mm and then adjust the starting exposure to 1/2" at f22 which is the shortest cycle for my timer/enlarger and lens. This usually does the trick. Using longer than about 1/2" can cause reciprocity failure problems. In fact, the 1/2" is probably too long but it will work. I have then gone on to dupe transparencies onto negative film (B&W and color).
PE's advice to keep time short like a half second makes sense...
For my purposes I take the speed as the rated speed when the curve meets the spec's.
I only question it when something doesn't make sense. Like... why am I getting EI smack between 50 and 64 when I hit the ASA gradient with Panatomic-X and D-76? Oh well I guess you can't trust the speed of expired film.
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I used a spare enlarger as a sensitometer for a few years. I found it almost impossible to account for all the variables even when I wrote everything down.
Exact same lens
Exact same aperture
Exact same head height
Exact same lens/light source (focus) distance
Exact same mixing box
Exact same filtration (all 'white' or did I give it some blue to make it more daylight ???)
Exact same bulb (did I use the EKG or the DED bulb??)
Exact same time
Exact same method of making the contact (ie exact same glass thickness, which contact frame did I use??)
Exact same step wedge (I have multiple, which one did I use ??)
Exact same location on the baseboard (did I put it the step wedge in the center of the carrier image projection, the center of the lightbox projection or the optical center of the lens???)
Using an enlarger might be just fine for you but it kind of depends on what you want to do. If you want to post some speed info on Shanghi film compared to Tri-x or just get your slope to help with determining negative development time or monitor the activity of your 5 year old replenished developer.
A few years ago sensitometers were inexpensive and I posted many times that everyone with a densitometer should have one, though, I think the inexpensive ones are getting harder to come by. 8 years ago I got an EG&G off ebay for $13 and I was the only bidder. My white-light Wejex was $50.
When someone posts about fantastic shadow density with their super concoction fine grained home-made developer I don't give it much credence unless I see a comparison curve with D76 or some other popular developer. A sensitometer is the best way to make curves quickly and easily.
Perhaps a stupid question, but... Why would you want to do that?
Originally Posted by Michael R 1974
The accuracy of exposure time with an enlarger is at least ten times worse than
the accuracy of a camera shutter, and much too long for most films.
So even with the best possible quality control, your results are just wild guesses.
You'll get far better (more accurate and repeatable) results shooting a Macbeth chart or step tablet with a camera.
Last edited by Leigh B; 07-12-2012 at 06:16 PM. Click to view previous post history.
“Wise men talk because they have something to say; fools, because they have to say something.” - Plato
I have used an enlarger, a contact frame setup, a professional 1B at EK and an EG&G unit. So, I've covered a lot of territory. I can just say that all of those work for me and have given useful and repeatable results that have been compared over several years. All I can say is use what works for you, and I am happy with the enlarger, contact frame and GG&G that I have. The 1B is probably in the dumpster at Ek.
According to the BTZS book, Delta 100 in D76 or ID11 1+1 with Ilford's recommended times is pretty much smack on the nose of ISO 100. You can therefore use that as a reference for the amount of light coming from your enlarger if you run some tests on Delta 100 at the same time (films exposed concurrently, preferably) as your other films.
Or, apparently (I have not verified this) if you get EV1 on a light meter, that implies a 1s direct exposure onto ISO 100 film to hit Zone V. Adjust accordingly depending on your film sensitivity and how far up the step-wedge you want to go.
You want to keep exposure to no more than 0.5s in order to prevent reciprocity failure from destroying your results, unless of course you're trying to make reciprocity-failure measurements. The other issue is that with many enlargers, the bulb takes a good fraction of a second to warm up / cool down, which means that a large part of a short exposure will be at a lower intensity, which means you're going to have reciprocity failure anyway; you can cure that if you put a good mechanical large-format shutter on your enlarger and use that to control the testing exposures (make them square-waves instead of trapezoidal).
Have you considered bouncing a known-power flash off the ceiling of your darkroom from a fixed location and using a flash-meter to get the power level at the tested film? Again, you can calibrate the exposure from your flash/room combination using Delta 100 as a reference-point. And a flash will be much whiter than an enlarger bulb, which is good when you're testing films (like Acros and CHS-25) that don't have a lot of red sensitivity and for which you would under-estimate speed using a tungsten light-source.
Originally Posted by Photo Engineer
PE- How is your EG&G holding up? Mine required a new bulb and one of the capacitor bleed resistors to ground fried. Other than that, seems like it should keep going a long time.
Michael R 1974-- The EG&G manual actually is very good reading for using and understanding any sensitometer, even an enlarger. If you want a PDF let me know. Even if you don't have a 'sensitometery laboratory' or calibrated sensitometer it is interesting to read about how to do an ASA PH2.5-1960 speed test in a step-by-step how-to manner.