Darkroom Light Meter Calibration
I have recently acquired a Mitchell Fotoval PM-17.
From what I have found online, I think I understand how to use it to determine paper exposure time and paper grade. However, a recurring statement is that the meter needs to be calibrated first.
Does anybody have any information about the calibration of a darkroom light meter? I may be able to get some value even from the instructions for different models or makes, so if you have experience with them, I will greatly appreciate your advice.
In particular, there are four adjustment knobs on the meter (see this picture) that seem to be intended to calibrate the meter, but I am not sure exactly how.
Again, any suggestions are appreciated!
It is NOT used to determine exposure time. It is used to set the f-stop of the lens at a specific number of seconds. The exposure time is constant, the f-stop is what changes.
As for the adjustment pots, you should email roger and get a copy of the instruction manual.
This is what I think it is from memory. But realize that the last time I read the instructions for this device was over 30 years ago, so I claim no accuracy to the following.
- To calibrate, you MUST turn off ALL the lights in the darkroom, including the safelights. The only light is the enlarger.
- The slide switch for 1 and 2 are probably for either of 2 program settings #1 or #2, each is independent of the other.
- I think the * is to calibrate to the black dot on the right of the meter for max brightness (no negative in the enlarger) at a particular enlarger light setting (combination of height of the head above the baseboard and the f-stop the lens is set to).
- I think the delta symbol is to calibrate for a max density. If I remember correctly, you stack 6 pieces of 1 stop ND filter pieces over the sensor. Thus done, from the dot to the 0 on the dial is 6 stops of density. What I do not remember is the exact number of filter to stack, might be 5, 6 or 7. The max number should cover the contrast range for paper grade 0. If you don't have the ND filters, you can order a small sheet of 1 stop ND filter gel from B&H or Adorama.
- Then you remove the stack of filters, and one at a time; put a piece of ND filter over the sensor and mark the scale, then repeat till you have all 6 pieces on the sensor. The meter scale now shows a scale of 6 stops, and you can see where each stop of density is on the scale. So you can determine the density/contrast range of a negative.
To use it, to determine density range for paper grade
- you put the sensor under the clear part of the image.
- dial the f-stop on the lens so the meter aligns to the black dot
- put the sensor under the DARKEST part of the negative
- The meter will show you the density range of the negative in f-stops, which you can then match to the appropriate paper grade.
Caution, some negatives do not go from white to black. When printed they may go from white to gray or gray to black. In other words the negative does not have the full tonal range, because the actual image does not go from white to black. Then you have to use your head to determine what the proper paper is for that image.
To use it set exposure
1- First you need to make a good print, ideally on grade 2 paper. There is no way around that.
2- Then you set the meter to read a particular point of density on the film. When I do it, I meter the clearest spot (where the print will be BLACK).
3- Then you note the meter setting for that; it could be any place on the meter. As long as you are consistent in where it is. In fact you could use that light level to do the calibration above.
4- You have now calibrated the combination of enlarger+paper.
When you put a new negative in the enlarger, you put the sensor under a clear part of the negative and close/open the lens till the meter reading is the same as above in #3. If everything works as it should, you should be close to a good print. You will still have to fine tune the exposure or contrast to be what your eye wants.
If you change to a different paper or grade of paper, you still do the above, but you have to experiment to determine what the appropriate exposure TIME should be for that paper.
Example grade 2 might be 10 seconds, grade 3 might be 14 seconds, grade 4 might be 20 seconds.
As with calibration, when you use it, you MUST turn off ALL the lights in the darkroom, including the safelights. The only light that is on is the enlarger itself.
That is a very cool device, that I would not mind having. But with the cheap price of used color analysers, those are an easier to find option.
Last edited by ac12; 01-31-2013 at 11:09 PM. Click to view previous post history.
Thank you very much! I really appreciate the time and effort you took to write all of this out! It is very useful information, and should get me well underway. I have read that these devices used to come with a calibration negative, and your suggestion about the ND gel effectively explains how to replace it. I have ordered the ND gel, so it will be a few days before I can try out your suggestions.
One follow-up question if you do not mind: you suggested that I email roger and get the instruction manual. Please excuse my ignorance: who is roger?
As I recall, it wasn't a calibration negative, it was a piece of ND filter gel that you cut into the required number of pieces. So you are doing exactly what the original method specified. You can verify when you get a copy of the manual.
Roger is the person who has the HM device that you linked to above. His email is below the pix of the HM device.
His site shows a picture of the instruction manual, so if you can pay him to copy the manual for you, then you will have the manual.
You could also do an internet search for the manual. But some of those old manuals are really hard to find, especially when it is not a mainstream item. So when you do find them, you have to jump on it. When it sells, Roger won't be able to copy the manual for you. Personally, if I had the manual, I would scan it so it could be easily sent to anyone who needed it.
Now one thing to remember, the HM stuff were kits. You could ask him to just copy the owners manual. But I think it is a good idea to also have the assembly manual. Then you have it, in case you need to have someone repair it for you.
Last edited by ac12; 02-01-2013 at 11:35 AM. Click to view previous post history.
OK, now I get who Roger is! Yes, I will ask him as you suggest. (The first time I looked at his site, I misread it as saying that the page had been up since 2005, but that is in fact not correct. So he may still have the manual).
I have spent some time searching online for a copy of the manual, and I could not find one. There was an article in Camera magazine back in the 60s that may be relevant. Apparently the library where I live has it, and I will look for it this weekend. I may also be able to reverse engineer the right steps using the information you gave me. If I succeed, I will write it up and post it here "for future generations".
Thanks again for your help!
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That you can't find anything online makes it more important to get a copy of both manuals.
Just remember that what I wrote is from memory, from over 30 years ago.
I don't trust my memory that well, much less something in detail that long ago.
Were you able to get a copy of the instruction and assembly manuals?
Was my calibration instructions correct?
It would be nice if you could scan the manuals.
I would like a copy of the scanned manuals myself, since you got my curiosity up.
Apologies for the late answer, ac12, I have been very busy with work lately.
I was not able to find a full copy of the manual. I did find an article in Camera magazine, December 1967, that describes the operation and calibration of this meter. My local library happened to have a copy of this article. The calibration procedure relies on a calibration negative, but, as you suggested, the same result can be accomplished with ND gel. I obtained ND gel in different grades, then work got very busy, as I mentioned, and I have not been able to test the calibration procedure yet.
I can send you a pdf of the relevant pages.
I sent you a PM with my email address.
This is helpfull
And gives me a hint about the functions of my timer, too (since I used to have the simplest Hethkit timer without any measurement system whatsoever, when I last time made print at 80's). However, if somebody had instructions for Wallner Report ( http://www.ebay.com/itm/WALLNER-REPO...vip=true&rt=nc ) and could share me a copy, I would be very happy and grateful.