Spot Meter Calibration

[William Lindley]

I am interested in your ideas concernig calibration of a spot meter. When I firs t started in large format, I assembled the best equipment I could aford, and par t of that was a spot meter from Adorama. I believe it is manufactured by Capitol , and seems to function well. I have only used the meter in my SLR previous to t his. I never calibrated my SLRs meter, and it seems to work well just as it is c alibrated by the manufacturer. In my reading about large format, I am encounteri ng much about calibrating meters, and indeed, this seems to be the thing to do i f one is going to follow the zone system. I attempt to follow a kind of modified zone syestem for color transparency film. I have had good succes with this in 3 5mm, but now want to be able to refine my exposers, as to not wast film. I would like to hear how you have tested and calibrated your meters. I have compared my meter based on the "sunny 16" rule, and found (to my delite and suprise) that i t was spot on, (if you will excuse the pun). I have found that in compareing it to my in-camera meter, it varies a good deal. But, of course my in-camera meter is a center weighted and not a spot meter.

Thanks, Bill

[N Dhananjay]

Probably the easiest way to callibrate is to use a Kodak grey card and compare different meters' readings. Place the grey card in uniform illumination. Put the longest lens you own on your 35mm camera. Fill the frame of your viewfinder with the grey card. Needless to say, do not cast a shadow on the grey card. Focus at infinity to avoid light loss due to lens extension (doesn't matter if the grey card is not in focus). Check the reading. Repeat with your spot meter with the spot meter in exactly the same position as your 35mm camera. The readings should match. Try to keep the light level on the grey card such that you get a reading at about the middle of the scale (oftentimes meters lose some sensitivity at very dim/bright light levels.

Finally, shoot some pictures and see if you're happy with the results. Don't be surprised if you find that you prefer slightly different exposures for transparencies than for negatives - many people like to slightly overexpose negatives to ensure good shadow detail or slightly underexpose transparencies to prevent blowing out the highlights. Good luck. DJ

[doc]

Hi Bill, if your spot meter is like mine, i think the only calibration possible (other than the zero adjustment) would be to send it into a laboratory that does that kind of thing, and the fine tuning would be accomplished by an electrical technician tinkering with its circuts; i could be wrong. but, if you mean by calabrating, synchronizing your LF camera's shutter/exposure to your light meter, and that so you can pick and chose your subject's shade of grey, then you might check The New Zone System Manual by White, Zakia, and Lorenz. Because, as it seems to me, sometimes calibration is used by some of these folks to mean some kind of harmonious relationship between the meter, the particular shutter, and the unique way to develop a particular negative.

[Bruce M. Herman]

Bill,

I think that you have already determined how to calibrate your meter - by using it with your camera and examining your results on a light table. If they are what you expect and want, then the meter is fine. I suppose that there could be some nonlinear response under very low light levels, but in that case you also have to worry about reciprocity failure in your film, and more importantly, the look that you're trying to impart. Take a look at John Shaw's book, "The Nature Photographer's Complete Guide to Professional Field Techniques". It's a 1984 book for 35 mm camera users, but is filled with lots of down to earth information like calibrating your meter by doing just what you did.

I work almost exclusively with color transparency film. Here's a tip that I learned from Pat O'Hara to help you relax a bit when you're unsure about the exposure. Make at least two exposures of each composition, and expose them in the same manner. In other words, if the light is unchanging, all are made with the same exposure. But if the light is changing, determine the exposure identically for each sheet. That may mean metering the same spot in the composition, or averaging the same two or three spots. Then, develop only one sheet for each composition. You can determine how much to push or pull the other sheets for that composition based on the results from the first. This method has the added advantage that if one sheet has a bad dust problem, or a bird flies across your field of view while you're exposing, you'll have some backups. Believe me, this method is guaranteed to lower your blood pressure.

Enjoy, Bruce