Hi folks, All this talk about calibration of leaf shutter speed on the 6x7 leaf shutter lenses reminds me of a question I was going to ask the group. Here's the question....
I recently obtained a used TTL prism finder for my 6x7. (Was sold as "meter inoperative", so the price was most excellent. Turns out that the meter circuit was perfectly fine -- I just needed to glue down a little reflecting optic inside the housing. But I digress....) I ran it through its paces with a 105mm f/2.4 lens and an evenly illuminated white test target, and I found that the exposure setting indicated by the meter is actually one stop underexposed -- compared with another exposure meter and two 35mm TTL metering bodies of known accuracy. My guess is that the metering cicuit of this used 6x7 finder was calibrated for a brighter screen than the one I have. Naturally, I'd like to calibrate the meter so that it gives the correct exposure with the standard screen in my 6x7. Seems like there are two alternative approaches to this, and I'm hoping someone in the group may have done this before. First approach would be to make a fine adjustment in the positioning of the shutter speed dial and/or the concentric, nested, ASA indicator wheel. There are a couple screws on the back of the ASA wheel of the finder, and a couple more on the inside of the shutter speed dial bushing-thingy of the finder. This is probably the "safest" way, but a bit tedious. Also, not sure if I can get a full stop of adjustment by loosening either pair of screws, rotating the corresponding wheel a bit, then retightening the screws. Second approach would be to adjust one (or more?) of the little potentiometers (or trimmers) on the metering circuit board, inside the housing of the prism finder. The question is, which one? I've got the cover off of the finder (the cover needs a touch-up paint job anyway), and I see the little metering circuit board glued to the left-hand side of the pentaprism itself. There are three little potentiometers on this board, lined up from the back (eyepiece end) to the front of the prism. My guess is that one of these probably sets a supply voltage for the metering circuit and should not be adjusted; another sets the "span" (or sensitivity) of the metering circuit -- that is, how far the meter deflects from center position if the light is changed by a stop, sort of like the slope of the calibration curve; and another sets the "zero" -- sort of like the intercept of the calibration curve. I think I want to change only the "zero", or intercept, adjustment. Has anyone done this before? Looks like only one of the potentiometers (the center one, IIRC) has a direct wire connection to the shutter speed/ASA potentiometer on the left side of the finder. I'm guessing that this is the one I want to adjust. Any help would be much appreciated. Thanks in advance. Bill Peifer Rochester, NY - This message is from the Pentax-Discuss Mail List. To unsubscribe, go to http://www.pdml.net and follow the directions. Don't forget to visit the Pentax Users' Gallery at http://pug.komkon.org .
