When I did this I recall I flipped the lens of the pentax over and taped it in place. To read I handheld the meter and moved it up or down until the ragged edge of the aperture was in focus, ensuring a consistent height each time. That worked for me for relative values and determining film speed.
Similar to Nate & Tim, I used a cheap filter attached to the front of my Pentax with a diopter in place to narrow the field. I was fortunate to have Phil around when I tested it so he could correlate the readings with his densitometer. Naturally they were somewhat what different, but the difference was consistent enough that I could use the numbers in the BTZS programs for analysis. The whole concept of the testing is to get you in the ball park, not to spend your life testing... :-) That's an easy trap to fall into.
On the other hand, densitometers are cheap enough on ePay that you will eventually find one in your price range.
Good luck, enjoy the process of building & testing, but then go photograph.
mj
“You can’t have everything. Where would you put it?”
There is an article on apug that discusses this subject, [URL="http://www.apug.org/forums/forum216/34433-spot-meter-measurment-transmission-densities-r-eugene-smith.html"]
J. K.
Check out http://www.largeformatphotography.in...cles/VIDEC.pdf page 77
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