Thanks sparky, That really is a photocell, not a phototransistor or photodiode. I'm not sure how they get the speed they get out of it. Now I've got to try hooking one up to my test fixture and see how fast they are. i know I have one laying around somewhere. How far set back would you say it is?
I'm with you grahamp, checking consistancy may be most important. I think I have to ask myself what I'm trying to do with this gadget? I'm not trying to make a lab grade tester. Just a cheap handheld thing that you don't have to hook up to your laptop and run audacity to check your shutter.
If you look at my previous post with the old Voightlander.. it's supposed to be 1/100 sec , but it measures out closer to 1/150. That is the kind of thing I'd like to know before taking it out and burning through a roll of film.