P3D Re: Shutter Testing (PHOTO-3D digest 3497)

[bobh@xxxxxxxxxxxxxxxxxx]

RE: John Goodman was playing with an electronic shutter tester..I 
suppose this is one that reads out a digital number for the speed. 
e.g. 0.010  for 1/100 sec. If it test focal plane shutters it might give 
readings at center and edges of frame as well. And he worries 
about scatter or 50% difference in exposure.. and muses "Do I 
have to repair my shutters or is exposure just a crapshoot?"
 Well, John, in your innocence you hit the nail on the head, if one 
knew all the factors contributing to exposure it is a crapshoot. But 
just the same we do remarkably well. Here are some things that 
the average person does see or know about:
	1. Shutter efficiency..essentially on a between the lens shutter 
that opens and closes like a diaphragm..it IS a diaphragm for part 
of the exposure if the lens is wide open. Oh a high shutter speed 
like 1/500 half the exposure may be at full f/stop and the rest 
dimmed as the shutter is opening and closing.
	2. Actual shutter speed is rarely as marked but often close. All 
one really cares is that it is consistant. One a curtain type focal 
plane shutter you can view the slit width with back open and 
tripping with no lens while looking a a bright TV screen. Each 
speed should halve the slit width as you increase speed.
	3. 1 & 2 even if perfect would have to face the fact that the 
diaphagm markings may not be perfect and only apply to infinity. 
So closeup or bellows extension changes real f/stop. (Behind the 
lens meters in camera compensate, but external meter can be 
fooled.
	4. If using meter cell linearity, age, battery condition and 
calibration affect both internal and external meters which if 
calibrated correctly only give the correct exposure with a scene 
that appoximates 18% gray reflectance. (snow and black cats 
need compensation.)
  	5. the ONE shutter speed  and ONE f/stop you can finally 
select hopefully allow the brightness of the scene that exceeds the 
film latitude to be placed somewhere on the 'straightline' part of the 
H&D curve so that the important part of the scene will be rendered 
well.
    So with the above one should not fuss about minor variations in 
shutter speed..One can improve things by using test film to find a 
best film speed setting for a meter internal or external. With DX 
settings that could mean an offset with the +/- control. 
BobH
 
Bob Howard (bobh@xxxxxxxxxxxxxxxxxx)
SEND NO PIX OR PROGRAMS TO LIGHTSIDE!!
 Instead send those to:
robhoward@xxxxxxxxxxx  or   bobh626@xxxxxxxxxxx