Film speed

Good evening,

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Is there a formula that generates the speeds betwwen 50 ASA and 100 ASA and 200 ASA and 400 ASA and...? I mean how do we get 64 ASA or 320 ASA...

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Many Thanks

12,20,25,32,40,50,64,80,125,160,250,320,400,500,640,800 Those are the

numbers. What's the pattern?

Doubling the iso doiubles the sensitivity (1 stop), so 1.414 times the

base should be 1/2 a stop, which would be 50, 70, 100, 140, 200, 280,

400.... The actual steps, (somewhat set by tradition as well) are

closer to 1/3 of a stop (three jumps gets you to a doubled speed), and

thus the cube root of 2 is the multiple (just under 1.26 by my K&E

slide rule -- honest -- but hte factors seem just under 1.25, for

convenience?). You won't find these absolutely precise, but be

cognizant every M before the M7 has a slop factor of at least 1/10 of

a stop for shutter speeds (i've got a well calibrated M6 and

everything is within 5%, save the 1/1000th), and given the vagaries of

metering, precision worrying over the difference between 1/3 and 1/2

stop is questionable. Whne people shoot Tri_x at 200 or 320, thety are

biasing their exposures to this end, but not guaranteeing precise

exposure for any particular shadow (like large format zone-system,

people may try for).

(To L Smith) Have you checked your M7 yet for shutter accuracy? Did

you have your M6 calibrated, or did you just get lucky and buy an

accurate one? The slow speeds of my M4-P are quite accurate, but

from 1/250 & faster, they're way off and came back from Leica CLA

that way. (1/250= 1/200; 1/500= 1/350; 1/1000= 1/550)

What L. Smith said. The mutiplier comes out near 1.26 on my Pickett

& Eckel, as well.

just lucky, i think, my second m6 (newer) was more typical, but when

I had it sent in for an unrelated issue, I had them tweak it as well.

I understand 1/1000th is typically left a little slow for wear

purposes. I don't see how anyone can really tell an exposure

difference unless two cameras were off significantly in opposite

directions, and mostly M errors are in the same direction -- a little

slow -- 5-15%, your case, 20%?. Even a 20% error is a fraction of a

stop.

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don't have, can't afford, don't know if I would buy an M7 (give me,

trade me, you bet). But, any electronically controlled shutter has

the potential for more reproducible performance, and I as read the M7

hoopla it is indeed more accurate. Frankly, I don't think I care that

much.