Film speed

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Good evening,

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...

Many Thanks

-- Arie Haziza (nhaziza@northrock.bm), May 07, 2002

Answers

12,20,25,32,40,50,64,80,125,160,250,320,400,500,640,800 Those are the numbers. What's the pattern?

-- Michael Darnton (mdarnton@hotmail.com), May 07, 2002.

What no iso 16?

-- Charles (cbarcellona@telocity.com), May 07, 2002.

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).

-- L Smith (lacsmith@bellsouth.net), May 07, 2002.

(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)

-- Frank Horn (owlhoot45@hotmail.com), May 07, 2002.

What L. Smith said. The mutiplier comes out near 1.26 on my Pickett & Eckel, as well.

-- Bob Fleischman (RFXMAIL@prodigy.net), May 08, 2002.


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.

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.

-- l smith (lacsmith@bellsouth.net), May 08, 2002.


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