- -----Original Message-----
From: Chuck Norcutt <norcutt@xxxxxxx>
To: Olympus mail list <olympus@xxxxxxxxxxxxxxx>
Date: Monday, June 21, 1999 7:00 PM
Subject: Re: [OM] Sunny 16?
snip
>
>I'll go just a bit further and assert that this is actually where our
>film speed ratings come from. The ISO rating is the inverse of the
>correct shutter speed using the "sunny 16" rule.
.... I think this is putting the cart before the horse....
> An ISO 400 speed film
>will yield correct exposure under bright sun at f/16 with a shutter
>speed of 1/400 second. Fortunately, the ISO rating is based on the old
>ASA standard and not the DIN standard. The DIN number was "just a
>number". It didn't have a useful meaning like the ASA/ISO value. Now
>the young whippersnappers will have to ask: "What's DIN?" "What's ASA?"
- the ASA system is an arithmetical progression based on the gradient of a
film's charasteristic curve at a point near the foot of the curve (the speed
point - determined at a constant density value just above base+fog ); the
DIN system calculates the speed in precisely the same manner, but records
differences in a logarithmic progression; at least, that's how I understand
it <donning flame-proof suit, just in case...>
The ISO system didn't do away with either system, but combined them, eg
100/21º. Convention now means the first half is often used alone.
IMHO this is a pity, because the DIN system gives an (apparently) more
consistent and comprehensible set of values than ASA - particularly with 1/3
stops, the generally accepted professional standard.
By way of example, Kodak's hype that it's Tmax 3200 film can be rated at ASA
50 000, is just a three stop push, from 36 to 45 DIN, always 9 numbers,
whereas if you push an ASA100 film 3 stops, you get ASA 800 (21 to 30 DIN) -
so the relationships look skewed with ASA values, unless you think in
multiplications :-)
Sorry for my long-windedness (an probably my lack of clarity)...
Cheers
Andy
< This message was delivered via the Olympus Mailing List >
< For questions, mailto:owner-olympus@xxxxxxxxxxxxxxx >
< Web Page: http://Zuiko.sls.bc.ca/swright/olympuslist.html >
|