Shutter speeds on modern cameras do not coincide to the film ISO, but we seen to "pretend" they do.
Film speed (ASA) equals the reciprocal of the "correct" shutter speed for exposure in normal daylight at f/16, i.e. ASA 100 = 1/100 sec. Whether or not such a shutter speed is available on a given camera is totally unrelated to the film speed.
The exposure error resulting from using 1/125 sec shutter for ASA 100 film in daylight is trivial.
Also, a shutter speed marked 1/125 may actually deliver 1/100*.
At higher speeds, mechanical shutters have a speed tolerance of +/- 30%.
Apertures have no specified accuracy that I know of.
These are some of the reasons why two people using identical equipment may get different results.
- Leigh
*NB: There are a couple of reasons for this.
1) Operator convenience.
Shutter speeds should progress by a factor of 2.
But depending on whether you start at 1 second or 1/1000 second, the numbers are different:
1, /2, /4, /8, /16, /32, /64, /128, /256, /512, /1024, or
/1000, /500, /250, /125, /62.5, /31.25, /15.625, /7.8, /3.9, /1.95, /1
A combination of the two series was deemed to be most "user friendly".
2) Advertising/marketing hype.
Some cameras could do flash sync only at a speed slower than the advertised/marked speed.
E.g. marking a sync speed as 1/125 when it was actually 1/100 could be good advertising.