Fri 14-Sep-12 09:29 PM | edited Sat 15-Sep-12 02:39 AM by avm247
I read the Strobist.com com article, and while I haven't really digested it, it appears upfront to discussing equivalent stops in exposure: 1/125s is 1 stop slower than 1/250s.
Likewise, ISO 200 is one stop faster than ISO 100.
Maybe I'm old school but John Shaw's Macro and Closeup Photography discussed the ability to adjust correct flash exposure (using film) by knowing the GN, flash to subject distance and equivalent exposures (adjusting f/stop and shutter speed. Likewise, one can also speed up or slow down exposure by adjusting ISO (its not like there is a film cartridge to rewind, respool, pull the leader out, mark the canister...yeah, fun times! ) Now its press a button and rotate a wheel. (I just need to remember to switch it back! )
I'm not sure I agree with the strobist.com argument, again, most DSLRs had a base ISO of 200; most GN are at ISO 100 for flash. Doing the math, at a given ISO, you might find that the 1/200s (1/3 stop slower than 1/250s isn't enough).
If you need to slow down the shutter speed to get to 1/200s because your meter is indicating overexposure (used to be read with HI in the shutter speed), you could stop down the aperture, decrease the speed, or reduce the amount of light entering the lens with an ND filter or even a Circular Polarizer. At least this is what I did with film, when I had say ISO 800 or even ISO 400 loaded.
I think this would translate the same way.
If I'm wrong, by all means correct me. (Been wrong before, won't be the last time, either, I'm sure.