200,100iso exposure q.

rmphoto · Jan 18, 2005

just a question thats been annoying me lately,

if someone doing long exposures was using 200iso, and the person beside him using 100iso, would 100iso's exposures have to be 50% longer to achieve the same lighting conditions as the 200iso's?

ugh..

Unimaxium · Jan 18, 2005

I believe in theory the 100iso user would have to use 2x the exposure time (or I suppose 2x the aperture size), which is the equivalent of 1 stop, so technically it would be 100% longer. However, on long exposures with film, I think you get into the issue of reciprocity failure, which is something I don't fully understand, but have read about in various places (you can google it and read webpages about it).

ksmattfish · Jan 18, 2005

The simple answer is that if they were both using the same aperture, and identical performing equipment, then the exposure time for the photographer using ISO 100 would be twice as long as that of the photographer using ISO 200. The not so simple reality is that there are a lot of variables that can affect this.

Another thing to consider is reciprocity failure, which typically occurs with exposures longer than 1 sec or so. For exposures longer than 1 sec you need to check out what the manufacturer says, because one ISO 200 film may be significantly faster than another at long exposures. For instance, old Tmax 100 (I don't know about the latest incarnation) was actually faster than Tmax 400 when shooting long exposures.