halving ISO speed?

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I've recently begun shooting b+w film. I read in a book, Perfect Exposure, that by halving the ISO of a film, the negatives would be more accurately exposed. This was suggested for cameras with reflected light meters. Why? Also, assuming that this is true, would it still be recommended to spot meter off of the shadows and give two or three stops less exposure. Would I be messing around with the exposure too much?

-- Ryan Fox (rfox90@hotmail.com), September 29, 1999


Ryan-- I am especially fond of Tri-X shot at 200. I used to use Microdol-X at 1:3 for 10-1/2 minutes (75 degrees F). Lately I am using diluted Xtol (either 1:2 or 1:3), and I have found that the Xtol times on the Kodak website for Tri-X at 200 give very fine negatives. When reading with a thru-the-lens meter on my Canon, I make a point of comparing shadow values and highlight values and setting the exposure so that it is adequate to give shadow detail -- much like you describe.

-- Sam (sselkind@home.com), September 29, 1999.

>> ...by halving the ISO of a film, the negatives would be more accurately exposed.

Rubbish. It is important to find your own EI, and this might happen to be one stop slower than the ISO speed, or one-third or a stop, or even perhaps faster. Once you have found the correct EI, you should stick with it.

The only merit in always using half this EI is that you will then err on the side of overexposure. This is better than underexposing, but it is even better to get the correct exposure.

-- Alan Gibson (Alan.Gibson@technologist.com), September 30, 1999.

Are you getting sufficient shodow detail? You need to adjust your personal EI to your equipment and metering style. Overexposing traditional B&W film just increases grain and reduces sharpness.

-- Tim Brown (brownt@ase.com), September 30, 1999.

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