Quote Originally Posted by keithwms View Post
For negs I usually treat 3.5 as 4, so as to err on the side of slight overexposure. For slide, I treat 3.5 as 2.8, so as to err on the side of slight underexposure. You can of course also change the EI just a bit to compensate, as needed.

Don't sweat the petty details, and don't pet the sweaty details.
But then you're overexposing neg film by 1/3 stop, which you will never notice, and underexposing slide film by 2/3s stop, which you WILL notice.

For neg film I too pretty much just treat it as f/4. If I'm shooting slide film hand held, I'm doing it in 35mm and have TTL metering anyway. If I'm shooting slide film in medium format I am probably working off a tripod or in decent light and won't be shooting my one medium format 3.5 lens (on my Yashicamat) wide open anyway. And in 4x5 I don't have any lenses that fast anyway. So for practical purposes it's a non-issue.

Even slide film can generally handle 1/3 stop overexposure though it will change the look, but if you treat it as half way between 2.8 and 4 you will be only 1/6th stop off and even in slide film you won't notice that.