Neutral density filters question

[ryanmills]

how accurate are Neutral density filter ratings? Going to toy with a 3 stop cheap hoya tomorrow. Not going to get a chance to test before I shoot a bunch through it. Is it safe to just set my meter 3 stop slower or do I need to add a little more than that?

[polyglot]

The ones I've used are accurate to better than 1/3 stop, I.e my camera meter read exactly 3 stops slower. Reciprocity failure would be the main thing to worry about IMHO.

[ryanmills]

The ones I've used are accurate to better than 1/3 stop, I.e my camera meter read exactly 3 stops slower. Reciprocity failure would be the main thing to worry about IMHO.

Thanks I will keep it in mind, im just using to slow down tri-x 320 in the sun.

[Leigh]

It depends entirely on the quality of the filter you buy.

Professional-grade filters like B+W and Hoya should be spot on, as would some of the cinematographic filters.

Good consumer-grade filters will likely be within 1/3 stop.

Chinese filters will be kinda dark, maybe, sort of, depending on what part of the filter you look at.

- Leigh

[Cletus]

As long as you are metering through the filter itself before you mount it on the lens and take the picture, it won't matter if its "precisely" 3 stops or whatever. There could be some slight deviation depending on the color and/or reflectance of the subject you're metering but you're basically safe if you meter through the filter rather than blindly relying on the filter factor.