400 rated at 320? probably a stupid question?

this is such a silly question, but I'm confused...

 

I've seen lots of posts on 400 speed film "rated at 320" - what does

this get me? and how do I process it?

 

such a rookie! thanks!

:what does this get me?

 

Depends...if shooting slide film it may make the difference b/w dead on exposures and just a little under-exposed. Sometimes manufacturers rate film at one speed (say ISO400) and after testing in the "real world" most shooters find that they get better saturation, or better exposure when they set the ISO manually to 320--essentially over exposing the film by 1/3rd stop. Likewise others rate film 2/3rd stop slower. Just depends on personal preference, how the film responds with your equipment, etc. Don't go out and start setting your camera to ISO 320 for every ISO400 film you shoot. You should do some structured testing to see where your preferences are with your gear.

 

and how do I process it?

You process normally.

 

BTW, Some pros will expose print film at different ISOs as well. This is getting pretty nit-picky, but sometimes for critical work it just helps make sure the highlights and shadows are exactly where you expect them exposure wise. But in most cases it is much less critical with print film.

Evan did not make perfectly clear that you have to strictly distinguish between print and slide films.

 

For all what I'm saying here development is normal.

 

Exposing a 400 ISO print film as if it were a 320 ISO film (=400 rated as 320) results in 1/3 stop more exposure and a slightly more dense negative. There are several reasons to do this, probably the most important is that C-41 print films (your everyday vanilla film) HATE underexposure, which makes them grainy and looses image detail esp. in the shadows, whereas slightly overexposure won't blow out the highlights most of the time, give you maybe a little mor shadow detail, is not harder to print and you're on the save side. Some print films really kick the bucket with 1 stop underexposure, others are more tolerant. They all tolerate 1 stop over (and maybe more) and you won't even see the difference in the prints most of the time.

 

Exactly the reverse is true for slide film, they *HATE* overexposure and tolerate underexposure better, but to a much smaller extent in both directions than print film. This is due to their higher contrast = smaller dynamic range = smaller latitude and higher maximum density (Dmax), so exposure has to be much more precise than with print film.

Keep in mind, too, that if you're testing with a particular camera or meter, you may make such an adjustment simply because your meter is a smidgen off.