Jeff Adler , Feb 03, 2008; 10:30 p.m.
ISO by itself does not cause grain. If I use Tri-X in a medium format camera and make an 8X10 I can have a print with very little grain. Higher speed films tend to have more grain because the particles of silver which capture the image are larger. Many things can affect grain and some things which people think affect grain do not. Overexposing the film can increase grain. Overdeveloping the film either with time or too much agitation will also increase grain. Grain tends to be more apparent in gray areas of a photo. It is less apparent in areas which are black or white. Underexposing film can also make grain more apparent. Low light conditions do not by themselves increase grain.
When light is very low it is more difficult to get the correct exposure. An area in the scene which is brighter than other parts can fool the camera's meter by inflating the reading. In very low light reciprocity failure can also be a factor in getting an incorrect exposure. It's the underexposure which causes the grain to be more apparent. Some developers also cause grain to be more apparent. This is especially true of Rodinal and similar developers. HP5+ is a nice film but it is not as fine grained as Tri-X or Neopan 400. The new T-MAX 400 is supposed to have even finer grain than Tri-X. I have some in 120 size but I have not tested it yet.
If your exposure is correct and your developing time is correct you will minimize the apparent graininess of the final image. The last link in the chain is scanning the negatives or printing them optically. Scanning can give modest improvement in apparent graininess or can make thngs a lot worse. Printing on different paper grades or on variable contrast paper with different filters will change both the contrast range and the apparent graininess of the final image.