The real speed of tri-x in D-76 1+1 is 320. I scientifically measured it with a densitometer, and my findings are agreed to by just about everyone who has done proper testing. If you shoot at 400, you'll get slight underexposure. Does it matter? If you want the quality I get, then yes, it does. You'll get people here who will insist that the small underexposure won't be visible. People tell me all the time that they want pictures like mine, but they have hangups like insisting that film must be shot at the 'box speed'. The ISO speed is determined using a developer that no one uses in the real world and developing times that give too much contrast. Few films give the box speed in normal developers at normal developing times, but some of those who say that may have meters that overexpose slightly, so they may be getting 400 with their gear!
Now in my earlier post, I mentioned testing for yourself. Thats because meters can vary. If yours overexposes a little, you'll be ok at 400. If it underexposes a little, you'll get bad results since 400 on a properly calibrated meter is underexposing slightly. All of my handheld meters and in-camera meters are calibrated to match each other and are accurate. I'm a perfectionist and have gone to the trouble of having them all properly calibrated and doing scientific testing of films because I want my images to be as good as they can be from a technical standpoint so that I can concentrate on the subject and the image without worrying if the photo will be ruined by a technical fault like poor exposure. Your results may vary though depending on how accurate your meter is and how you meter. You might get best results at 400, or 200 or 320 or somewhere else!
