Focus is one of the most important elements of a good presentation, and one of the easiest mistakes for your audience to spot.
Starting a show in focus is both easy and fun: The first step is to grab a loop of SMPTE RP40 and run it with the lenses you're going to use for a show. Adjust focus as best you can using just your eyes. Now, get out your binoculars and look at all four corners and the center of the frame, and adjust the focus until there is no blurring of the line pairs located on the corners and in the center of the frame.
During a show, it is your job to monitor the focus as much as possible (while cleaning the projector gate between reels, threading the next reel, monitoring the sound, and keeping an eye on the film as it goes through the projector). Film can shift focus at a lab splice where two different stocks are joined together, from dirt buildup on the gate, from warping, or any other number of factors (including stupid stuff, like you forgetting to lock the lens in place), and a few minutes of an out of focus film feels like an eternity for your audience. Check focus with binoculars after the start of each reel, and periodically throughout the reel.
Focus on grain, not text or subtitles. Grain is the smallest element of projected film, and when the grain is in focus, the image is in focus.
Check focus with binoculars. Even if you have perfect vision, you cannot see as well as the person sitting in the first row. Binoculars will allow you to see better than they can.
- Test loops and starting in focus
- Adjusting lenses, etc.