Hi James.
It is hard to get focus right if you are pretty wide open, like between f/1.8 and f/2.8. How wide are you when having trouble?
If you are shooting a person, you need to pick the eye closest to camera and put the reticle over that eye. If you are shooting 2 or more people you need to shoot stopped down to get a wider depth of field, or else you won't get everyone in focus.
Why not do a simple test, though? Set a cardboard box on a flat surface and poke a pencil into the side of it, near the front. Then lean a ruler onto the pencil, touching the box, so the bottom of the ruler is ahead of the box, and its top is behind the front of the box. Set the aperture wide open to f/1.8 to get the narrowest DOF. Shine a couple of lights onto the box and ruler because you need a lot of light. Focus on the front of the box, but close to where the ruler is. Shoot straight at the front of the box, not at an angle, and not above or below the box. Shoot at short, medium and long distance, and Take multiple shots at each distance. Using tripod or resting camera on something helps. If the box does not have writing on it near the ruler scribble on it or tape newspaper tight onto it so the camera has detail on which it can lock focus.
Look at the ruler in the picture, blown up for detail. Where on the ruler are the hash marks in focus? If it is in front or behind the box, you are front focusing or back focusing.
Unfortunately Rebels do not have autofocus micro adjustment, or you could just set the camera to compensate. You could send camera and lens in to Canon to have them calibrate if it is worth it.
Scott
Canon 5d mk 4, Canon 6D, EF 70-200mm L f/2.8 IS mk2; EF 16-35 f/2.8 L mk. III; Sigma 35mm f/1.4 "Art" EF 100mm f/2.8L Macro; EF 85mm f/1.8; EF 1.4x extender mk. 3; EF 24-105 f/4 L; EF-S 17-55mm f/2.8 IS; 3x Phottix Mitros+ speedlites
Why do so many people say "FER-tographer"? Do they take "fertographs"?
