Ill have to check into that.
edit: I feel dumb! I just tried that by focusing in on an object center mass in my frame, then while continuing to hold the af lock i moved that object to the left side of my frame and it remained focused!
Would anything cause the AF lock to unfocus? Say if you move the focus point into lower lighting in the frame etc?
In good lighting you can use all your AF points... cross or not, they will all work well in good light.
Only in dimly lit situations will you need to focus and recompose using the centre-point.
The only times focus and recompose can be a problem are in some edge cases...
- If you're shooting at an incredibly shallow DoF like f/1.2-1.4 where moving the camera even slightly can move the focal plane enough to put your intended subject out of focus. In those kinds of situations I would suggest not recomposing... use your centre point and then crop in post if the composition is not to your liking.
- Sometimes it's possible to dramatically change the lighting/exposure when you recompose. Most people have AE lock and AF lock tied together on a half-press of the shutter for convenience. Using this button to focus and recompose will also lock your exposure to the initial scene, not the final one. You can get around this by using back-button focus "*-button" to lock focus separate from exposure, but I've never bothered with this technique and it hasn't really caused an issue.
As for cropping, don't worry about it. There's no do's and don'ts when it comes to cropping. Obviously, getting it right in the camera provides you with the maximum resolution image but for images that get posted on the web, this is hardly a big constraint. If you're shooting for a magazine or Times Square billboard, then it's more important