Today’s Question: To achieve focus is zooming in on “Live View” and focusing better than zooming in with the lens itself to focus?
Tim’s Quick Answer: With most zoom lenses if you change the zoom setting you do not maintain focus. So if you zoom in to set focus, then zoom out again, the image would no longer be in focus. When zooming in using the Live View display rather than the lens itself, you don’t have to worry about this issue.
More Detail: There are some zoom lenses that will maintain focus when you zoom. Those lenses are referred to as being “parfocal”, but they are in the minority of zoom lenses. Note that in professional video production it is indeed a common practice to zoom in with the lens to establish focus, and then zoom out again. But that only works with zoom lenses designed to maintain focus when the zoom setting is changed.
If you know that one of the zoom lenses you are using is parfocal, you can certainly zoom in using the lens to establish focus. When in doubt it is best to leave the zoom setting on the lens fixed, and zoom in with the Live View display to establish manual focus (or to confirm accurate focus). And frankly, I would feel completely comfortable trusting that a lens that is parfocal will really perform such that there isn’t any change in focus when the zoom setting is changed, so to me Live View is always the best approach.
It is worth noting that the Live View display will also generally enable an exposure simulation, so that even when the scene is relatively dark you’ll be able to see a reasonable (though possibly noisy) preview on the camera’s LCD display. And keep in mind that if you enable the depth of field preview on your camera (if so equipped), the combination of that depth of field preview and the exposure preview can be tremendously helpful for evaluating your overall camera settings.