relationship: zoom & focus???
 

I was kinda surprised to see in a instructional video about how to shoot great video, when using manual focus.....zoom in on the subject first, then adjust the focus and then zoom back out to compose the shot. I just sorta found this odd at first since zooming changes the focal length of the lens. Still being a laymen I have in my mind that when you zoom in/out the focus would have to be adjusted to. Can anyone elborate on this? I was just kinda curious about the relationship between Zoom and Focus when talking about a fixed subject.

If the lens is properly designed and calibrated, focus should stay the same throughout the zoom range. However, depth of field is very broad at the wide end of the zoom (distant and close objects will both be in focus), but at the telephoto end of the zoom depth of field is shallow. That's the reason for suggesting that you zoom all the way in, focus, then zoom out to the desired setting. You can't focus very accurately at wide zoom - and that doesn't matter a lot since you have so much depth of field. But if you focus at wide, then zoom in on something, the chances are you'll be out of focus.

Thanks Boyd, That helps. I don't know why I was so puzzled by this.