One note, most cameras auto focus on the center of the image because that’s where most know-nots put their point of focus. If you know about the rule of thirds or have ever composed a decent looking shot, you should know that the center is rarely where you want your main focus to be.
“however to try and blur the background a bit so the actors stand out more, it seems like this is the only way to do it. “
Manual and auto-focus don’t really deal with this issue; it’s depth of field you’re concerned about. If you haven’t looked at it, check out the “Filmmaker’s Handbook” by Edward Pincus (
http://www.amazon.com/exec/obidos/t...f=sr_1_5/103-2197005-4606215?v=glance&s=books) I would recommend it. There’s a newer version of it (
http://www.amazon.com/exec/obidos/A...8731786/sr=2-1/ref=sr_2_1/103-2197005-4606215) but I haven’t read it. The original deals almost exclusively with celluloid film, but it has very very good pictures and demonstrations of how zoom, lighting, and depth of field affects picture composition and other theory that’s just as applicable for video as film. I think it’s worth it just for the pictures and when I was at on Amazon someone was selling it used for only $.88. Anyway, depth of field is the area in front of and behind the plane of critical focus that is still in acceptable focus. I’m trying to do this from memory so I apologize if I get confused because I don’t use this knowledge that often. There are 2 ways to adjust depth of field: distance to subject and iris. The best way to affect depth of field is to change the camera (focal plane) distance from the subject. If the subject is far enough from the camera you will reach the point on the focus ring that has the infinity mark. This means that from that point on everything is in focus. As you get physically closer to the subject (because zoom does not affect depth of field) you will start to have to adjust focus to find optimal focus. The closer you get to a subject, the shorter the depth of field. Most consumer lenses have a minimum distance of 2 to 3 feet (professional lenses are different in several ways). So if you are as close as you can get to the subject but still be in focus your depth of field will be as small as possible and your background will look as out of focus as possible.
Adjusting the iris will affect your depth of field. This is an inverse relationship, so the larger the aperture the, the smaller the depth of field and vice versa. There are some extremely good drawings about this in the book above also. So if you can’t get closer to the subject, reduce the lights and open the iris.
Now, that being said, most video lenses are poor quality and aren’t as accurate as professional video mean they have a larger depth of field. A Panavision 35mm prime can have a subject’s eyeball in focus but the tip of the nose and the ears will have soft focus. You just can’t do this with a cheap lens (by cheap I mean less than about $4000). Consumer quality lenses have an almost infinite depth of field except at extremely close ranges. That’s about all I have to say about that.