Silly question about apertures...a bit confused!

[sugar_babygirli]

So I know that the larger the aperture, the better you can shoot in low light, but also notice that the more you open it up, the less your depth of field is (which is why you'd use, say, f/22 for a landscape to have everything in focus)

So my question is, if I were to use f/1.8 to shoot a full body shot of a person about 30 feet away, would they be completely out of focus since the range is so far away? I notice the DOF is very shallow when using it to shoot very close objects, (such as a single flower) but does it still apply to full body shots far away?

See this full body photo: http://www.photosig.com/go/photos/view?id=1452190&forward=browse

And this face shot:
http://www.photosig.com/go/photos/view?id=1457096&forward=browse


Notice how they are both shot at the same aperture, but the on the first, all the body is in focus and the second, mainly the eyes? Is it because the closer you are to the subject, the more it tends to focus on only a small spot?

I have NO clue if this makes any sense! :rolleyes:

[Fills]

Yes depth of field is affected by distance. If you are using a DSLR with a 1.6 factor and I'm assuming you are using the 50mm, at 30' your DOF is 7' while at 3' it is only a few inches. Play around with a dof calculator such as here: http://www.dofmaster.com/dofjs.html

[sugar_babygirli]

Thanks I'll look at the link! :) I have the 85mm 1.8 actually with the 20 D.

[k2kv]

Sugar_baby -

To answer your question specifically, your point of focus will always be sharp, regardless of the distance. Depth of field does not affect point of focus, only the area in front of, or in back of the subject.

If you shoot a vertical body at thirty feet, the entire body should be in focus, as there is little front-to-back difference, and depth of field will not be so shallow at thirty feet unless you are using a very long lens (in which case, thirty feet might be considered rather close). You are correct in that the closer you are to your subject, the narrower the depth of field (all other factors being equal).

Hope that helps,

Jeff

[robertwgross]

Sometimes it can be a problem to shoot a full body shot at thirty feet. If you have a wide open aperture, then the depth of field is very narrow. On some lenses, it might be so narrow that you have to hit the center of the subject perfectly to get them all in focus. Then the other problem is that if you budge the autofocus spot just slightly off, it might find a new autofocus point behind the subject, which might put the subject completely out. So, you have alternatives. You might want to close down the aperture at least a stop or two, which will provide a deeper depth of field. Unfortunately, that will leave your background less blurred, which is not exactly what you had in mind, maybe. By moving the subject forward from the background, you can get that effect back again.

---Bob Gross---