how to focus for a very sharp image

[nosw]

Hi friends,

I just wonder what is the best way to focus in order to get the sharpest image possible. I always set the autofocus point in the middle, press half-way to lock focus, then recompose. However, I was told that this technique can move a focal plane a bit resulting in the image not sharp as it could, especially with a very shallow DOF and large image sensor of DSLR. A DPS post 3 Ways to get Better Control of Autofocus suggested me using back-button focus (technique #3) but I think this technique also suffers the same focal plane change as in the press half-way and recompose technique (correct me if I'm wrong). I know that I can move my autofocus point to the subject and not in the middle, but this is quite cumbersome as I need to move the focus point manually for every shot. Manual focus is out of the question for me as I personally don't like it. Do you guys have any suggestion on how to focus for a very sharp image?

Thanks,
Ben

[jli]

Yeah, it's tough. I pretty much exclusively use the recompose technique, but it definitely throws the focus off when you're really close to the subject and shooting with a big aperture.

One thing I thought of is to recompose by moving your body and camera parallel to the focal plane (so the position changes but the angle doesn't). If I'm not mistaken, this will still change the distance, but not as much as changing the angle of the camera. I sort of try to do this sometimes, but it's much slower and more cumbersome than angling the camera. And I'm not totally sure if it works or not .

Using a smaller aperture of course makes it less important to get the focus exactly correct.

And finally, there's manual focus. I find it really hard to focus manually while looking through a small viewfinder. My dad's old Canon film SLR has this sort of "fuzzy" texture in the middle of the viewfinder, and as you focus, the textured part gets sharper or fuzzier, so it's much easier to tell when something's in focus. I really wish my 1000D/Rebel XS had this, but I think I read that it makes the viewfinder dimmer.

I agree that the back button focus doesn't help with the distance change caused by recomposing.

I believe most people just bump up their aperture enough so that recomposing doesn't affect the sharpness of the areas they care about. Someone correct me if I'm wrong!

[OsmosisStudios]

Originally Posted by jli

Yeah, it's tough. I pretty much exclusively use the recompose technique, but it definitely throws the focus off when you're really close to the subject and shooting with a big aperture.

This is true with VERY close distances and VERY large aperture. f/1.8 at minimum focus distance, for example. That generally means that getting focus is hard enough as it is so as to make it not worth it. Most people also dont shoot at f/1.8

You can also turn the camera around the nodal point, like a pano, and not have any problems.

[vandergus]

You can also turn the camera around the nodal point, like a pano, and not have any problems.

This doesn't have anything to do with error associated with focus and recompose. For an ideal lens, the area of focus is a plane. Any sort of rotation moves the subject out of the plane of focus. You would have to use a straight translation during recomposition to keep the subject in the plane of focus and minimize errors. Of course with a real lens that won't work either because they don't have perfectly planar planes of focus. They're usually curved, more so at large apertures, sometimes toward the camera, sometimes away from the camera. So who knows what sort of camera movement you would have to use during recomposition to keep the subject in the plane of focus.

But, yeah, it usually doesn't matter unless you're really close to the subject.