As I understand it, 'focus and recompose' is often used to ensure the subject is in focus and at the same time composition is right. It has been estimated that some lenses are best in terms of image quality at the center and soft/blurry towards the edges at wide open apertures. When I am using these lenses if I focus and recompose a portrait at a lens's lowest aperture (not necessarily the sweetest aperture value) and put the person to one side will the portrait be sharper or softer in this case?
The problem with lenses wide open (gross generalization coming) is softness moving toward the corners - and this will be there no matter the exactness of your focus.
The problem with focus and recompose is that it’s very easy to minutely change the distance to which you are focusing...not that you’re actually refocusing but that by changing the camera angle, the distance to your new subject is minutely different.
The problems compound when using a lens at a large aperture where DoF is mere millimeters anyway (now your shot is OOF from sloppy recompose technique) and compounded by putting the subject in a soft portion of the lens.
All that being said, I’ve missed more shots to OOF than lens sharpness. Of the two, that’s the more critical issue in my book.
Depends at least on the depth of field.
For example, if you have 85mm f/1.2 on a full frame camera, and do a head&shoulders portrait (distance: 1.65 meters), the depth of field is 12.3mm in front of the focal plane and 12.5mm in rear of the focal plane.
What are the chances that the camera moves so that the subject is no longer close enough to the focal plane? I'd say pretty high, even though I don't have a full frame camera or a 85mm f/1.2 lens.
Use the right tool for the job. Your camera probably has several autofocus points, even though in some cases the center one is the most accurate.
On the other hand, 135mm f/2.8 head&shoulders portrait on a Canon 1.6x crop sensor body (distance: 4.26 meters) has 48mm DoF in front of the focal plane and 49.2mm DoF in rear of it. I'd say in this case the danger of subject no longer being in perfect focus is less significant.
There are several factors that all combine to determine what effect, if any, "focus and recompose" will have on a particular image.
- The depth of field as determined by aperture and magnification. Magnification includes things such as subject size, subject distance, lens focal length, sensor/film size, intended display size and viewing distance, etc.
- The absolute acutance ("sharpness") of the lens. If a lens is already fairly soft at the focused subject distance before recomposing, it won't have near the same effect as if a lens is razor sharp at the focused distance.
- The design (and actual construction/alignment) of the lens with regard to how curved or flat the field of focus is rendered for that lens.
- The accuracy at which the camera is rotated around the optical center of the lens when recomposing occurs. Most photographers tend to rotate around their own torso's center, rather than rotating themselves around the center of the camera's "no parallax" point.
... if I focus and recompose a portrait at a lens' lowest aperture (not necessarily the sweetest aperture value) and put the person to one side will the portrait be sharper or softer in this case?
Sharper that what? If you mean than the subject when it was in the center of the frame and focused properly, recomposing to move the subject away from the center will almost certainly not make the subject sharper. The real question is more along the lines of, "How much less sharp will it be? Will that amount be noticeable in the intended display conditions?"
Well-designed and properly aligned lenses will always be "sharpest" at the center. The degree to which things not in the center of the frame are less sharp is determined by pretty much the same list posted above: depth of field, the overall optical performance of the lens, the shape of the lens' field of focus compared to the shape of the subject (this is particularly applicable when the "subject" is a flat test chart). etc.