It can be challenging for compact digital camera users moving to a digital single lens reflex camera like the Canon XSi/450D to take sharply focused images.
Because it’s difficult to break the habit of locking focus and exposure (AE/AF Lock) and then recomposing to take the shot. If the same steps are taken with a DSLR, the area you think is in focus can actually end up out of focus.
Switch to single area focus
Most digital cameras, including entry level DSLRs, have automatic focus modes with multiple focus points. But leaving focus up to an automated system frequently results in misfocused images.
Do yourself a favor if you want accurate focus. Switch your camera to single area focus mode so you control where the camera focuses. Then check the manual about how to change the location of the focus point when necessary.
Compose > adjust focus point > lock focus
With a DSLR, it is often necessary to compose first. When necessary, manually change the default focus point, usually the center point, so it’s over the area of the subject you want in sharp focus. Then take the picture.
For example, when taking a vertical portrait shot you typically want to lock focus on the eyes. If you focus with the center focus point and recompose, the center focus point may move away from the eyes when you change the orientation of the camera.
Instead, compose, select the appropriate focus point and then lock focus and shoot.
DSLR aperture and depth of field
Compact digital cameras have sharp focus across the entire frame because they have a wide depth of field (DOF). DSLR lenses have a narrow depth of field, sometimes only a few inches.
When the aperture on a DSLR is wide (lower F-number such as f/1.8, f/2.8, f/3.5), depth of field is narrow and portions of an image appear blurred. Using a wide aperture to blur the background when taking a portrait or close-up shot is often desirable, but the blur can be unsightly for other scenes, giving the appearance of poor focus.
Lens “sweet spot”
Every lens has what’s known as a “sweet spot,” an aperture range where the lens produces the sharpest images. To find the “sweet spot” for a lens, experiment using different apertures, or ask other owners of the lens.
Though the actual “sweet spot” varies among different lenses, it is generally sharpest in the middle range.
To help ensure sharp images, set your lens to a mid-range aperture (higher F-number such as f/5.6, f/8, f/11) and/or shoot at a longer distance from the subject. Watch the shutter speed too. If it is too slow for the focal length used, images will be blurred due to camera movement.
Canon’s ZoomBrowser software can display DSLR autofocus points, like the nine points in the above photo taken with the XSi. The camera orientation was changed, then the focus point was changed (red box indicates the manually selected focus point). When the shutter button was half pressed, the camera locked focus in the area of the eyes, rather than on the wrist in the center.
On Canon DSLR cameras with Basic and Creative zones, the AF Point can only be manually changed in Creative zone. Check the manual for details (pg.61 for XSi users).