 BetterPhoto Member |
Focus When Using a Telephoto Lens I have trouble getting my subject in sharp focus when using my Olympus E-500 with Zuikor 45-150mm lens. When shooting a subject far away using autofocus, with a tripod, I bring my lens all the way out, then a hair in (as suggested), and I just don't get a sharp image. I have better luck closer to the subject. Does this have anything to do with focal length? I feel like I'm missing a simple step, or a bit of knowledge. Please enlighten me, as I'm losing many good shots! Thanks, Rick
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Alan N. Marcus |
Hi Ricky, Some tips on focusing: As you know, the aperture actually used for the exposure effects the sharpness. Generally a lens will be sharpest when stopped down about 2 f/stops. Stated another way, consider a lens with a maximum aperture (lowest number) of f/2.8 and a minimum aperture of f/22 (largest number). Now, f/2.8 allows in more light and f/22 restricts much of the light. Two stops down from f/2.8 is f/5.6. This will generally be the sharpest setting. Why? The central region of the lens has the best correction. The peripheral or edges of the lens yield the worst performance. Additionally, as we stop down to tiny apertures, a high percentage of the light rays are forced to brush by the edges of the restricting iris. This close encounter causes many rays to go off course resulting in a phenomenon know as diffraction. Diffraction degrades sharpness. While stopping down yields greater depth-of-field, overall sharpness is diminished. When you focus a single-lens-reflex (SLR,) the mechanism forces you to focus at maximum aperture because in this position depth-of-field is at minimum. When the shutter actually clicks the camera stops down to a pre-selected taking aperture, generally quite a bit stopped down. This stopping-down gains greater depth-of-field that improves the odds of getting a sharp picture. Now you need to know that short lenses have greater depth-of-field than long lenses. We can take advantage of this fact when we focus a zoom. If we focus at max magnification, we are focusing under conditions of minimum depth-of-focus. Now after focusing, we zoom out to compose (reduce magnification), depth-of-field increases thus improving our chances of getting a sharp in-focus picture. Stated another way, zoom to max position to focus, and then reduce the zoom for actual picture taking. Camera shake must be controlled. Higher shutter speeds and a well-mounted camera are the rule when working with a zoom at high magnification. When photographing a group of people, don’t focus on the center row, instead, focus on the next closest row to the camera. When focusing on landscapes, don’t focus on the horizon (infinity); instead focus on an object 100 feet or so away. This allows the depth-of-field zone to carry to infinity plus encompass most of the foreground. This allows depth-of-field to work for you. The zone of acceptable sharpness is not split down the middle. This zone extends further away from the point of focus than it extends back towards the camera. As a rule of thumb, depth-of-field extends 1/3 towards you and 2/3 away from you as measured from the point focused upon. Happy holiday,
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