MAXIMISE LENS SHARPNESS
Understanding lenses and focusing technique is important for obtaining critical sharpness
No lens is perfectly sharp, especially when stopped down to small apertures, resulting in diffraction – the bending of light rays as they hit the aperture blades. The smaller the aperture, more light rays get bent, so images get progressively softer as you stop down.
The dilemma is that it is difficult to achieve enough depth of field without stopping down to a relatively small aperture. A balance has to be struck, where the aperture is small enough to get the depth of field you need, but not so small that diffraction becomes a problem.
Most lenses are sharpest at mid-apertures, usually around f8. With good lenses, f11 will be only marginally worse and at f16, you will see a difference, though not an extreme one. Beyond that, images can become noticeably soft. The sensible advice, therefore, is to shoot in the f8 to f11 range where possible, but not to be too concerned about stopping down to f16; however, try to avoid stopping down further.
Knowing where in the scene to focus is key as this allows you to place the zone of sharpness where you need it, so that depth of field extends from foreground to background. Rather than the traditional focusing ‘a third of the way in’ to the scene, which is rather imprecise, try ‘double-distance’ focusing. Assuming you’re shooting with a wide-angle lens and a smallish aperture, estimate the distance from the camera to the nearest object you want to keep sharp and then focus at double that distance. In practice, this delivers consistently good results.
You need to make sure that once you’ve focused, focus is locked. So either focus manually, or set up ‘back-button focusing’ where focusing is decoupled from the shutter button and assigned to one of the buttons on the rear of the camera.
Finally, to make sure your lenses perform at their best, keep them and your filters clean and free of dust and be sure to switch off vibration reduction when the camera is mounted on a tripod, as VR on tripod-mounted lenses can actually introduce shake.