Reason No. 9 Your Photos Aren’t Sharp: Depth of field too shallow
With a few creative exceptions, when you’re shooting landscapes you usually want the image to be sharp throughout the whole scene, from the foreground to the background. This requires using a small aperture to get lots of depth of field.
Using a small aperture and a low sensitivity setting for maximum detail and sharpness means also using a fairly slow shutter speed, making a tripod essential.
Reason No. 10 Your Photos Aren’t Sharp: Aperture too small
Although using a small aperture increases depth of field (the size of the focused area in the shot) it also increases the impact of diffraction, the bending of light as it passes over the edge of the aperture blades.
This bending prevents the light from focusing on the sensor and thus softens the image. The smaller the aperture, the greater the proportion of unfocused light reaching the sensor, so the greater the softening effect.
This means that there is a compromise between depth of field and sharpness. Rather than closing the aperture right down to its minimum setting, it’s better to open up by stop or two.
To discover how aperture size influences sharpness with your lenses, set your camera up on a tripod and focus on an object with lots of detail. Then take a shot at every available aperture setting.
When you’ve finished transfer the images to your computer and examine them at 100% on the computer screen.
Look carefully at the point of focus and you’ll see that the images go from being a little soft when the aperture is wide open, to being progressively sharper as the optimum aperture is approached.
Beyond this the image becomes softer again as diffraction becomes more of an issue.