How do you focus to ensure as much of the image as possible is sharp?

We regard a lens as focusing light to a point (on the film or sensor) but this is not strictly true. A perfect camera lens does not exist and most of the light is focused down to a tiny circle, the so called “circle of confusion”. This tiny circle is so small the eye regards it as a point and we consider it to be sharp.

If you focus a lens on any point then both that point and a certain distance in front of and behind that point will appear to the eye to be sharp. The distance that appears sharp is widely considered to extend roughly twice as far beyond the focus point as it does in front. (This is true only in certain circumstances buy we need not be concerned with them here).

How do we apply this knowledge?

Well in the good old days lenses were supplied with f numbers engraved on the barrel and a focus scale so you could set this up instantly by focusing on a point in the landscape and then adjusting the f number until the scale showed both the near and far points were within the scale for the aperture chosen. A few years ago Canon built in an electronic equivalent to some of their cameras, but not now. So how do we set the camera up to achieve what we want?

Focus on infinity, look through the camera and find the nearest point that is sharp (take a picture and zoom in on the screen if you have time, just to make sure), refocus on that and you are now certain everything up to infinity is sharp and also roughly half the distance from the focus point to infinity will also be acceptably sharp in front of the focus point. Why? Because you have focused on exactly the hyperfocal distance!

And one word of warning, just in case your eyes are not as sharp as you would like (as mine) close the lens down one stop more just before taking the picture, depth of field increases with smaller apertures. This will extend the depth of sharpness a bit more, so, when you print it large you won’t be disappointed because your eyes were not seeing clearly through the camera.

Try to avoid the smallest aperture of the lens as image quality will degrade due to optical errors.
If the whole scene cannot be rendered sharply it is usually better to let the far distance go out of focus than to blur the foreground. Also a slightly blurred horizon in a picture adds to the overall impression of depth of the scene.


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