Small apertures can mean soft images – why?


Photography Phactoid number 005.

A small aperture, lets say f16, normally gives us sharpness all the way through the picture. Actually, there are some circumstances where that is not true. At very narrow apertures the image becomes soft… explanation below!

Normally the widest aperture size is marked on a lens. People want to know if it is a fast lens – one that works well in low light. Fast lenses, with wide apertures, may be in the range of f2.8 to really fast at f1.2. Find out more about aperture sizes in: “What is the aperture range of a lens?”.

The narrowest size of the aperture is not quoted on lenses. Why not? At extremely narrow apertures the lens partially loses it ability to create a sharp image. This is due to a phenomenon called diffraction. As the light wave enters the aperture the edges of the light wave are bent very slightly as they touch the edge. In the case of wider apertures this does not have a very significant effect on the overall image. However, t very narrow apertures, say f22 or smaller, the light bends significantly and the resolution of the image is damaged.

See the diagram below..

Explanation of the diagram:

  • Top image (cross) the wide aperture blurs the image (bokeh)
  • Second image (cross) the image is quite well resolved
  • Third image the narrow aperture has softened the image to an interference pattern of concentric circles
  • In the bottom image the blur is almost complete – the image is blurred out

A whole range of aperture sizes resolve the image normally – our image is sharp. However, as the aperture gets very small the image will get softer as the diffraction effect becomes more pronounced. This happens despite correct focussing. At the narrowest aperture of the lens the image may be unrecognisable. In other words there is an optimum size of minimum aperture.

When your lens is stopped right down it may create a softness in the picture. There is a simple way to correct it. Open the aperture one half to a full stop wider to enable the lens/aperture focusing to restore the resolution.

By Damon Guy (author and Photokonnexion editor)

Damon Guy - Netkonnexion

Damon Guy (Netkonnexion)

Damon is a writer-photog and editor of this site. He has run some major websites, a computing department and a digital image library. He started out as a trained teacher and now runs training for digital photographers.
See also: Editors ‘Bio’.

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