Introduction to Sharpening

Tim Parkin

Amateur Photographer who plays with big cameras and film when in between digital photographs.

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Quite a few readers have asked us about sharpening over the last few months. It’s such a big subject that it’s probably best to split up into a series of posts which means that this issue we have an introduction to blur and sharpening.

The first step is to understand what blur is, where it comes from and what it looks like. Let’s take a look at two types of blur.

Types of Blur

The first type of blur is probably familiar if you have ever used the blur function in photoshop - this is technically known as gaussian blur because it follows a ‘gaussian’ distribution.

There are only a few types of real world causes of gaussian type blurring, spherical aberration is one - a blur caused by a lens with spherical surfaces - and ink spread on your printer is another.

Interestingly your typical out of focus blur of a good quality lens is not a guassian type blurring. This is why photoshop has an extra type of blur called ‘Lens Blur’. If you look at the image below you can see an example of lens blur on the same image used previously. This time the point source has become a circle whose brightness is greatly reduced.

We showed you a diagram in the article on depth of field recently that demonstrated why lens blur looks like it does and we’ll repeat it here.

As you can see, when your point source is slightly out of focus it is spread over a circle related to our previous ‘circle of confusion’ distance.