10 - Defocus, depth-of-field and focussing techniques

Summary

Introduction

In the discussion of image formation so far, we have always assumed that the image is correctly focussed, with only one exception – in the reference to the telecentric pupil in the preceding chapter. However, in this chapter we will show that focussing is not usually exact and therefore in any imaging system, there is some residual defocus. One reason for this focus error is that there is always a range of positions of the “image” plane within which the image appears to be correctly focussed. This range is called the depth-of-field or depth-of-focus. A defocus outside this range will, by definition, be noticeable and hence must have some effect on image quality, but within this range, there will be some loss of image quality that may be detectable under different circumstances.

Sometimes we defocus a system intentionally. For example, we may wish to understand the effect of a defocus on some particular target structure or image quality criteria. In these cases, we may need to control the level of defocus accurately and quantify it.

Therefore, whatever the cause of defocus, it is important to understand the different ways in which a defocus can occur and how to quantify the level of defocus. To meet these needs, in this chapter, we will investigate the defocussing of optical systems, the effect of a defocus on the image, sources of defocussing, and depth-of-field and finally discuss a variety of methods that can be used to focus an optical system accurately.

Quantification of defocus

Any study of defocus must look at the ways of quantifying the level of defocus.