Depth Of Field And Working Aperture - XIIMUS 1024CTI User Manual

XIIMUS R-GB-IR CL User Manual. Version 1.2
7.2.3

Depth of field and working aperture

Optimum sharpness of the image is achieved only when the object is in the focus plane.
Behind and in front of this plane the sharpness is worse. Depth of field is defined by how un-
sharp a point is allowed to be. Depth of field is thus the distance from the plane of focus
where the unsharpness stays tolerable. So the depth of field depends on the smallest appli-
cation dependent feature size to be recognised. Depth of field considerably increases with
smaller aperture sizes.
Depth of field is also affected by the focal length used. A longer focal length with a given
aperture will result in a shorter depth of field. Therefore, as a guideline, if the focal length is
increased the aperture has to be stopped down by the same factor to retain the desired depth
of field.
A lens improves optically when stopped down. At large apertures most of the area of the lens
is used, resulting in a slight blur caused by unavoidable imperfections in the lenses. When
stopping down, only the central area of the lens is used. The optical picture is more correct
and resolution improves. Considering this fact the lens to be used should have a large
maximum aperture and should be operated at a mid-range aperture.
In reality, stopping down does not improve optical quality endlessly, since diffraction starts to
affect image quality at small apertures. According to the law of diffraction a sharp edge turns
light slightly off. The aperture forms such a sharp edge and light closest to the edges causes
fuzziness in the image. When using wider apertures the percentage of light passing along the
edges decreases in relation to light passing through the centre. So, at small apertures the
ratio of light passing close to the edges increases and thus small apertures result in a lot of
diffraction.
Sharpness is therefore limited not just by imperfections in the lens but also by diffraction.
Thus, the use of mid-range apertures (f5.6 - 11) results in optimum picture quality. Normally a
lens is at its sharpest at aperture 8.
TVI XIIMUS cameras are very sensitive to light and have a wide range of user programmable
gain factors, so it is possible to use mid-range apertures without significantly increasing
lighting costs.
© TVI Vision, 18 October 2006 page 24 ( 57 )