The resolution of an optical system indicates the discernibility of two minute structures that can still be perceived separately. The resolution of optical instruments is limited by diffraction. The typical
specification is made in line pairs per mm (Lp/mm). The dependence of the resolution from the contrast describes the modulation transfer function.
Modulation Transfer Function (MTF)
MTF (also called contrast transfer function) describes the comparison of the contrast of the edges of a (real) object with the contrast of its image at a certain resolution (MTF = image contrast/object contrast). There will always be some contrast loss.
The contrast describes the relationship between light and dark areas of an image, indicated in per cent (%).
Depth of Field
The depth of field (DOF), often erroneously dubbed “field of depth”) describes the expanse of the focussed area (within the object space) in front of and behind the optimal focus level. The smaller the aperture (i.e. the larger the f-number) the greater the depth of field area.
The hyperfocal distance is a special instance in which the depth of field area extends to infinity.
In case of telecentric lenses, the telecentric area is essentially the depth of field area.
The distortion (or visual distortion) is a geometric image defect
(aberration) of lens systems. It leads to different magnifications when
the distance to the optical path of the system increases and creates the
image of straight lines that do not cross the centre of the image to be
reproduced in a bent way.
Decreasing magnifications (reductions) with respect to the image
field edge are called barrel-shaped distortions (fish-eye effect) with
an increasing pulvinate distortion. The distortion is generally
indicated in per cent (%).