Lens flare

[imxputs@knoware.nl (Erwin Puts/imX)]

Most contributions on this subject equate flare with ghost images or
secondary images. This is only part of the story. The official definition
of 'flare' is: non-image forming light, more or less evenly distributed on
the film plane. This light enters the lens, reflected from the subject
and/or from lightsources illuminating the subject and obliquely entering
the lens directly.
This type of flare is degrading the image quality by lowering the overall
contrast, and washing out the very fine details by destroying the micro
contrast. You can easlisy see this phenomenon for yourself if you compare
two pictures taken in exactly identical situations, one with a lens prone
to image degrading flare (like the Summilux 35mm) and one which has good
flare control (like the Summilux-ASPH 35mm). In the first image the deep
shadows are lighter (more deep grey than ink black), very fine subject
detail can not be detected (which the ASPH easily shows) and the overall
contrast is lower, where the ASPH has brilliance and deep saturated and
clear colors (in the small subject areas that is). The ASPH also suppresses
light fringes around the small subject outlines in strong backlighting and
sidelighting, where  the older lens exhibits strong halo around the subject
contours.
The second way to show the image degrading by flare is measuring the deep
shadows with a densitometer. You will then find that the older lens has
actually a higher reading in the deep shadows than the ASPH, which you
could interprete (wrongly) that the older lens is better in the area of
light transmission.
Ghost images are a totally different kind of phenomenon and yes, my new
Summicron 35mm has its share of it if I take pictures with the sun or
another strong lightsource shining in the lens. This is a fact of life and
you can only get rid of it by building custom shades long enough to shield
the lens (look at the Hasselblad shades).
Hope this helps
Erwin Puts