There are a number of effects, that are generally described as flare. One must however be careful in distinguishing between veiling glare, that produces a drop in image contrast. the cause of this effect is a quantity of stray light that enters the lens and is produced by a uniform additional illumination level. This is best seen when one takes a picture of a local black spot illuminated by a diffuse overcast sky when one takes a landscape picture. In most cases however there are local bright sources in the scene (like the sun or some structured light source) that produce ghost images, secondary reflections, bright spots and diaphragm ghosts. The causes of these effects are numerous and very difficult to analyze in a systematic, reliable and reproducible manner. The rims of the lens elements, the shape of the lens surfaces, the interior of the lens barrel, the diaphragm blade surfaces may all contribute to the perceived phenomena.
Basically the flare problem is caused by the amount and intensity of light energy in the pupil area of the lens. The intensity of the bright light source and the oblique angle of the rays of that light source are the main causes of the visibility of the ghost images. The point is that the light rays produce a sharp image in the pupil, but the resulting reflections are in the unsharp area of the image and the light energy is much reduced, because the energy is spread out in the out-of-focus image. The intensity and the position of the source in the image field are extremely important parameters and even small changes in both parameters may alter drastically the visibility. In addition the chosen aperture also influences the result. The upshot is that it is almost impossible to draw reliable conclusions from incidentally observed flare situations, especially when there are no comparisons and when the flare situation is not protocoled or standardized. As example: the sun may have on face value the same intensity, but small changes in illuminance may have significant influence. The same is true when you take a picture with the sun shining through foliage of trees. A small change of light rays because you have moved the camera bit, may have unforeseen effects. Over-and underexposure also have a major impact on the results. The role of the sensor surface cannot be disregarded. Below is a picture with large purple effects. These are not related to the lens, but to the sensor. One must be very careful to separate these differences.
A comparison was made between the Leica Apo-Summicron-M 2/50 ASPH, the Summilux-M 1.4/50 ASPH, Summarit-M 2/50, Summicron-M 2/50, and Zeiss Planar-ZM 2/50. The main grouping is the classical Double Gauss version and the modern 8-element type with floating elements. The set-up used for this comparison was a bright light source of known intensity and a change of camera position compared to the stationary light source from center to edge. Every position was photographed with standard exposure at f/4 and one and two stops over exposure. This aperture has beens selected because one can compare a larger range of lenses at the same aperture and it is not often the case that you will use the widest aperture for normal landscape pictures. When shooting a stage performance of the Rolling Stones, the widest possible aperture may be used, but then most of the image will be underexposed and then the effects of flare are not visible and maybe even add to the atmosphere. The most realistic version of the series of photographs is the one with the correct exposure. The best photograph for visually grasping the effects of the flare is the one with one stop over exposure. I will present both pictures. The comparison images presented here show a severe amount of flare that is unlikely to be encountered in normal photographic conditions.
Below is the performance of the Summilux-M 1.4/50 ASPH.
Note the curved shape that indicates the reflection from the rim of a lens element or the mechanical mount.
The microscope image shows the depiction of the pupil and the construction of the bright spot that is seen in the image.
This lens had at the moment of its announcement the best performance in the domain of flare reduction. It still is excellent. The conclusion so far.The SX shows some rim reflections and the microscope image of the pupil shows the cause. It is the rim of one of the lens elements or the mount (maybe the additional mounting for the floating elements group). The same behavior, but much less pronounced is seen in the ASC image. The microscope shows the cause, but here the effect is artificially enhanced to be able to see it.
The ASC 50 has hardly any propensity to flare, and given its eight elements, compared to the six of the standard lenses, the performance is outstandingly good and the lens can be used in all but the most exotic conditions. Remember that it is always possible to create flare and ghost images, secondary bright spots and so on with every lens and camera. The SX is very good, given the larger aperture and does not have to hide behind the others. The reflection seen in the images, may be visible in several situations. The built-in lens shade of the ASC 50 and SX 50 is ineffective when used to reduce flare. They are only useful as a mechanical protection for the front lens surface.