This page provides supporting evidence for the occurrence of filter flare as
detailed on the main flare page. I captured a
residential area at night with the Planar 1.4/50 equipped with and without filters.
Dark pastures in front of bright street lights formed a merciless setting
for the observation of flare spots. The exposure was kept constant for all shots at
f/2 and 30 seconds on an ISO 100 film, and it speaks for itself that the filters were clean.
Figures 1–3 show a section of 5.7×4.0 mm of the original slides, around the image center. In the absence of a filter the meadows are clean: Fig. 1. By contrast, in Fig. 2 the addition of a high-quality UV filter comes with ghost images of the street lights. When a cheap UV filter is used (Fig. 3) the flare is worse with more and brighter spots, and even a window that stands out clearly against the dark grass. In all cases the highlights in the upper half and the ghosts in the lower half are mirror images of one another with the image center as the center of symmetry.
It is also interesting to examine the influence of the lens aperture on filter flare. To this end I made a larger selection of the original slide, measuring 18.7×8.6 mm. Fig. 4 is the no-brand filter again with the lens at f/2, whereas for the photograph in Fig. 5 the aperture was f/8, with an increased exposure time to maintain the same overall exposure. As it appears there is a clear influence of the aperture. At f/8 the flare is less pronounced, and more noticeably, there are fewer spots. The smaller aperture prohibits the occurrence of filter flare outside a circle of a certain radius from the image center. As a side effect of the smaller diaphragm opening the street lights are now saddled with the typical diffraction stars of nighttime photography.
The risk of being confronted with filter flare reduces with a smaller focal length, a
smaller aperture (larger F-number), an increased separation of highlights from the image
center, and with a better filter quality. However, by far the most secure way to avoid
this type of flare is to omit the filter altogether.
© Paul van Walree 2003–2014