"Dirty" Filter Resolution LossTest


I had cause recently to test the effect of a moderately "dirty" UV filter on image quality. One of my workshop participants was having trouble getting sharp images from a new 400 f5.6L canon lens which has a reputation for sharpness. None of images were sharp, irregardless of shutter speed or aperture. The lens appeared to be focusing fine and no amount of back or front focus was noticeable. It was clear from comparing images in the field with my old 300 f4L, that without a filter in place the 400mm was slightly sharper than the 300 x1.4 as one might expect. With the filter in place all images were low in contrast and had odd color fringing with a slight yellow cast around hi contrast areas. To test the effect of the filter I shot six frames  with the filter ON the lens, six with no filter and six after cleaning the filter. The target chart is the USAF 1951 standard test chart that I have used for years and is capable of providing lines per mm figures when used properly. The filter as tested was moderately dusty and may have had a slight film on it though the amount of dust did not seem excessive and no significant film was noticeable. However it is clear from the the test that cleaning the filter (Isopropyl alcohol and lens tissues) improved the image sharpness to a point that the test with no filter  looks similar to the one with a clean filter. Only the extreme corner of the test chart is shown below. Other areas of the chart, including the center were actually much worse than this corner.



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