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| p.3 #7 · p.3 #7 · Thanks to those that commented on No Filter (UV or similar) on 100-400 |
Lenses with longer focal lengths and larger entrance pupils (which typically go together) have a double-whammy increase in sensitivity to the quality (optical flatness/uniformity) of filters --- the lens both magnifies the small angular changes caused by nonuniformity more, and each point in the image "sees" a larger area of the filter (so the filter needs to be flat over a much larger scale). A filter that will be just fine with a wide/normal lens may cause trouble on a telephoto (the plastic Singh-Ray grad NDs I've used are great on wide angles, but will turn a shot with my 280/4 to mush). Issues with flare/contrast can strike any lens.
Not doing apples-to-apples comparisons with focal length and entrance pupil often adds confusion to the "filter-or-not" debates --- one person sees degradation on their fast telephoto (using a high-$ filter), which another disputes because their wide-angle slow zoom works with the cheapest window-glass filter. The level of filter quality necessary to avoid noticeable compromise in image quality might vary by an order of magnitude or two based on the lens (length, speed, intrinsic sharpness, filter reflection interactions).