Originally posted by yucatanPentax Just in general, I wouldn't use any filter on the PLM 55-300mm.
I only use a UV filter or similar when there's a possibility of physical damage like salt from sea spray or sand on a windy beach or desert.
This^^^. Using no filter will maximize the potential image quality your lens is capable of. A CP filter is for special circumstances, like reducing reflections as off water, glass, etc. and for increasing blue in the sky on a sunny day when the sun is at a certain angle. It does reduce the light coming in through your lens, how much depending on the rotational setting of the filter, making it necessary to adjust your exposure parameters to compensate. If your camera is set to one of the automatic exposure modes, the camera will make the adjustments, but the degree of adjustment needed for good exposure can be a compromise against a best quality outcome, unless a real need for that filter is there.
If you do use a UV filter for protection against hazardous conditions, get a high-quality multi-coated one in order to minimize the presence of the filter and loss of image quality. The other main use it is formulated for, is for greater clarity against distant haze. This type of filler will not meaningfully reduce incoming light.
Like Pentax Syntax recommends above. However, the CP filters you show, having protective coatings, are most probably multi-coated and of good quality.