MTF program line is one of those interesting little gems of the Pentax K system, where the camera will prefer to use the aperture at which the lens is sharpest. However, I should note the following (correct me if I'm wrong):
- MTF information is communicated by the lens, not stored by the camera. Pentax F lenses do not provide this information and therefore do not support the MTF program line. The lens communication protocol was expanded in the FA series to add MTF information. MTF program line is available when using FA, FA J, D FA, DA, and DA L lenses, including all modern Pentax lenses.
- Because MTF information is communicated by the lens, third-party lenses can support MTF, as suggested by this thread.
My DA* 60-250mm is indicated to be best at f/4 (wide open!) at 108mm and below, f/5 from 118mm to 140mm, and f/5.6 at 153mm and above, as reported by the Exif data. These are mostly consistent with
the Photozone review of the lens, which uses the much older K10D—note that on 16+ MP sensors, diffraction is likely to start degrading sharpness earlier than what the review suggests. I'm not really a fan of shooting wide-open, though, so I'm still inclined to stop down to at least f/5.6 in most situations as I do get slightly better contrast and sharpness this way.
Questions, thoughts, or comments?
—DragonLord
Last edited by bwDraco; 07-26-2015 at 05:33 AM.