Great wide angle from Sigma. This specific lens is in Pentax K mount, I used it on my Pentax K1000 and it worked wonderful.
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The filters built in the lens, include a 1A (skylight), Y48 (Yellow), O56 (Orange), and a 82C (color correction).
It also takes 62mm filters. The diaphragm is from 2.8 to 22. Focus distance 20 cm to infinity.
Good image quality, small softness in the corners, barely no vignetting.