The Pentax smc FA 645 33-55mm f/4.5 AL Lens is a wide-angle zoom lens designed for use with medium format Pentax 645 mount cameras. It features an aperture range of f/4.5-32 and has a 35mm equivalent focal length of 26-43.5mm, providing a very useful range of wide-angle lengths.
This lens in particular utilizes two aspherical elements that greatly reduce distortion, field curvature, and coma aberrations common with wide angle lenses.
The 33-55mm features a lens construction of 11 elements in 8 groups, including some high refraction low-dispersion glass. To maintain image quality, it uses a 3-group floating zoom to reduce field curvature throughout the range. Also, the 33-55mm has a minimum focus distance of 15.7".
For smooth out-of-focus areas, the lens has an eight blade aperture diaphragm. Additionally, it features an 82mm front filter thread.