Size and Weight Comparison
Size of a lens is an important deciding factor when comparing two lenses.Tamron 17-50mm f2.8 XR II LD is the longer of the two lenses at 82mm. The Pentax DA 18-50mm F4.5-5.6 DC WR RE with a length of 71mm is 11mm shorter. Besides being longer, the Tamron 17-50mm f2.8 XR II LD also has a larger diameter of 74mm compared to Pentax DA 18-50mm F4.5-5.6 DC WR RE's 41mm diameter.
Weight of a lens is as important as its external dimensions, especially if you are planning to hand hold your camera and lens combination for longer periods. Pentax DA 18-50mm F4.5-5.6 DC WR RE weighs 158g, which means it is 272g (172%) lighter than the Tamron 17-50mm f2.8 XR II LD which has a weight of 430g.
Lens Mounts
Both Pentax DA 18-50mm F4.5-5.6 DC WR RE has the Pentax KAF3 lens mount whereas the Tamron 17-50mm f2.8 XR II LD has the Pentax KAF lens mount. Some of the latest released cameras that are compatible with these mounts are Pentax K-3 III, Pentax KP and Pentax K-70 for the Pentax KAF3 Mount and Pentax K-3 III, Pentax KP and Pentax K-70 for the Pentax KAF Mount.
Tamron 17-50mm f2.8 XR II LD is also available in Canon EF , Nikon F (DX) and Sony Alpha mounts.
Focal Range
Pentax DA 18-50mm F4.5-5.6 DC WR RE has a focal range of 18-50mm and 2.8X zoom ratio which has a 35mm equivalent focal range of 27-75mm when used on a APS-C / DX format camera. Tamron 17-50mm f2.8 XR II LD has a focal range of 17-50mm and 2.9X zoom ratio which has a 35mm equivalent focal range of 25.5-75mm when used on a APS-C / DX format camera.