Size and Weight Comparison
Size of a lens is an important deciding factor when comparing two lenses.Pentax D FA 70-200mm F2.8ED is the longer of the two lenses at 203mm. The Pentax FA 70-210mm F4 ED SDM WR with a length of 175mm is 28mm shorter. Besides being longer, the Pentax D FA 70-200mm F2.8ED also has a larger diameter of 92mm compared to Pentax FA 70-210mm F4 ED SDM WR's 79mm 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 FA 70-210mm F4 ED SDM WR weighs 819g, which means it is 936g (53%) lighter than the Pentax D FA 70-200mm F2.8ED which has a weight of 1755g.
Both Pentax FA 70-210mm F4 ED SDM WR has the Pentax KAF4 lens mount whereas the Pentax D FA 70-200mm F2.8ED has the Pentax KAF3 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 KAF4 Mount and Pentax K-3 III, Pentax K-1 II and Pentax KP for the Pentax KAF3 Mount.
Pentax FA 70-210mm F4 ED SDM WR has a focal range of 70-210mm and 3.0X zoom ratio which has a 35mm equivalent focal range of 105-315mm when used on a APS-C / DX format camera. Pentax D FA 70-200mm F2.8ED has a focal range of 70-200mm and 2.9X zoom ratio . When it is mounted on an APS-C sensor camera with 1.5x crop, it provides an 35mm (FF) equivalent of 105 - 300mm.