The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Tamron AF 28-200mm f3.8-5.6 XR and the Sigma 18-200mm F3.5-6.3 DC for their suitability to various photography scenarios:
Size and Weight Comparison
Size of a lens is an important deciding factor when comparing two lenses.Sigma 18-200mm F3.5-6.3 DC is the longer of the two lenses at 78mm. The Tamron AF 28-200mm f3.8-5.6 XR with a length of 75mm is 3mm shorter. On the other hand, the Tamron AF 28-200mm f3.8-5.6 XR has a larger diameter of 71mm compared to Sigma 18-200mm F3.5-6.3 DC's 70mm 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. Tamron AF 28-200mm f3.8-5.6 XR weighs 354g, which means it is 51g (12%) lighter than the Sigma 18-200mm F3.5-6.3 DC which has a weight of 405g.
Below you can find the dimension and weight table of the 'SuperZoom Lenses in Pentax KAF Mount for size comparison purposes.
SuperZoom Lenses in Pentax KAF Mount for Size Comparison
Both Tamron AF 28-200mm f3.8-5.6 XR and Sigma 18-200mm F3.5-6.3 DC has the same Pentax KAF lens mount. Some of the latest released cameras that are compatible with the Pentax KAF lenses are Pentax KF, Pentax K-3 III and Pentax K-1 II.
Tamron AF 28-200mm f3.8-5.6 XR is also available in Canon EF , Nikon F (FX) and Sony Alpha mounts.
Sigma 18-200mm F3.5-6.3 DC is also available in Canon EF , Nikon F (DX), Sony Alpha and Sigma SA mounts.
Tamron AF 28-200mm f3.8-5.6 XR has a focal range of 28-200mm and 7.1X zoom ratio . When it is mounted on an APS-C sensor camera with 1.5x crop, it provides an 35mm (FF) equivalent of 42 - 300mm.
Sigma 18-200mm F3.5-6.3 DC has a focal range of 18-200mm and 11.1X zoom ratio which has an effective (full-frame 35mm equivalent) focal range of 27-300mm when used on a APS-C / DX format camera.