Size and Weight Comparison
Size of a lens is an important deciding factor when comparing two lenses.Sigma 18-125mm F3.8-5.6 DC OS HSM is the longer of the two lenses at 89mm. The Tokina 16.5-135mm f3.5-5.6 DX with a length of 78mm is 11mm shorter. On the other hand, the Tokina 16.5-135mm f3.5-5.6 DX has a larger diameter of 84mm compared to Sigma 18-125mm F3.8-5.6 DC OS HSM's 74mm 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. Sigma 18-125mm F3.8-5.6 DC OS HSM weighs 490g, 24% (120g) lighter than the Tokina 16.5-135mm f3.5-5.6 DX's weight of 610g.
Below you can find the dimension and weight table of the 'SuperZoom Lenses in Nikon F (DX) Mount for size comparison purposes.
SuperZoom Lenses in Nikon F (DX) Mount for Size Comparison
Lens Mounts
Both Tokina 16.5-135mm f3.5-5.6 DX and Sigma 18-125mm F3.8-5.6 DC OS HSM has the same Nikon F (DX) lens mount. Some of the latest released cameras that are compatible with the Nikon F (DX) lenses are Nikon D3500, Nikon D7500 and Nikon D5600.
Tokina 16.5-135mm f3.5-5.6 DX is also available in Canon EF mount.
Sigma 18-125mm F3.8-5.6 DC OS HSM is also available in Canon EF and Sigma SA mounts.
Focal Range
Tokina 16.5-135mm f3.5-5.6 DX has a focal range of 17-135mm and 7.9X zoom ratio which has a 35mm equivalent focal range of 25.5-202.5mm when used on a APS-C / DX format camera. Sigma 18-125mm F3.8-5.6 DC OS HSM has a focal range of 18-125mm and 6.9X zoom ratio which has a 35mm equivalent focal range of 27-187.5mm when used on a APS-C / DX format camera.