The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Sigma 50-500mm F4.5-6.3 DG OS HSM and the Pentax 18-135mm F3.5-5.6ED for their suitability to various photography scenarios:
Size and Weight Comparison
The size of a lens is a crucial factor to consider when comparing two lenses. Sigma 50-500mm F4.5-6.3 DG OS HSM is the longer of the two lenses at 219mm. The Pentax 18-135mm F3.5-5.6ED with a length of 76mm, is 143mm shorter. Besides being longer, the Sigma 50-500mm F4.5-6.3 DG OS HSM also has a larger diameter of 104mm compared to the Pentax 18-135mm F3.5-5.6ED's 73mm diameter.
The weight of a lens is equally significant as its external dimensions, particularly if you intend to handhold your camera and lens combination for extended periods. Pentax 18-135mm F3.5-5.6ED weighs 405g, 79% (1565g) lighter than the Sigma 50-500mm F4.5-6.3 DG OS HSM's weight of 1970g.
Filter Threads
The Sigma 50-500mm F4.5-6.3 DG OS HSM has a filter size of 95mm whereas the Pentax 18-135mm F3.5-5.6ED has a 62mm diameter. Larger filters are generally more expensive than the smaller ones given all the other features are equal.
Below are links to the 95mm filters that we recommend you to consider for the Sigma 50-500mm F4.5-6.3 DG OS HSM:
And these are the 62mm filters that we recommend for the Pentax 18-135mm F3.5-5.6ED:
Lens Mounts
Both the Sigma 50-500mm F4.5-6.3 DG OS HSM and the Pentax 18-135mm F3.5-5.6ED has the same Pentax KAF3 lens mount. Some of the latest released cameras that are compatible with the Pentax KAF3 lenses are , and .
Sigma 50-500mm F4.5-6.3 DG OS HSM is also available in Canon EF , Sony Alpha, Sigma SA and Nikon F (FX) mounts.
Focal Range
Sigma 50-500mm F4.5-6.3 DG OS HSM has a focal range of 50-500mm and
10.0X zoom ratio . When it is mounted on an APS-C sensor camera with 1.5x crop, it provides a 35mm (FF) equivalent of
75 - 750mm.
On the other hand, the Pentax 18-135mm F3.5-5.6ED has a focal range of 18-135mm and 7.5X zoom ratio which has an effective (full-frame 35mm equivalent) focal range of 27-202.5mm when used on a APS-C / DX format camera.