The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Sigma 120-400mm F4.5-5.6 DG HSM and the Pentax 18-270mm F3.5-6.3 ED SDM 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 120-400mm F4.5-5.6 DG HSM is the longer of the two lenses at 203mm. The Pentax 18-270mm F3.5-6.3 ED SDM with a length of 89mm, is 114mm shorter. Besides being longer, the Sigma 120-400mm F4.5-5.6 DG HSM also has a larger diameter of 92mm compared to the Pentax 18-270mm F3.5-6.3 ED SDM's 76mm 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 18-270mm F3.5-6.3 ED SDM weighs 453g, 72% (1187g) lighter than the Sigma 120-400mm F4.5-5.6 DG HSM's weight of 1640g.
Filter Threads
The Sigma 120-400mm F4.5-5.6 DG HSM has a filter size of 77mm whereas the Pentax 18-270mm F3.5-6.3 ED SDM 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 77mm filters that we recommend you to consider for the Sigma 120-400mm F4.5-5.6 DG HSM:
And these are the 62mm filters that we recommend for the Pentax 18-270mm F3.5-6.3 ED SDM:
Lens Mounts
The Sigma 120-400mm F4.5-5.6 DG HSM has the Pentax KAF3 lens mount whereas the Pentax 18-270mm F3.5-6.3 ED SDM has the Pentax KAF lens mount. Some of the latest released cameras that are compatible with these mounts are Pentax K-3 III Mono, Pentax KF and Pentax K-3 III for the Pentax KAF3 Mount and Pentax K-3 III Mono, Pentax KF and Pentax K-3 III for the Pentax KAF Mount.
Sigma 120-400mm F4.5-5.6 DG HSM is also available in the Sony Alpha mount.
Focal Range
Sigma 120-400mm F4.5-5.6 DG HSM has a focal range of 120-400mm and
3.3X zoom ratio . When it is mounted on an APS-C sensor camera with 1.5x crop, it provides a 35mm (FF) equivalent of
180 - 600mm.
On the other hand, the Pentax 18-270mm F3.5-6.3 ED SDM has a focal range of 18-270mm and 15.0X zoom ratio which has an effective (full-frame 35mm equivalent) focal range of 27-405mm when used on a APS-C / DX format camera.
