The type of photography that you are planning to use is an important decision factor. Here we have evaluated the HD Pentax-DA 11-18mm F2.8 and the Rokinon 8mm f3.5 Fisheye 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. HD Pentax-DA 11-18mm F2.8 is the longer of the two lenses at 100mm. The Rokinon 8mm f3.5 Fisheye with a length of 77mm, is 23mm shorter. Besides being longer, the HD Pentax-DA 11-18mm F2.8 also has a larger diameter of 90mm compared to the Rokinon 8mm f3.5 Fisheye's 75mm 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. Rokinon 8mm f3.5 Fisheye weighs 443g, 37% (261g) lighter than the HD Pentax-DA 11-18mm F2.8's weight of 704g.
Lens Mounts
The HD Pentax-DA 11-18mm F2.8 has the Pentax KAF3 lens mount whereas the Rokinon 8mm f3.5 Fisheye 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.
Rokinon 8mm f3.5 Fisheye is also available in Canon EF , Nikon F (FX), Four Thirds, Sony Alpha and Samsung NX mounts.
Focal Range
HD Pentax-DA 11-18mm F2.8 has a focal range of 11-18mm and
1.6X zoom ratio which has an effective (full-frame 35mm equivalent) focal range of
16.5-27mm when used on a APS-C / DX format camera.
On the other hand, the Rokinon 8mm f3.5 Fisheye is a prime lens with fixed focal lenght of 8mm which has an effective (full-frame 35mm equivalent) focal range of 12mm when used on a APS-C / DX format camera.
