The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Tamron 18-270mm f3.5-6.3 II VC PZD and the Sigma 15mm F2.8 EX DG 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. Tamron 18-270mm f3.5-6.3 II VC PZD is the longer of the two lenses at 88mm. The Sigma 15mm F2.8 EX DG Fisheye with a length of 65mm, is 23mm shorter. On the other hand, there is no difference between the max diameters of these lenses, both coming at 74mm.
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. Sigma 15mm F2.8 EX DG Fisheye weighs 370g, 17% (80g) lighter than the Tamron 18-270mm f3.5-6.3 II VC PZD's weight of 450g.
Lens Mounts
The Tamron 18-270mm f3.5-6.3 II VC PZD has the Nikon F (DX) lens mount whereas the Sigma 15mm F2.8 EX DG Fisheye has the Nikon F (FX) lens mount. Some of the latest released cameras that are compatible with these mounts are Nikon D3500, Nikon D7500 and Nikon D5600 for the Nikon F (DX) Mount and Nikon D6, Nikon D780 and Nikon D3500 for the Nikon F (FX) Mount.
Tamron 18-270mm f3.5-6.3 II VC PZD is also available in Canon EF mount.
Sigma 15mm F2.8 EX DG Fisheye is also available in Canon EF , Pentax KAF, Sony Alpha and Sigma SA mounts.
Focal Range
Tamron 18-270mm f3.5-6.3 II VC PZD 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.
On the other hand, the Sigma 15mm F2.8 EX DG Fisheye is a prime lens with fixed focal lenght of 15mm. When it is mounted on an APS-C sensor camera with 1.5x crop, it provides a 35mm (FF) equivalent of 22.5mm.