The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Irix 45mm F1.4 and the Pentax DA 18-50mm F4.5-5.6 DC WR RE for their suitability to various photography scenarios:
Size and Weight Comparison
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 DA 18-50mm F4.5-5.6 DC WR RE weighs 158g, 82% (767g) lighter than the Irix 45mm F1.4's weight of 925g.
Filter Threads
The Irix 45mm F1.4 has a filter size of 77mm whereas the Pentax DA 18-50mm F4.5-5.6 DC WR RE has a 58mm 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 Irix 45mm F1.4:
And these are the 58mm filters that we recommend for the Pentax DA 18-50mm F4.5-5.6 DC WR RE:
Lens Mounts
The Irix 45mm F1.4 has the Pentax KAF2 lens mount whereas the Pentax DA 18-50mm F4.5-5.6 DC WR RE has the Pentax KAF3 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 KAF2 Mount and Pentax K-3 III Mono, Pentax KF and Pentax K-3 III for the Pentax KAF3 Mount.
Irix 45mm F1.4 is also available in the Canon EF and Nikon F (FX) mounts.
Focal Range
Irix 45mm F1.4 is a prime lens with fixed focal lenght of 45mm. When it is mounted on an APS-C sensor camera with 1.5x crop, it provides a 35mm (FF) equivalent of
67.5mm.
On the other hand, the Pentax DA 18-50mm F4.5-5.6 DC WR RE has a focal range of 18-50mm and 2.8X zoom ratio which has an effective (full-frame 35mm equivalent) focal range of 27-75mm when used on a APS-C / DX format camera.
