if you go to the in depth review of the D FA 100mm F2.8 WR, you will find a discussion about that lens and the other 3 - D FA 100mm F2.8 Macro lens, which in turn had inherited the optical design from the well-respected FA 100mm F2.8 and F 100mm F2.8 macro lenses
Pentax-D FA 100mm F2.8 WR Macro Review - Introduction | PentaxForums.com Reviews
Pentax-D FA 100mm F2.8 WR Macro Review
Specifications
The lens covers the full 24x36mm format, not just APS-C. The aperture blades are rounded, delivering a smoother bokeh which is important for a macro lens. However, unlike its predecessor, it has no aperture ring.
The optical design is the tried-and-proven formula of the previous generation D FA 100mm F2.8 Macro lens, which in turn had inherited the optical design from the well-respected FA 100mm F2.8 and F 100mm F2.8 macro lenses introduced in 1991 and 1987, respectively. None of these earlier lenses had rounded aperture blades.
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Pentax-D FA 100mm F2.8 WR Macro Review - Specifications | PentaxForums.com Reviews
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so, no WR and straight aperture blades for older F2.8 lenses
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D FA 100mm F2.8 Macro lens
This lens features a clamp and 1:1 macro magnification. Optically it is the same as the predecessor, the smc Pentax-FA 100mm F2.8 Macro, and optically also the same as the Weather Resistant successor.
Diam x Length
67.5 x 80.5 mm (2.7 x 3.2 in.)
Weight
345 g (12.2 oz.)
w/ Hood: +38g
Read more at:
SMC Pentax-D FA 100mm F2.8 Macro Reviews - D FA Prime Lenses - Pentax Lens Reviews & Lens Database
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the FA 100mm F2.8 Macro
Description:
The SMC Pentax-FA 100mm F2.8 Macro is a high-quality macro lens designed to be well above the SMC Pentax-FA 100mm F3.5 macro in image quality. It features a clamp and a focus limiter. The optics are seated that far from the front that no separate lens hood is required.
Optically this lens is identical to its F-series predecessor and the D FA-series successors.
Diam x Length
74 x 104 mm (2.9 x 4.1 in.)
Weight
600 g (21.1 oz.)
Read more at:
SMC Pentax-FA 100mm F2.8 Macro Reviews - FA Prime Lenses - Pentax Lens Reviews & Lens Database
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FA 100mm f3.5 Macro
Description:
This is the slower of the two FA 100mm macro lenses.
Diam x Length
68 x 72 mm (2.7 x 2.8 in.)
Weight
220 g (7.8 oz.)
Read more at:
SMC Pentax-FA 100mm F3.5 Macro Reviews - FA Prime Lenses - Pentax Lens Reviews & Lens Database
it is possible that there were advances made in the SMC coatings as well