best pentax lens for bokeh?

[wmmk]

Hey everyone! Sorry for all of my threads in the DP forum lately, I'm just getting really excited about photography!
I'm looking to get some primes and macros over the next few years. Someone in another thread mentioned that better bokeh can help a lot.
The 31mm f/1.8 is wonderful to lust over, but is rather expensive. I also like the 50mm f/1.4, which is quite affordable. I'll probably end up getting that at some point, but something that has perspective close to the human eye would also come in handy quite a bit.
Thanks,
wmmk

 

[dllavaneras]

Bokeh quality is a combination of the maximum aperture of the lens and the amount of blades it has. The aperture controls the DOF (which is why a f2.8 will give you more blurred out areas than a f4.5) and the amount of blades affect the "roundness" of the out of focus areas. So more blades mean better bokeh, in most cases.

For an example of the amount of blades thing, see this pic I took. The highlights in the out of focus parts come out as hexagons, since my lens has 6 blades. If it had more blades, the highlights would have been rounder and more pleasing to look at.

So when looking for lenses with good bokeh, check for both maximum aperture and number of blades. Many recent lenses have completely round OOF areas when used at maximum aperture. It has something to do with a circular diaphragm (look it up on FredMiranda or DPreview)

BTW, fredmiranda has a nice review of lenses, although I'm not sure if many pentax lenses are reviewed.

NOTE: in my pic I didn't take aperture into consideration because it was taken with my macro lens at a high magnification, and as such I stopped down to f8 (smallest aperture my P&S has) to get some decent DOF.

 

[wmmk]

Interesting. Does amount of aperture blades also affect lens flare? On my Tamron 28-200, lens flare is always a very ugly hexagonal shape. yuck. anyways, the Pentax FA 50mm has 9 aperture blades and a minimum aperture of f/1.4. Too bad I can't afford it...

 

[balofagus]

The best lenses for bokeh that are made by Pentax are:
1) FA 50mm f/1.4 (priced okay and the cheapest of this lot). There is also f/1.7 and f/2 versions of the 50 which are pretty decent as well
2) FA 77mm f/1.8 Limited (pricey and nowhere to be found but I guarantee you...it's fantastic)
3) FA 85mm f/1.4 (uber pricey and very near to extinction)

The newer - less expensive - DA 70mm has bokeh arguably close to the 77 Limited also.

I'm not sure what the bokeh's like on the 50 and 100 Macro but I imagine its rather nice