[micromacro]

I received the lens yesterday tested it, it's not razor sharp at 1.8, not at 2.0 either. It gets much better from about f 2.8 and up.
Actually, I like it that way for not being so cruel for portraits wide open, but I read mixed reviews about this lens : some tells it's not sharp wide open, some states that it is razor sharp wide open.

How should this lens be wide open? Razor sharp, or just "acceptably" sharp?

[UncleVanya]

https://flic.kr/p/BmL9WD
Pretty sharp but like most lenses it will improve up to f5.6 or f8. The depth of field can be razor thin at f1.8.

[blan01]

The FA31 is plenty sharp wide open in the center, but not across the frame,however I think it's more than enough for portraits.
You down have to go 5.6 to get really sharp results, just stop down a little bit to about 2.2 or 2.8, it will jump to razor sharp.
Honestly, I rarely use my 31 at 1.8, especially on K1 the depth of field is so thin, and at 2.2 and up, my 31 always deliver the goods.

[BigMackCam]

I don't own the lens so can't say with any authority. However, test data suggests it has very good resolution from f/1.8, then around f/2.8 the centre reaches excellent levels... by f/4 the centre is peaking and the borders have reached excellent levels also... then, at f/5.6 the borders are peaking, so this is technically where the best frame-wide resolution can be found. The borders drop back into very good levels from f/8 onwards.

[micromacro]

Honestly, I rarely use my 31 at 1.8, especially on K1 the depth of field is so thin, and at 2.2 and up, my 31 always deliver the goods

I forgot to mention that I tested it on K-1 as well, and yes, I like this lens from f2.2 and up. At f4 it's great. It works fine, IQ is awesome.

The only portrait lens wide open on K-1 I love is SuperTak 50mm f1.4, my copy obviously improved sharpness after de-yellowing.

Thank you, guys, gonna keep the lens, and keep testing.

[osv]

make sure to evaluate the sharpness using live view for focusing, not af.

[oxidized]

I have it and it definitely is on the softer side between 1.8 and 2.8 (also past f8 due to diffraction). Soft vs sharp though is purely a matter of opinion and very subjective. If you are used to using Zeiss glass, at 1.8 you would consider it softish. If you are used to using the kit lens then it will be bitingly sharp even at 1.8. It also doesnt helpt that there is a ton of mis-information about this lens such as claims the MIJ > AIV version. FYI, mine is AIV (though i dont think that matters).

I remember I was also a bit dissapointed when I first got it and started shooting rulers (to fix some of the back-focus) and walls to check for corner sharpness and de-centering. At the end of the day, it is an old legacy glass and I just adjusted my expectations accordingly. Dont expect biting sharpness across the frame wide open and if you want sharpness stop down a bit . If you want wide open sharpness just return it and go for the sigma art 35 1.4. You buy the 31 primarily for the 'look' and rendering (aka pixie dust ). To see it though, you would have to start shooting in real world situations.

Resolution is also better at 1.8 when the focus is between minimum and probably a meter or so. 1.8 at infinity is softer.

These are both at 1.8

Attached Images

   

[wissink]

Can someone explain this to me? I often hear wide open is okay for portraits because its still quite sharp in the centre. ??? Most portraits are composed in thirds are they not?

[oxidized]

Can someone explain this to me? I often hear wide open is okay for portraits because its still quite sharp in the centre. ??? Most portraits are composed in thirds are they not?

I think they mean head and shoulders portraits

[UncleVanya]

Can someone explain this to me? I often hear wide open is okay for portraits because its still quite sharp in the centre. ??? Most portraits are composed in thirds are they not?

Also with portraits even if you are shooting in 3rds you likely have an eye past the 1/3 mark towards the center. The eye snapped in focus is all that must be razor sharp. A little softness is flattering.

[tomO2013]

I shoot a Sony FE 35mm 1.4 Distagon on Sony system and that is what I consider bitingly sharp wide open.

I've found that the FA31mm limited has a tendency to front focus a little on the K1 - I've had to dial in +3 on my K1 with focus tune and lens align. When I dialled this in, I found a huge difference in wide open performance and wide open is very sharp.
However in context, the FA limited is not nearly as sharp as the Zeiss distagon wide open and I shoot both for very different purposes. Where I want clinical wide open precision I use the Distagon. If I want artistic 3d pop rendering and creamy bokeh I shoot the FA limited. The two lenses have very different design goals from different eras.
However by F2.8 and up the FA31mm sharpens up considerably to be practically as biting. That being said the Zeiss Distagon to my eye does not render as nice the out of focus areas. Quite literally the FA limiteds make subjects jump off the background!

This is not to say that the FA 31mm F1.8 is soft on a digital Pentax - it's not. Results will appear soft in bright high contrast out door situations wide open. It depends heavily on the lighting that you are shooting in (it is susceptible to coma wide open and fringing).
The 31mm has been optimized for a film plane rather than a digital sensor. On film it is probably one of the sharpest lenses out there.

Where the FA Limited comes into it's own in my opinion is how all the little deliberately designed 'imperfections' - coma/field curvature/vignetting - come together to create a very artistic organic looking image that pops far more than the 31mm focal length would suggest.
The Bokeh on the FA 31mm is creamy smooth - more so than the Art or Distagon 35. It has plenty of sharpness wide open but lends itself towards a 3d pop.

This is pretty close to out of camera with a 20 second black and white conversion in LR, no sharpening. Open full screen to see sharpness.

Evan by tom.ohle, on Flickr

[Medex]

Pentax smc FA 31 mm f/1.8 AL review - Image resolution - LensTip.com

[ChatMechant]

The first copy I had I thought was awesome, but not crazy sharp wide open. The copy I have now is crazy sharp wide open. To the point where the only consideration for which aperture to choose has to do with how much dof I want. However that opinion is just from putting the lens on the camera and shooting, no tests, charts, adjusting ff/bf etc. So ymmv.
f/2.0

[Parallax]

Some of the lack of sharpness you're seeing at 1.8 and, to a lesser extent, 2.0 is actually due to lack of contrast. I suspect that if you do a curves adjustment to bump the midrange contrast it will appear sharper.

[Shodra]

Center sharpness at F1.8 is good, but it doesn't blow me away. The main issue is not a lack of sharpness as such, but low contrast. As you stop down, it should get better rapidly. At F2.8 or even a bit earlier, mine is razor sharp.

The 31 is a tricky lens to evaluate, really. It is not designed to be an overcorrected sharpness machine and once you start complaining about your copy, a lot of people will tell you that. OTOH, there is (was?) a lot of sample variation and a few people with true lemons have been told to accept the lens characteristic in this very forum, when they just had a very bad copy.