[Edgar_in_Indy]

I have seen enough shots from the 30mm f/1.4 to know it is has strengths, but corners sharpness isn't among them

It seems as though Sigma was on a mission to maximize center sharpness when they designed the 30mm 1.4. As a result, it's one of the sharpest lenses around at f/1.4. In fact, when Photozone tested the 30mm and the 31mm on the same body, the 30mm edged out the 31mm for center sharpness wide open. Pretty impressive considering that the Sigma was at f/1.4 while the Pentax was at f/1.8.

The fact that Sigma was able to accomplish this in a lens as inexpensive and as small as the 30mm 1.4 is even more impressive. The downside, of course, is that extreme border and corner performance suffers. But I think that Sigma knew that with how people would typically be using the 30mm, it would not be an issue for many users.

I myself haven't been bothered by the corners at all. But then again, I mostly shoot people, portraits, and events with my 30mm, and rarely use it for landscape or architecture. If I were shooting more in situations where edge-to-edge sharpness was vital, then I would own the 31mm. But for me, I value the larger aperture of the 30mm more than the corner advantage of the 31mm.

[Edgar_in_Indy]

I have heard of sigma lenses requiring calibration on camera bodies. Why not look into that with your local sigma service centre?

I really don't think that Sigma lenses are the only ones that can benefit from calibration. I just went from a K-x to a K-30, and I can tell you that my lenses focused differently between the two camera bodies. So sometimes it may be the camera that is causing the focus issues. Calibration issues are most evident with larger aperture lenses, especially as focal length increases.

My 30mm 1.4 front focused on my K-x, so I sent it to Sigma in NY with my camera body to be calibrated, and the whole process was painless. They calibrated the lens to my K-x the same day they received it, and they had it back in the mail that afternoon. It came back focusing perfectly on my K-x, and has since become one of my favorite lenses.

[wlachan]

I have read somewhere years ago that designing any lens with high center sharpness was actually rather easy, but it became a challenge when trying to bring even sharpness across the frame plus a whole bunch of other corrections.

[lytrytyr]

The answer to that question is obvious - de-centering, and its effects of field curvature

Decentering and field curvature are entirely separate phenomena.
A flat-field lens may be decentered,
and a perfectly centered lens may exhibit field curvature.

[Rondec]

Honestly, I was pleased with my Sigma when I owned it, I am more pleased with the FA 31 limited. I didn't take many landscape photos with the Sigma 30, because it just wasn't strong in the corners (read into that what it is). Out of focus rendering is more pleasing with the FA 31.

Since there was a comment about not seeing FA 31 photos in the thread, I'll just add a couple:

[Designosophy]

I have seen enough shots from the 30mm f/1.4 to know it is has strengths, but corners sharpness isn't among them - and the bokeh from it is a real mixed bag too because it has some pretty horrendous coma which isn't helped by the fact that it commonly has a poorly polished aspherical element in it's design.

One of the oddities of the Sigma 30 is triangular- or chevron-shaped bokeh near the edges. I sort of like it, myself. Check out the left edge of this photo:

[joe.penn]

I kind of prefer my Sigma 28mm 1.8 EX over my Sigma 30mm 1.4... OP: Have you considered the 28 1.8? [I know, someone had to throw a curve-ball in]

[lytrytyr]

Since there was a comment about not seeing FA 31 photos in the thread, I'll just add a couple:

Nice shots, Rondec, especially Nos. 1 and 3.

Comparing No. 3 with PCarfan's famous frog shot,
the FA Limiteds will soon be getting a reputation as reptile lenses!

[Digitalis]

Decentering and field curvature are entirely separate phenomena. A flat-field lens may be decentered, and a perfectly centered lens may exhibit field curvature.

indeed they are but when combined both effects can cause unusual behaviour, for example the left AF points being more accurate than the right.

[snake]

Is that the same Foka as the one in Rotterdam?

foka.nl

[123jippo]

Is that the same Foka as the one in Rotterdam?

foka.nl

I dont think it is same even have same name.

www.foka.fi

[Edgar_in_Indy]

Honestly its AF was working better before calibration so i do not recoment calibration also at least here in finland company named Foka is doing that for sigma lenses and their work is bullshit

I sent my K-x and 30mm to Sigma's US headquarters in New York, and they got the calibration perfect. They currently have my K-30 and my four sigma lenses (10-20mm, 30mm, 50-150mm, and 100-300mm) so we'll see how they do this time.

[Winder]

I just ordered the 31mm after trying a 30mm F/1.4 and trying to locate a FA 24mm F/2.

My one issue with the 30mm Sigma was that I really only use the center AF point and I recompose. With the softness of the 30mm Sigma and the field curvature of the focus I found that my subject was often moved out of center and into the soft zone or the curvature issues presented a problem after re-composition. For people who always shoot dead center the Sigma 30mm is an excellent lens. For me its softness and curvature were an issue. I do hope that Sigma will take the success they have had with the 50mm F/1.4 & 85mm F/1.4 and apply it to a 24mm f/2 & 35mm F/1.4 EX DG HSM. My 50mm & 85mm have both been stellar performers.

[Arrvon]

I realize this thread has pretty much run its course, however I don't recall the following link being posted here. Its a french website so I can only read it using an online translate page, however it is pretty much the only direct comparison I've found between these two lenses. Here it is:

Test_31ltd

I realize its easy to say I am biased based on the fact that I own the Sigma 30, however, I am of the opinion that it gets some additional critisism because it says Sigma on it.

I am curious how its negatives would be viewed if it had 'Pentax' written on it instead.

Either way, if anyone is after a fast normal prime, you can't go wrong with either. I chose the Siggy largely because of the extra half stop, being sharp at f1.4, and the price difference. (Not to mention 10yr warranty vs. 1 or 2)

[gtl]

The sigma is great for what it costs but the Pentax would support the eventual FF k mount.... This is one of the reasons why it costs so much more.