Pentax 21mm Ltd vs Sigma 17-50mm full-size samples

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left eye
left eye Senior Member • Posts: 2,553
Pentax 21mm Ltd vs Sigma 17-50mm full-size samples
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For those of you who are interested, here we go again! For those of you who have the lenses they need and are happy shooting with them, you can pass this post by.

I've ascertained that the Sigma 17-50mm on the whole has better IQ than the Pentax 18-135mm, not always but mostly (of course the 18-135mm has a lot more reach in it's favour). The Pentax at the wide also has noticeably more barrel distortion than the Sigma, correctable but it's always better to have less (and so be closer to OVF framing).

But the Sigma is a big lens, fractionally longer and fatter then the Pentax making it just a bit ungainly on the KP. So what about the tiny 21mm Limited?

Overall the Sigma is better, mostly. With all the lenses I've been using there can be focus inconsistencies from shot to shot. I used to get this with my old K-5, I think it's a Pentax thing. So I'm weeding those out, not to pit a mis-focused shot against a well focused one.

That said, the 21mm has more barrel distortion and field curvature than the Sigma zoom. It also, despite my best attempts is often front-focusing at infinity (though not for close-up subjects). I've set the AF tune to -4 to push the focus plane back, but it still occasionally 'likes' foreground.

Fully open at f2.8, the Sigma shows a high degree of hazing and smearing - not something to be proud of.

The 21mm Limited fully open at f3.2 also shows hazing and smearing though not as bad. Stopped down to f3.5 and the Limited is immediately helped. So comparing the Sigma zoom to the Limited both at f3.5, the Sigma is more even across the frame, while the Limited is better on the left and worse on the right (and seems to be front focusing). With perfectly focused shots at f3.5 I'd give it to the 21mm Ltd.

Both stopped down a bit more to f5.0, hazing and smearing is now gone and we're hitting the beginning of best performance (which goes on until f8). At f5.0 the Sigma is better.

First an infinity test and close-up at a solid f6.3 aperture :

Pentax Ltd 21mm f6.3...

Sigma 17-50mm at 21mm f6.3...

Pentax Ltd 21mm f6.3...

Sigma 17-50mm at 21mm f6.3...

Now with the aperture nearly fully open, a stress test...

Pentax Ltd 21mm f3.5...

Sigma 17-50mm at 21mm f3.5...

And now stopped down to the start of good performance...

Pentax Ltd 21mm f5.0...

Sigma 17-50mm at 21mm f5.0...

All the Pentax lenses I've been using seem to need about +0.7 EV comp in camera, and also often some slight EV lifting during development. At least they are consistent with themselves.

The Sigma is fine with 0 EV comp in camera and no EV lifting during development. In all the Pentax lenses are delivering nearly a stop less light (or the Sigma a stop more). I'm really not sure why this is, but is a slight hassle when swapping lenses - remembering to readjust the EV comp.

I found the 21mm Ltd to produce cooler less saturated results with a slight haze. Detail was more finely rendered. The Sigma had a coarser rendition of detail - more prominent and contrasty - nothing wrong with it, just an observation.

The physical size (and feather-light weight) of the 21mm is incredible, especially when compared with the Sigma 17-50mm.

In the end I'm in two minds about the 21mm Ltd, great FL and a beautiful lens (like all Ltds) but I'm not totally convinced by the IQ. The Sigma still holds its head high, it might not be perfect but it does deliver.

 left eye's gear list:left eye's gear list
Fujifilm X100V Olympus E-300 Fujifilm GFX 50S Panasonic GH5 Pentax KP +11 more
Pentax K-5 Pentax KP
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