Pentax 18-135 vs Sigma 17-50 full-size samples

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left eye
left eye Senior Member • Posts: 2,528
Pentax 18-135 vs Sigma 17-50 full-size samples

The Sigma 17-50mm (non OIS) lens arrived, it looks brand new. Just carried out my usual horizon test, to see if there are any serious faults. There are none, all good.

Overall it is a better lens than the 18-135mm (as long as one doesn't need a FL over 50mm!), that said the 18-135mm really is not far behind. I'd say the 18-135mm in parts of the frame is less than good, while the Sigma 17-50mm is usually more than good. The 18-135mm also seems to produce files half a stop underexposed, but that's more of an observation than an issue (I matched the exposures when developing).

The Sigma has better edge performance at all FLs except 35mm - where the 18-135mm is excellent, at it's best. The Sigma seems to have a particular AF inconsistency at 35mm, sequential shots being almost alternating slightly in or out (sometimes at 35mm it's sharpness is outstanding - far better than the Pentax, but not every time).

The DOF (at the same f-stop) seems much greater with the Sigma than the Pentax, an odd phenomena though one I've seen before. The Pentax 18-135mm blurring quicker outside of the focal plane (whether curved or not) - which I think is often a trait of complex 'super zooms'. So although the edge performance of the Sigma at 35mm is not as good as the Pentax, the Pentax is quickly more blurry outside of the focal plane.

The Sigma is slightly larger than the Pentax 18-135, and does look slightly too large on my KP, though doesn't look ugly, more like a serious tool.

From memory my old Pentax 17-70 was maybe fractionally sharper at 17mm, though no where near as sharp at 50mm. I'd say the Sigma is an upgrade. AF is quick and quiet. I might be seeing some slight AF inconsistencies from shot to shot at the longer end 35-50mm, not a lot but could be due to the AF motor being 'stepped' rather than totally linear. The DC motor in the Pentax always seems to exactly hit the same spot each time.


Just to mention the Tamron 17-50mm (non VR - which is known to be better than the VR vers), it seems to have better corner performance at 17mm than the Sigma (possibly they might be equal), though at the long end the Tamron is rather hazy and blurred. The 'field tests' here of the landscape scene I'd say are quite true... need to click in the scrollable sample windows to view them at 100%.


I include my tests from today here, full-res, developed with CA correction on. At f6.3 as this'll be around where I normally try and shoot. I could do further more demanding tests with the aperture fully open, but I'm happy enough not to go any further!

Focal-lengths are approximate, judged via the markings on the lens's barrel, near enough to compare.

Pentax 18-135 @ 18mm...

Sigma 17-50 @ 17mm...

Pentax 15mm Limited (poor edges esp left)...

Pentax 18-135 @ 28mm...

Sigma 17-50 @ 28mm...

Pentax 18-135 @ 35mm...

Sigma 17-50 @ 35mm...

Pentax 35mm Macro Limited (never stellar performance, but great for macro!)...

Pentax 18-135 @ 50mm...

Sigma 17-50 @ 50mm...

I might compare them tomorrow with a closer subject, mainly to check my AF tune settings!

I hope this comparison has been of some help to anyone considering these two zooms.

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