For my Pentax K20D I recently bought the Sigma 28 f2.8. I wanted to use it as a standard 50mm equivalent lens for standard photos and especially for low light portrait applications. Since there isn’t that much information about this lens out on the web I compiled this little review to help everyone out there who is wondering whether to buy the lens or not.
A definite plus of the Sigma. It is quite heavy for a prime but this makes it feel really solid. I like the EX finish, other people might not. Nothing is wobbly or cheap-made. The lens hood is also firmly attached to the front, reducing the risk of the objective being dropped during exchange. The lens cap however is not so well designed, because it is hard to get off while the hood is still on.
The autofocus for medium to wide distances is pretty fast for a lens on a Pentax. Only if the autofocus misses and goes all the way to makro you might have to wait a second or two until it is back at infinity. Autofocus accuracy exceeds that of my Sigma 17-70 DC, although in low light it can take a moment until the autofocus locks. Surprisingly it works until it is really dark. In the streets at night the focus works without any assist lamp being necessary. Under all conditions one might experience some “hunting” of the focus, it changes a few times the focus direction until it is really on-target. What is a bit disappointing about the AF is, that you lose the autofocus points at the very left of the frame. The lens does not work with those and focus will give completely back-focused results. The 9 points inside the inner AF-frame do work, but only the middle sensor works really well. Please note, that although my lens doesn’t show it, many users have reported terrible front or back focus! Considering the age of the design Sigma might not be able to adjust it. If you have a K20D you can circumvent this problem by using the build-in AF-correction since the lens’ ID is transmitted properly. In terms of movement the AF does extend the barrel only slightly, eliminating “pump action” dust suck up. The front element does not rotate. But considering the price for 77 mm polarizers it doesn’t really matter
Sharpness is pretty good already at f1.7. It is a bit blurry, but for portraits and many other situations it is sufficient. I found my lens to reach optimal sharpness already at f2.8. At higher f numbers sharpness decreased again.
Scene for crops
Sharpness at different aperture settings. 100% crops at 6 Megapixels. Please not the flare appearing at f16 in the upper left corner.
What is remarkable, is the even distribution of sharpness. This is due to the DG design, which is intended for full frame sensors. So on the K20D’s APS-C crop sensor edge sharpness is good. It by far exceeds the performance of the Sigma 30 mm f1.4, which shows terrible edge sharpness.
Contrast could be better, highlights tend to eat away fine structures like branches and the sun drowns quite a bit of the image in white. A specific problem of the lens is it’s tendency to flares. At the open end flares are pronounced even with the lens hood. Stopped down the optimum is around f8, but stopped down further, another type of flares appears. In my opinion the lens hood is too small, it can be extended a bit with black paper which helps with flares without increasing vignetting.
Only visible wide open.
CA’s are low and can be corrected very well. Purple fringing can occur wide open, I found it to be pretty light however. The Sigma shows some colored edges for out-of-focus objects with high contrast but I have seen worse. Here is what it looks like:
Overview of cropped scene at f1.7
Crop from center of the frame
Crop from edge of the frame with chromatic aberrations.
The chromatic aberrations seen in the above pictures can be complitely eliminated by the Photoshop or Lightroom CA filter.
Here are a few samples. The night time photos are all taken without a tripod at ISO 1600. Enjoy!