Testing 17-70 lenses - Sigma vs Pentax

Hi Everyone

I recently acquired a Sigma 17-70 F2.8-4 HSM OS Macro lens from another Forum Member, with the intention of keeping either this or my Pentax DA17-70 as my default walkabout lens. However, since getting the Sigma I have struggled to get sharp images from it, so today I decided to calibrate it and do some testing. In short, there's nothing wrong with the lens, it just needed a '+5' adjustment on both my K-5IIs and my K-3. After sorting out the focussing accuracy, I decided to do some comparison tests with the Pentax DA 17-70 at F4, nothing scientific here, just my own thoughts after comparing images side by side in Photoshop with no processing applied. Bear in mind, these results reflect the copies of the lenses I own and may be different for other copies. I was mainly interested in sharpness at different focal lengths, but also looked at contrast and colour. The controls applied were:

- Camera on tripod, pointed at identical subjects, sharpness assessed on a road sign approx. 70m away
- 2 second self timer used to avoid shake
- F4 used for all images on both lenses
- Centre focus point only
- Images shot at 70mm, 35mm and 17mm on each lens
- Same comparisons were shot with the K-5IIs and the K-3, all results were consistent between the two cameras
- Images were compared in Photoshop at 200%, side by side in a vertical tiling arrangement so it was easy to see the differences

Here's what I found:

Contrast
- The Pentax lens has much higher contrast than the Sigma at all focal lengths

Colour
- The Sigma is 'warmer'- most noticeable with Reds - the Siggy renders reds with more yellow / green in them, making a more 'orangey' red than the Pentax, which in turn has more magenta. The Sigma is probably marginally more accurate with reds, but the downside is that this tendency makes greens look too yellowy! No conclusive win for either here, but because of the Pentax's contrast, the colours look more 'punchy' straight out of the camera with the DA 17-70.

Sharpness
This is where it gets interesting!

70mm - The Pentax blows the Sigma away completely. At 70mm on the Siggy, there are halos around objects and a fuzziness even when accurately focussed, whereas on the Pentax everything is crisp and sharp; quite a surprising result.

35mm - The Sigma blows the Pentax away! This surprised me even more than the first result, the Siggy has better edge definition, overall a sharper image.

17mm - A closer match, but the Sigma just beats the Pentax for overall sharpness.

I would suspect that if both lenses were stopped down to F7.1 or so the results would be almost indistinguishable where sharpness is concerned, but colours and contrast would tell them apart. The other thing I noticed is that the Siggy exposes higher than the Pentax does, at the same aperture with the Siggy at 1/125, the Pentax lens produced a shutter speed of 1/250.

So, what does all of this mean? Which is the better lens? I still have no idea, it depends on how you like images to look and also what focal length you tend to use the lens at. I still don't know which one to keep and which I should sell.

I hope this is useful information, not sure how relevant it is to anyone else but thought it worth posting anyway. Both lenses are very well made and focus at about the same speed, both are silent focussing, the Siggy has a wider aperture at the wide end and also has smoother bokeh than the Pentax does. Pentax has a 67mm filter size, Sigma is 72mm.

Cheers
David

Before looking at the rest of the your testings I must comment on this.

It was indeed a Sigma 17-70 that taught me the lesson to ALWAYS Check focus calibration as soon as you get a new lens and to ignore anyone who says otherwise

When I had my K20D some years ago, I decided to get as second body and a K10D with 17-70 came up on ebay for a great price at the time. I won the auction, received the K10D and lens and took it out on a shoot.

Got home and was horrified at the soft images. It as onyl then I discovered AF Calibration, which the K20D had but the K10D needed enabling debug mode.

After calibrating the images from 17-70 turned really crisp and to this day it's still one of my most used lenses.

here's the AF calibration my 17-70 has needed on all m ypentax cams


K10D 210um
K20D +8
K-5 ( orig ) -5
K-5 1 0
K-5 2 0
K-5IIs +3


As for your tests, yes the 17-70 does fall off IQ Wise at 70mm, but I would try comparing at 60mm jsut incase it's not an extreme end problem, my 135-400 sigma is great up until 350



Have you compared the close focussing ability. One of the reasons I love my 17-70 is it's incredibility close focussing. very useful

Thanks for the comparison -- I'm looking for most consistent sharpness for the buck. If you had to choose, which?

That's a really difficult question. I've always found the Pentax DA 17-70 to be a reliable performer, except on my first K-5 on which it didn't focus properly, but that was the camera, not the lens. The Sigma is cheaper and based on my tests it is as sharp or slightly sharper than the Pentax at most focal lengths. It's not all about sharpness though, the Pentax has a way with colours and contrast that eludes the Sigma, but this can be adjusted in post processing.

If I had to give an opinion based on value for money, the Sigma would win, but if cost is not a big factor it would be a draw for me.

Yes, good point Smeggy, the Sigma focusses much closer than the Pentax and is almost like a macro lens. This adds versatility and the nice smooth bokeh is another advantage for close focussing.

Cheers
David

Value for money is the biggest plus with the Sigma. Indeed it's not all about sharpness ( although too many it seems seem to be too focussed ( excuse the pun ) on that aspect ).

Lenses for me is all about character and that is usually a combination of factors.





Although I love my DA* 50-135 it's close focus ability is terrible

Smeggypants wrote

"it was indeed a Sigma 17-70 that taught me the lesson to ALWAYS Check focus calibration as soon as you get a new lens and to ignore anyone who says otherwise"

As a relative newcomer with a new camera + lenses what is the best way to check focus calibration? Not yet shooting in raw but expect to do so soon (and will be asking for advice on best package - I use a Mac computer).

Thanks