Dissappointed in my Sigma "upgrade" lens

twocolor · Sep 16, 2008

Okay, when I bought my Pentax K20D, I was given an "upgrade" from the kit lens (pentax 18-55mm 3.5-5.6) to a Sigma 18-50mm lens. I was intrigued at the idea of having an "upgrade lens". I only own one lens that didn't come as a kit on one of my pentax cameras.

So, my K20 arrives, and I start shooting. I am EXTREMELY dissappointed in this Sigma lens. The crispness in my photos dropped to below what I would pass off to clients. So, I tried opening up my aperture a hair which I've been told that the middle ground on your aperture is the crispest place on your lens (don't know if I worded that correctly) and still saw terrible results.

My actual first fear was that it was the blame of the K20D. I had read every review of this camera, and had a hard time believing that it could put out such bad photos. Plus, we all know, that your glass is more to blame for good or bad photos than your camera body.

Well, Saturday I shot some senior portraits, and today I did some Maternity shots. This was my chance to test out the lenses, and cameras and decide what the issue is.

The decision . . .
my kit pentax lens is far superior to the Sigma lens that the camera company "upgraded" me to.

Pentax wins out again! Now I'm saving up for my 50mm 1.4! yeah!

So, here's the Sigma Lens (these are SOOC straight out of camera, enlarged to see detail)

and here's the Pentax lent (also SOOC, enlarged for detail)

what's your opinion?

keith204 · Sep 16, 2008

Sigma doesn't necessarily show pride in their quality control. If you're consecutively getting soft photos no matter what you do, send it back to Sigma for a new copy...or, if it's not too late, take it back to your camera store.

Edit: I wonder if camera shops get more commission from Sigma/Tamron than from the big brands. My local shop is always pushing their "Promaster" lineup (Tamron) and is making some irrational and unreasonable statements trying to get people to buy them.

Bifurcator · Sep 17, 2008

The Sigma image isn't just suffering from being out of focus? That's what it looks like to me. It looks like it's focused about 2 inches in front of your nose at something like f/3 or something. <shrug>

Garbz · Sep 17, 2008

The sigma outperforms the Pentax in controlled sharpness tests, not by a large margin, but significantly enough off the centre of the frame. If you get consistent shots like that I suggest getting a replacement.

twocolor · Sep 17, 2008

Bifurcator said:
The Sigma image isn't just suffering from being out of focus? That's what it looks like to me. It looks like it's focused about 2 inches in front of your nose at something like f/3 or something. <shrug>

okay, so if it's just suffering from being out of focus, then why can't I get the sigma lenses to focus, but my 3 other lenses I can hit spot on?????

I agree it looks out of focus, I guess that would be the issue here. I honestly don't think it is a user issue, I've been shooting semi professionaly, and then professionaly for about 7 years, and this is the only lens that gives me these results.

I think I'm going back to the camera store!

joecoulsonphotography · Sep 17, 2008

Sometimes companies like Sigma let some bad ones through the line, like keith said I would just imagine that this is a bad copy and you just need a replacement. Always check lenses after you get them before gong on a shoot with them. I have learned the hard way on taking fresh untested gear to a shoot. I would put the lens and camera on a tripod, take multiple images at different focal lengths and apertures, then do the same with the Pentax. If you get results that are less than desirable from the Sigma, take it back. If the new one has the same problems, I would suggest getting the Pentax "upgrade" lens like a 16-45 f4. I have owned Sigma's pro lenes for years and got great results so I know it's not the company as a whole.

FYI it gets a better rating than the Pentax you have here: http://www.photozone.de/pentax/146-sigma-af-10-20mm-f4-56-ex-dc-pentax-k-review--test-report?start=2

Big Mike · Sep 17, 2008

I've got focus issues with a Sigma lens, it seems that this isn't uncommon.

tirediron · Sep 17, 2008

Try a home-made lens-resolution test. (Using a laser printer preferably) print an 8x10/A4 sheet of paper with nothing but the letter 'X' in capitols, 12pt. Make sure you get as close to the edges as possible.

Tape that to a wall, set your camera up on a tripod at just a couple of inches greater than minimum focusing distance wide open, and measure to make sure that the camera is level and square to the wall. Make three exposures of the paper with your Sigma, at wide open, mid aperture and fully stopped down. Repeat with the Pentax lens.

Download the images and compare 100% crops from the centre and upper-right/lower-left edges. This is by no means a comprehensive test, but it will give you a very good idea of the relative focusing characteristics of the two lenses.

twocolor · Sep 17, 2008

tirediron said:
Try a home-made lens-resolution test. (Using a laser printer preferably) print an 8x10/A4 sheet of paper with nothing but the letter 'X' in capitols, 12pt. Make sure you get as close to the edges as possible.

Tape that to a wall, set your camera up on a tripod at just a couple of inches greater than minimum focusing distance wide open, and measure to make sure that the camera is level and square to the wall. Make three exposures of the paper with your Sigma, at wide open, mid aperture and fully stopped down. Repeat with the Pentax lens.

Download the images and compare 100% crops from the centre and upper-right/lower-left edges. This is by no means a comprehensive test, but it will give you a very good idea of the relative focusing characteristics of the two lenses.

Awesome Idea, thanks! I'll post the results!

twocolor · Sep 17, 2008

The results are in!

At 26mm with an aperture of 4.0, the Pentax lens is superior to the Sigma Lens. Quite extremely so.

At 26mm with an aperture of 11.0, the two are almost identical. If I were to pick one being just a hair better, I would honestly say the Pentax lens was again the better lens.

At 26mm with an aperture of 27.0, the two are even closer to being identical, but, quite honestly, neither of them are "crisp" at that aperture.

Verdict, when stopped all the way down, the Pentax is crisper. It is also very slightly crisper in the middle ground, and both are terrible stopped up.

So, I'm switching back to my Pentax lens, calling the dealer that swindled me into the "upgrade lens" in the first place, and waiting for my Pentax 50mm 1.4 lens to arrive in the mail. Just finished ordering it after I ran these tests!

pez · Sep 17, 2008

My Pentax 18-55 seems very sharp. There is a new Pentax 17-70 kit lens out there...

twocolor · Sep 17, 2008

pez said:
My Pentax 18-55 seems very sharp. There is a new Pentax 17-70 kit lens out there...

I've never had a single issue with mine either! I have never used it at an aperture as high as 27.0 though. Doesn't do much for us portrait photographers! I guess I didn't realize how good I had it, until I had something else!

Bifurcator · Sep 17, 2008

twocolor said:
okay, so if it's just suffering from being out of focus, then why can't I get the sigma lenses to focus, but my 3 other lenses I can hit spot on?????

I agree it looks out of focus, I guess that would be the issue here. I honestly don't think it is a user issue, I've been shooting semi professionaly, and then professionaly for about 7 years, and this is the only lens that gives me these results.

I think I'm going back to the camera store!

I think I would too! In the mean time if you have yard stick or a blank length of stucco wall place your camera at one end and at a 45˚ to it and manually focus for dead center. Then check the results to see if there isn't some section that's very sharp.

Then do the same thing but with a piece of tape as the focus point in the center-spot to see if the focus it on track or off somehow.

Repeat at different distances too. One from 1m, one from 3m, and one from 6m or 8m.

twocolor · Sep 18, 2008

Bifurcator said:
I think I would too! In the mean time if you have yard stick or a blank length of stucco wall place your camera at one end and at a 45˚ to it and manually focus for dead center. Then check the results to see if there isn't some section that's very sharp.

Then do the same thing but with a piece of tape as the focus point in the center-spot to see if the focus it on track or off somehow.

Repeat at different distances too. One from 1m, one from 3m, and one from 6m or 8m.

Another test to try out! Thanks! I think by the time I'm done with all these findings, I'll have a pretty strong case on my side to take with me when I talk to the camera company!

I should find how to send these results to Sigma!

JerryPH · Sep 18, 2008

Its not on a Pentax, but I do have the 18-50 Sigma EX IF HSM MACRO F/2.8 lens. Sigma made 3 versions of this lens... the latest is incredible and in several photography magazine shootouts, beat out lenses that were as much as 3 times more expensive.

Sharpness or focus is NOT an issue with my copy of this lens:

However, it is not unheard of that they let slip a lens out that either front or back focuses. To test for this, there is a PDF document out there on the web that can help you know if this is what is happening or not (http://focustestchart.com/focus21.pdf).

Follow the directions to the "T" and even though this chart was made with a Nikon D70 in mind, it should work on all cameras and all lenses with little to no changes.

I used it to test my Sigma 18-50 F/2.8 and Sigma 30mm F/1.4 lenses. They were if nothing else, spot on or so close as to make no difference in real life.

If you find yours is poorly adjusted, show them the results of this pic and how you did it and it can be used to easily send it back for adjustment or replacement... depending on how long you have owned the lens.

THIS is the result of my tests on the Sigma 30mm (please open this in another page, its the larger res picture). It will give you something to compare against. Note, that I did this test very fast and my paper was not properly rotated. The RIGHT side is the proper side to use in this picture to see the focus performance of this lens in particular.

Good luck!