Dissappointed in my Sigma "upgrade" lens

Discussion in 'Photography Equipment & Products' started by twocolor, Sep 16, 2008.

  1. twocolor

    twocolor No longer a newbie, moving up!

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    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)
    [​IMG]

    and here's the Pentax lent (also SOOC, enlarged for detail)
    [​IMG]

    what's your opinion?
     
  2. keith204

    keith204 No longer a newbie, moving up!

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    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.
     
  3. Bifurcator

    Bifurcator TPF Noob!

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    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>
     
  4. Garbz

    Garbz No longer a newbie, moving up!

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    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.
     
  5. twocolor

    twocolor No longer a newbie, moving up!

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    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!
     
  6. joecoulsonphotography

    joecoulsonphotography TPF Noob!

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    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
     
  7. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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

    tirediron Watch the Birdy! Staff Member Supporting Member

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    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.
     
  9. twocolor

    twocolor No longer a newbie, moving up!

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    Awesome Idea, thanks! I'll post the results!
     
  10. twocolor

    twocolor No longer a newbie, moving up!

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    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!
     
  11. pez

    pez No longer a newbie, moving up!

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

    twocolor No longer a newbie, moving up!

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    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!
     

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