A possible explanation for the Sigma 50 f/1.4 focus issues.

I have a Sigma 50 as stated and was finding that it does miss focus from time to time. After extensive testing I narrowed the majority of the miss focused shots down to when I use spot/center focus with my 7D. Since I happen to run a surfacing/coatings department for a lens manufacturing company I took my Siggy to work and decided to get a better look at what was going on with the glass itself.

The short story is there appears to be a defect that is located almost dead center in the front element. It appears as a line about 10mm and has a ~~\__ look to it. My best guess, without actually removing the front element is either that it's a generator mark from a bad diamond/cutter or a bad mold from the manufacturer. If it is in fact a generator or mold issue this same defect could appear in many other lens of this type so it is something that should be checked for by us end users from now on.

I sent the lens to the Sigma Service center in New York and explained what I was seeing in both verbal and written form so the issue could be addressed. I received the lens back from them last Friday and the only remark they left was that they made adjustments for front/back focusing issues. I checked the lens for the defect and sure enough the defect is still there so they chose to ignore my request to replace the front element or even acknowledge it is even there. Needless to say it is going back and I will be talking to a manager this time about it.

If anyone has the inclination to check their lens for the defect as I described all you need to do is hold the lens under a fluorescent light and look for a break in the reflection as you rotate the lens. The defect in my lens can be seen when held at about a 30 degree angle and about 4-6 feet from the light in normal room lighting conditions.

Thanks for the reply Thorrulz!!! I knew there had to be something more to it!

By the way... Just got home with my 50mm 1.8... It is spot on. It is SHOCKING to see the sharpness difference between two images taken with both of these lenses compared.

Does anyone know an email for Sigma Japan? I can't sit on my hands and wait for them to contact me anymore...

If there is a defect in the center that affects AF it should also be visible if you manually focus in liveview and shoot a test chart (or a newspaper for that matter). Might be worth trying.

I found mine relatively easily because I have the final say in any lens that is shipped or written up and rejected for a defect at the company I work for. What doesn't make it through inspection comes back to me to see if I can save it by refining/repolishing if possible. Needless to say I look at so many defects a shift I am very proficient at finding any defect quickly.

So what happened in the end? Did they replace the front element? Does the lens perform acceptably now?

Thorrulz wrote in post #15311860
I found mine relatively easily because I have the final say in any lens that is shipped or written up and rejected for a defect at the company I work for. What doesn't make it through inspection comes back to me to see if I can save it by refining/repolishing if possible. Needless to say I look at so many defects a shift I am very proficient at finding any defect quickly.

So what ever happened with Sigma? Did the manager acknowledge the defect on the glass?

Good investigative work! I never looked that closely at the 3 versions I tried.

Sigma said the defect appeared initially to be minor and within tolerances. Upon reinspection it could cause focusing issues but they reassured me it was an isolated case and would ship out a replacement if I requested one. I told them I wanted my money back and wound up taking an overall loss of $65. I did not like how when I started questioning how many misfocusing complaints there are regarding the 50 f/1.4 and why there isn't a direct company response I was met with the "oh really, we weren't aware of such a common issue with this particular lens". We hope you will find your next Sigma purchase satisfactory.

Keep waiting Sigma, keep waiting.

i'm curious, was yours a recent purchase? the three i tried were just after this lens came on the market, i attributed the problems i was seeing to a new product - and had hoped they'd cleaned up their QC.

rick_reno wrote in post #15312405
i'm curious, was yours a recent purchase? the three i tried were just after this lens came on the market, i attributed the problems i was seeing to a new product - and had hoped they'd cleaned up their QC.

I believe mine was bought new from B&H Photo in March or April of this year. It was one that had the newer finish as well. To bad the quality control got lost in the manufacturing of this lens. It could have been a real winner.

Sometimes if there is an adherent design flaw in how the element was originally produced. Or the machinery can't stay within the tolerances needed on that particular element on a reliable basis, the same problems will keep reappearing during the surfacing of the lens.

I'm on my phone while Christmas shopping so bear with me if some of what I type looks odd.

Tsk tsk. Sigma quality control.

Do you think it would have been possible to photograph the defect? Would be great if we could get the Sigma owners to post images of the glass to see if it truly is a recurrent marking.

I am still curios whether you can make the glass imperfection in the center show up in e.g. a picture of a wall map or something else flat.

The strange thing about the copy that I am trying to get replaced now after being calibrated twice is that it focuses perfectly every time with live view on my 5D2 no matter where I have the microadjustment set to. It is however, painfully cumbersome to shoot live subjects at 1.4 with live view...

Another member of this forum said his behaved the same way. Shots through the viewfinder are all frustratingly out of focus. Microadjustment only helps at fixed differences and as soon as you shoot anything that is not a laboratory shot it all goes pear shaped.

If you're in live view and not using quick AF it uses a whole different kind of AF (contrast vs phase detection)