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

mannetti21 wrote in post #15313556
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.

uOpt wrote in post #15313686
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.

Since the glass elements are clear and not polarized it it becomes more difficult to see a defect such as what I'm describing. The best way to look for the defect without specialized equipment at home would be to place the lens under a light and hold it at roughly a 30 degree angle while slooooowly rotating the lens.

Here is what the specific defect I am talking about looks like from a direct overhead viewing angle once you locate it on a bad element. Not a perfect description or illustration but now you have an idea of what I have been talking about in my earlier posts.

I shoot from the outer focusing points 80% of the time on the 7D so it was hard for me to initially blame a defect such as this particular one. Only after I started scrutinizing my focusing points where most of my OOF shots were coming from did I start to do the detective work that showed a surfacing defect.

Wow, 5-7mm seems like it would actually be a fairly large defect. Does it look like a scratch, discoloration, haze, etc? I can't wait for people to start examining their lens, especially the people who have problems, to see if this is present on their glass. Nice work.