Does this lens mantain focus as you zoom in/out?

The technical term is "parfocal", whereas "varifocal" is when the focus point changes throughout zoom range.

Anyways, are either of the two lenses parfocal?

Sigma 17-50 OS f2.8
Tamron 17-50 non-vc

Couldn't find much online on the issue. Reason it matters is because I'm going to be using it for video, where parfocal is important.

I doubt information like that can be easily found, though I know for a fact that my 70-200 f/4 IS is varifocal, even if by a fraction. This is something still photography lenses are rather weak at, since it is assumed focus is re-acquired every shot, and there is no backfocus adjustment on the lens itself like there is on video lenses.

By why is this so important to you? I don't often zoom while filming even when using my camcorder, unless I really have to re-frame the shot during a live event, and on a dSLR this is even less of an issue. On a video camera this is mainly useful for focus confirmation, as you can just zoom in, focus, and zoom back out to get the right framing, but dSLRs have 10x magnification built-in... At the very least, cameras with large sensors have much looser tolerances than a camera with a small sensor, so even if it is varifocal it may go unnoticed.

A friend of mine has the tamron.
When you change the range, like going from 50 to 30 or 17, you lose the focus.
My 15-85 also lose focus.

I have three Canon "L" series zoom lenses - the (original model) 16-35 f/2.8L, 24-70 f/2.8L, and (original model) 70-200 f/2.8L IS. NONE of these three lenes is truly parfocal either.

I doubt that you're going to find EOS autofocus lenses that are parfocal, as the reason to make them parfocal is essentially negated by autofocus and the fact that EOS cameras, from their inception, are basically still cameras.

Now that video has been introduced in the EOS line, I suspect there could be some development money spent to make new model "L" series zoom lenses parfocal. I wonder if the new 16-35 and 70-200 MkII versions are truly parfocal. The marketing web pages for the two new lenses does not mention "parfocal" or anything like it in either the overview or specifications sections.

i own the tamron 17-50 non-vc and it's not parfocal. hth