SIGMA 100-400/5-6.3 OS Contemporary is a (relatively) light weight, affordable and high performance telephoto lens. I found the EF mount version is very suitable for adapting to X mount through Fringer adapter. I think Its sharpness is as good as EF100-400L II.
Although its optical performance is excellent, a lot of user found their Sigma 100-400's OS function is not so good. At the beginning I felt the same. I can barely tell whether the OS is working or not. But after some research and tests, I found the key, i.e. setting the OS to dynamic mode. You need a USB dock from Sigma to do that. Use the customized setting and set the OS mode to dynamic!
Remember to save your setting to C1 or C2 and turn the switch to corresponding position. At the same time you may set the focus distance limit for that custom setting to improve AF speed.
Now you can try it. Half press the shutter release button, the image in the EVF immediately become stable! I did some tests at 400mm handheld. When shutter speed set to 1/50s, I got about 80% sharp images. When it's set to 1/25s, I got about 40%. Even at 1/13s I got 20%! Although it's still not as good as some latest Canon lenses' IS function, a level of 3 or 4 stops are not bad!
Be noted, if you upgrade your 100-400 lens to firmware 2.0, it may not work on Fringer adapter with v1.80 or earlier firmware. Please update your adapter. See the descriptions below.
Fringer adapter users may try the latest firmware version (v1.83) which has some enhancement regarding Sigma100-400's support. The PDAF accuracy has been improved. Meanwhile, the OS function only work during your half-pressing of the shutter release button. And the lens with latest (v2.0) firmware will work normally, too. V1.83 is a minor update as its only difference from v1.80 is Sigma100-400's support. Later, these enhancements will be integrated into next major update, i.e. v1.90.