This is the original lens (3.6mm) that came with the camera. It gives a view angle of 70°. Simply screw it off and screw another lens in (with the same M12x0.5mm thread).
This is the lens I'm going to replace it with. It's a 2.1mm 2MP HD lens (since the FI9821W is a HD camera after all) and gives a view angle of 115°. I got it from eBay from a seller called Andy.lig.
Unfortunately this seller is as of November/December 2016 not active anymore on eBay. But I found that you can buy a similar lens on Banggood that works just as good and is cheaper as well! I'm using this lens myself on my "LIVING ROOM CAM", so check that out to see what the quality is like.
Now on the camera it looks like this. It's a pretty small lens with a rather short screw-thread so it's recessed somewhat into the casing of the camera when screwed in.
This creates the problem of the lens being behind the IR leds causing IR light to shine directly into the lens when the IR leds are on. A distorted image full of glare is the result.
|Original lens 70°|
|Wide-angle lens 115°|