divemask distorting view?

[seaflower]

hello,
maybe someone can shed light on a problem I have.

I own several different goggles and masks, and one of them- a clear Mares X-Vision mask-
distorts my underwater view quite strangely everytime I use it.
It looks as if the entire floor/bottom were deeply curved although the bottom is perfectly flat (a pool, for example).
It's incredibly hard to judge distances due to this- the view sometimes changes from being concave to convex, making me dizzy and almost motionsick.

I have never experienced this with any of my other masks (although I usually use big goggles, not diving masks).
So...is this normal? The lens looks flat, so this shouldn't happen, right?

 

[Pinniped72]

If the lens is flat it shouldn't be happening, dive masks with curved plastic lenses are well known for distortion but flat tempered glass should not distort your vision, magnify maybe but not distort. For me personally clear undistorted vision is a must in a dive mask as I dive alone and have no back up if all goes wrong, so do everything I can to swing the odds in my favour