Flat sapphire does not have optical properties.
Single domed sapphire has a magnifying effect that depends on the height of the dome. It works like a magnifying glass. When you look through it from an angle, the view is also distorted due to the dome. Thicker glass, height of the dome: these factors count in the magnifying effect and the distortion. With the smaller Seiko5 models, the glass is thin and the dome is very low so the effect is marginal. With the SKX007, the glass is thick and the dome is high, so you see an enlarged dial when you look straight in the watch and much distortion when you look from an angle.
Double domed sapphire has almost no optical effect, due to the inner dome. No enlarged dial, no distorted image.

 

I always regulate mechanical watches after I have opened them, so when I install sapphire, I regulate the movement. If you send in a watch for sapphire, I first check how the movement runs. When it does not run OK, I contact you first to see how to continue. All sapphire that I install has AR coating (usually blue) on the inside. The original hardlex is thrown away, I have no use for it. If you want to have the hardlex too, add that in the comment of your order. I love sapphire, that is why I am specialized in it and have it for so many models (so no need to ask my opinion about sapphire).

When sapphire is asked for, mind that the order will take one to two weeks longer to be fulfilled. The watch has to be dis-assembled, hardlex removed, sapphire installed and the watch has to be assembled correctly again (it still has the Seiko warranty after I install sapphire!).

When sapphire is installed in a new Seiko watch by me, the Seiko warranty stays valid. When you open your new Seiko, remove the glass and install sapphire, the Seiko warranty is void. This applies to ALL modifications that involve opening the case.

The water-resistance is not affected, it can even be improved for used watches because I replace old gaskets.