Specs - other issues
Bahn-Jeaux,
I returned to flying recently after a break of many years. I had worn varifocals for twenty odd years previously and continued to wear them when I resumed flying. I am still wearing them now and would not like to have to convert onto bi or tri focals. Yes, you can get a distortion when your eyes are turned well to one side or the other, but I find the horizontal line of bi-focals much more annoying.
There is little of what other posters have said that I would disagree with. I would merely add that I feel you should stick to using in the air whatever you are used to using on the ground. I would, however, add the following.
First of all, remember that for flying varifocal specacles need to have good big lenses with plenty of depth. The modern shallow styles are OK for fashion purposes, but might be too sensitive to the angle of your head for flying in.
Go for thin, metal frames. These are much more comfortable under a headset and less likely to interfer with the seal of your headphones.
Plastic lenses are a lot lighter than glass (especially the ultra-thin ones I use), and are probably safer also in an accident. Plastic lenses can also be provided with three coatings all at once; i.e. anti-scratch, anti-reflection and anti-UV: get all of them, they are worth while. (The cheaper opticians will insist only two can be applied together - rubbish)!
Some authorities don't like photo-chromic lenses because of the time it takes them to clear after bright light is removed, so I don't use these. I have had graduated tints added to my varifocal sunglasses with the angles chosen to ensure that my eyes are shielded from the sun, but the instrument panel is unobscured.
When I go flying, I have three pairs of specs with me, first the fashion specs I wear all the time and that I drive to the aerodrome in, second a pair of large lensed clear specs, third a similarly large lensed pair of sunglasses as described above. All of these are varifocals.
Broomstick