CASA has some good stuff on sunnies in the DAME handbook
http://www.casa.gov.au/manuals/regulate/dame/index.htm
2.1.13 Sunglasses
Glare is often a cause of significant discomfort when flying above cloud or when flying into the sun. Sunglasses may be required in such circumstances.
There are two basic factors to consider when selecting sunglasses, namely the frame and the lenses.
Any spectacle frame reduces the field of vision. Narrow frames that carry large lenses are desirable. The most critical problem with frames arises from the presence of wide sidearms which significantly impair the peripheral visual field.
Sunglass lenses should protect the eyes from glare while not adversely affecting the visual cues necessary for safe flight. Accordingly, lenses should not be too dark, and should transmit at least 15% of incident light. The tint used should be "neutral density" (N.D.), that is, a greyish tint that does not distort colour perception or adversely affect red signal
detection and recognition. The recommended tint is N.D.15.
Lenses of polycarbonate are preferred because of their impact-resistance and ability to absorb ultra-violet and infrared rays. However, these lenses can scratch readily and any scratched spectacles should be discarded.
To ensure that sunglasses provide adequate protection from solar radiation that may damage the eyes, only those sunglasses that conform to the current Australian Standard should be worn.
Sunglasses that conform to the current Australian Standard also meet acceptable standards for lens quality, frame strength and lens retention.
For aviation use, those sunglasses marked "Specific Purpose Sunglasses" are
recommended, provided their frames are appropriate. The lenses of these sunglasses have been specifically designed for use in conditions of intense glare, such as in flight above cloud. At high altitude, atmospheric absorption of ultra-violet radiation is reduced.
Polarising sunglasses should not be used when flying. The polarising filter interacts with the cockpit transparency to produce a distorted and degraded visual field that poses a threat to air safety.
The pilot who already wears prescription spectacles for flying can choose from a number of options for glare protection. Prescription sunglasses with N.D.15 lenses can be obtained, or N.D.15 clip-on or flip-up sunglasses may be worn over prescription spectacles.
Pilots who require correction of their near vision only and who wear "look-overs" are advised to obtain bifocals and a plano upper segment. Clip-on or flip-up sunglasses can then be worn. However, the dangers of flip-ups previously mentioned should be recalled.
I wear prescription sunnies ND15 tint with a graduated focus to read maps