Sunglasses aka shades aka sunnies are ubiquitous: (i) part eye protectors, (ii) part disguise, (iii) part fashion statement.
(i) They offer protection against excessive exposure to light, including its visible (e.g. glare) and invisible components. The most widespread protection is against ultraviolet radiation, which can cause short-term and long-term ocular problems. For adequate UV protection, experts recommend sunglasses that reflect or filter out 99 % or more of UV light with wavelengths up to 400 nm.
(ii) Sunglasses can also be worn to hide one's eyes. They can make eye contact impossible, which can be irritating to those not wearing sunglasses. Eye contact can be avoided even more effectively by using mirrored sunglasses. Sunglasses can be used to hide eye movements and emotions since many facial expressions involve the eyes. In all cases, hiding one's eyes has implications for nonverbal communication: sunnies may slightly obscure our view of the world, but they also hide the world's view of how we are feeling.
(iii) Fashion trends can be another reason for wearing sunglasses, particularly designer sunglasses from high-end fashion brands. Sunglasses of particular shapes may be in vogue as a fashion accessory. Fashion trends can also draw on the association with a particular lifestyle. Worn equally by men and women, sunnies can simultaneously say we are funny, cool, trendy, mysterious, intimidating... Once more, the map isn’t the territory and human projection prevails.
For taking street portraits of people with sunglasses, aperture-priority mode (f/2.8 ... f/4.5) and single-point auto focus might be a practical beginning, before one decides to shoot in manual mode. If necessary, changing the angle/position of the subject and/or the camera might help to avoid unwanted reflections.
(iv) Mirrored sunglasses with a reflective optical coating on the outside of the lenses are also useful impromptu props for self portraits.
Portrait photographs with exif data, geotags and other specs in Matt Hahnewald's