UV protection comes first.
Lens darkness does not tell you how well a sunglass lens protects against ultraviolet radiation. Look for appropriate UVA and UVB protection from a reputable product. A very dark lens without proper UV filtering can be counterproductive because the pupil may enlarge behind the tint.
Polarization is about reflected glare.
Polarized lenses are designed to reduce glare reflected from horizontal surfaces such as roads, water and snow. They can make driving, boating and other outdoor activities more comfortable. They are not required for every person or situation, and they can affect how some digital displays appear, so consider where you will actually use them.
Prescription sunglasses are often worth considering.
If you normally depend on prescription eyewear, standard non-prescription sunglasses can create an awkward compromise. Many frame styles can accept prescription sun lenses, although very curved frames or stronger prescriptions may require specific designs. Ask about what is practical before choosing the frame.
Tint and environment
Grey tints tend to preserve colour relationships, while brown or amber tones may increase perceived contrast in some conditions. Green and other tints create their own visual character. Choose based on comfort, driving needs, outdoor activities and personal preference rather than fashion alone.
Fit still matters outdoors.
Sunwear is often worn for longer periods and in more active conditions than fashion eyewear. Check temple stability, bridge comfort, lens coverage and whether the frame interferes with hats or helmets you regularly use. Good coverage can also reduce light entering from the sides.