Sunglasses span an enormous price range and the protective function does not scale with it. A cheap compliant pair and an expensive one may offer identical protection.
What actually matters for the eyes
Ultraviolet filtering, which is the entire protective function.
Standards exist in most markets requiring lenses to block ultraviolet up to a specified wavelength, and compliant products are marked accordingly.
The relevant point is that this is achieved through the lens material and coatings, both of which are inexpensive. A compliant lens costs very little to produce.
Which means protection is a threshold that is either met or not, rather than something that improves with price.
The dark lens danger
The one genuine hazard worth understanding.
Pupils dilate in low light. Dark lenses reduce visible light and cause dilation.
A dark lens that does not filter ultraviolet therefore admits more ultraviolet to the eye than no glasses at all, because the pupil is wider.
Which is why unmarked novelty sunglasses are worse than none, and why the marking matters more than the darkness.
Tint depth and ultraviolet protection are independent properties, and a very light lens can offer complete protection.
The category markings
Worth knowing since they are printed and rarely read.
Lenses are classified by how much visible light they transmit, in numbered categories from clear through to very dark.
The darkest category is unsuitable for driving, and this is stated on the product and widely ignored.
Mid categories are the general purpose range for ordinary conditions.
Categories intended for high glare environments — snow, water, altitude — are darker for a functional reason rather than an aesthetic one.
Polarisation
A genuine functional feature that is frequently confused with protection.
Polarised lenses reduce glare reflected from horizontal surfaces — water, roads, snow — by filtering light oriented in a particular plane.
This produces a real and immediately noticeable improvement in those conditions and has nothing to do with ultraviolet protection.
The trade-offs are that some digital displays become difficult or impossible to read at certain angles, and that pilots and some machine operators are advised against them for that reason.
Where the money actually goes
Being clear about what a premium buys.
Frame materials and construction, which affect durability, weight and comfort, and where the differences are real.
Optical quality of the lens, meaning freedom from distortion, which matters for extended wear and is genuinely better in higher-end lenses.
Coatings — anti-reflective on the inner surface, scratch resistance, hydrophobic treatments — which are real and incremental.
Fit and adjustability, which determines whether they are comfortable and whether they stay on.
And brand, which is a substantial share of the price at the upper end.
The market structure
Worth knowing because it explains the pricing.
Manufacturing, distribution and retail in eyewear are concentrated to an unusual degree, with a small number of companies producing for a large number of brands.
Which has attracted competition scrutiny in several jurisdictions.
The practical consequence is that two very differently priced products may come from the same factory with the same lens supplier, and the difference is design, materials and name.
What I would actually check
The ultraviolet marking, which is the only thing that matters for protection and which takes a second.
The category number, appropriate to what you will use them for.
Coverage, since light entering around the edges is unfiltered, and a wraparound shape protects considerably better than a small lens.
Fit, which determines whether you wear them.
And whether prescription lenses can be fitted, if relevant, which some frames accommodate poorly.
Beyond that the choice is aesthetic, which is a perfectly good basis and is not what the marketing claims to be selling.
Children specifically
Worth noting because the case is stronger than for adults.
Children's eyes transmit more ultraviolet to the retina than adults', since the lens becomes less transparent to it with age.
They also spend more time outdoors on average.
Which means protection matters more for them, and novelty sunglasses without proper filtering are a genuinely poor idea rather than merely useless.
Fit and durability matter more than anything else, since glasses that are uncomfortable or fragile do not get worn.
Prescription and the cost consideration
Since it changes the calculation for a substantial number of people.
Prescription sunglasses cost considerably more than either component alone, which pushes people toward clip-on solutions or contact lenses with ordinary sunglasses.
Photochromic lenses, which darken in ultraviolet, are a genuine option and have a specific limitation — most do not darken adequately behind a windscreen, since laminated glass blocks the ultraviolet that triggers them.
Which is worth knowing before assuming they cover driving.
Damage and replacement
Scratched lenses scatter light and reduce clarity, and the coating on cheaper lenses degrades. A scratched pair is worth replacing rather than persisting with, since the optical quality is the function.