Sunglasses: Proportion, Lens And The Fit That Fails

Sunglasses are bought by looking at the front and worn from the sides. The measurements that decide whether a pair works are not the ones anyone looks at in the mirror.

The bridge decides where the frame sits

Bridge width determines how high or low the frame rests. Too wide and it slides down the nose; too narrow and it perches high and presses on either side.

Because this is a millimetre-scale fit, two frames of the same apparent shape can behave completely differently. The number is printed on the inside of the temple arm.

Adjustable nose pads solve much of this on metal frames. Moulded acetate frames have a fixed bridge, so the fit is decided at purchase and cannot be corrected.

Temple length and curve hold them on

The arms should run straight back to the ear and only then curve down. A curve that begins too early presses on the side of the head; too late and there is nothing holding them.

Temple length varies more than frame width does and is the usual reason a pair slides forward. It is not visible in a mirror and is easily confirmed by touch.

Acetate arms can be heat-adjusted by an optician, and metal ones bent. Most poorly fitting sunglasses are adjustable, and most owners never have it done.

Proportion relates to the face, not a shape name

The useful guidance is about scale rather than geometry. The frame should be roughly as wide as the face and should not extend beyond the outer edge of the cheeks.

Vertically, the top of the frame sits near the brow line. Covering the brows entirely or leaving a wide gap both look unbalanced, independently of the frame's shape.

Shape advice about matching angular frames to round faces has some basis in contrast, but it is far weaker than the scale relationship and often produces poor results applied rigidly.

Lens function is separate from darkness

Ultraviolet protection is a coating property and is unrelated to how dark a lens looks. A dark lens without it opens the pupil while admitting the light that matters.

Tint colour changes contrast rather than protection. Grey preserves colour relationships, brown and amber lift contrast, and very strong tints distort colour noticeably.

Polarisation removes glare reflected off horizontal surfaces, which is valuable near water and roads. It also renders some screens hard to read, which is a genuine trade rather than a defect.

Where they are worn should drive the choice

A pair for driving needs a lens that does not darken excessively and arms slim enough not to block peripheral vision. Broad arms create a genuine blind spot at junctions.

A pair for bright open conditions can be much darker and can wrap further round to reduce light entering at the sides, which the lens itself does nothing about.

One pair rarely covers both well. Recognising that they are different tools explains why a favourite pair can feel wrong in one setting and perfect in another.