Spectacle lens designs explained: single vision, bifocal, progressive, occupational

Single vision: one power, the whole lens

A single vision lens has the same prescription at every point. It is the simplest design and still the most common one. It corrects distance or near vision, or it is made as a dedicated reading or computer pair.

Because the power is uniform, there is nothing to adapt to beyond the new prescription itself. The main choice is between a conventional spherical surface and an aspheric one. Aspheric surfaces flatten towards the edge, which makes the lens thinner, reduces the magnified look of strong plus lenses and the shrunken look of strong minus lenses, and keeps the image sharper when you look through the edge of the lens rather than the centre.

Single vision suits almost everyone under about forty, and many older wearers who prefer separate driving and reading pairs to one compromise lens.

Bifocal: two zones with a visible line

A bifocal carries the distance prescription in the main lens and a reading segment, usually a D-shaped window, set into the lower part. The line between the two is visible and the jump in power is abrupt. Look at the floor through the segment and the image jumps.

Bifocals have lost ground to progressives but are not obsolete: the reading area is wide and undistorted, the price is lower, and some wearers who never settled into a progressive find a bifocal effortless. The missing piece is the intermediate distance: a computer screen at arm’s length sits between the two zones and can be sharp through neither.

Progressive: a smooth change from distance to near

A progressive lens changes power gradually down the lens with no visible line. The top gives distance, the middle intermediate, the lower part the full reading addition. The channel through which this change happens is called the corridor.

There is a cost, and it is set by physics rather than by the manufacturer. When power changes smoothly along a surface, unwanted astigmatism appears either side of the corridor. This is the soft blur you notice in the lower left and lower right of every progressive lens. Premium designs push it further out or spread it more gently; none removes it.

Adaptation

Most first-time wearers settle in within days; a minority need two weeks. The trick is to point your nose at what you want to see and let the eyes drop into the reading zone, rather than sweeping sideways through the blurred edges. Full-time wear from day one speeds this up.

Why the fitting matters

A progressive is only as good as its position in front of your eyes. The optician measures your pupillary distance for each eye, the fitting height from the lens bottom to your pupil centre, and ideally the tilt and wrap of the frame on your face. A lens fitted two millimetres too high puts the intermediate zone where distance should be.

Corridor length and frame size

The corridor has to fit inside the frame. Designs come in corridor lengths from roughly 11 mm to 18 mm, and the frame needs enough depth below the pupil to contain the chosen corridor plus the reading zone. Very shallow frames force a short corridor, which compresses the change in power and narrows the intermediate zone. Slim frame, short corridor, smaller reading area; deeper frame, wider zones.

Occupational and office lenses

An occupational lens, often sold as an office or computer lens, is a progressive turned upside down in priority. It is built from the reading power upwards and only partly reduces power towards the top, so it covers near and intermediate distances with wide, comfortable zones and gives up true distance vision. Designs are offered in room depths, such as 1.3 m, 2 m or 4 m, matched to how far you need to see from your desk.

They are superb for people who spend the day between a screen, a keyboard and a colleague across the desk, and for anyone who finds the intermediate zone of their everyday progressive too narrow for work. They are not for driving, and most wearers keep them as a second pair.

Digital surfacing and freeform design

Older progressives had the design moulded into the front surface. Freeform lenses are cut point by point by computer-controlled machinery, usually on the back surface, so the design can be calculated for your exact prescription, frame shape and fitting measurements. The gains are wider usable zones and less peripheral blur, especially for high or astigmatic prescriptions. The physics is unchanged, so a freeform lens is a better compromise, not a lens without compromise.

Which design suits which wearer

What to tell the optician

• Your real working distances: screen height and distance, how far you hold a phone, whether you read music or dashboards.
• Any previous progressive that did not work, and what felt wrong: narrow reading, swim when walking, blur at the sides.
• The frame you want, so the corridor length can be chosen to fit it rather than the other way round.
• Whether you want one all-purpose pair or two specialised pairs; two single-purpose lenses often beat one compromise.