Diopter

A diopter (also spelled dioptre, symbol D) is the unit of optical power of a lens or curved mirror, equal to the reciprocal of its focal length in meters, so that 1 D = 1 m^-1. The same unit is used for eyeglass and contact lens prescriptions, where a minus value corrects nearsightedness and a plus value corrects farsightedness. Vision scientists also use diopters to express viewing distance, as the reciprocal of the distance in meters.

In virtual reality and augmented reality, diopters come up in three places. Headset optics place the image at a fixed focal distance, and vision researchers express that distance, and the size of the vergence-accommodation conflict it produces, in diopters. Many headsets and display glasses have "diopter dials" or myopia adjustment that let nearsighted users focus the image without wearing glasses. Research varifocal displays give their focus range in diopters as well.

Reviewed 27 September 2026. Checked every figure, date, quote and product range against the cited OpenStax, AAO, de Jong (Eye 2021), Shibata et al. 2011, Xiong et al. 2021, Turnbull and Phillips 2017, Microsoft, Meta, SIGGRAPH, UploadVR, MIXED, HTC, Shiftall, Rokid, VITURE and Apple sources. About review dates.

Definition

The power P of a lens is defined as the inverse of its focal length, P = 1/f, with f in meters. Converging (convex) lenses have a positive focal length and positive power; diverging (concave) lenses have a negative focal length and negative power. A magnifying glass with a focal length of 8.00 cm (0.0800 m), for example, has a power of 12.5 D.

The eye is itself a strong optical system. With the typical 2.00 cm lens-to-retina distance used in the OpenStax physics textbook, the power of the eye ranges from 50.0 D for relaxed distance vision to 54.0 D when fully accommodated for close vision. The same textbook notes that the near point, the shortest distance at which the eye can focus sharply, increases with age, becoming meters for some older people.

Distance expressed in diopters

Because optical power and distance are reciprocal, a distance can be written in diopters by taking 1/d with d in meters. Shibata, Kim, Hoffman and Banks express both vergence distance and focal distance this way in their 2011 study of stereo display comfort. On this scale, an object at optical infinity is at 0 D, one at 2 m is at 0.5 D, and one at 25 cm is at 4 D. Meta's 2023 varifocal prototype paper uses this equivalence, describing an accommodation range "from 0 to 4 diopter (i.e., infinity to 25 cm)".

Prism diopters and meter angles

Two related units appear in eye care and vision science. The meter angle, a measure of vergence distance, is mathematically equivalent to the diopter. The prism diopter (symbol Δ) is different: it measures how far an eye's line of sight is rotated, with 1 Δ displacing gaze horizontally by 1 cm at a distance of 1 m. Shibata and colleagues note that, unlike the diopter and the meter angle, the prism diopter depends on the observer's interocular distance. In VR and AR hardware, prism diopters appear in prescription handling. Apple's Digital Prism Correction feature for Apple Vision Pro, for example, supports prism prescriptions up to 7.75 prism diopters horizontally and/or vertically.

History

The word comes from the Greek dioptra, a sighting instrument for measuring angles; in a 2021 editorial in Eye, ophthalmologist Paulus de Jong traces how the term moved from instruments to a unit of lens power. Before the metric unit, F. C. Donders expressed the dioptric power of lenses in Paris feet or inches, and lenses were customarily numbered by the radius of their curvature.

At the 3rd International Congress of Ophthalmology in Paris in 1867, a commission was set up to replace the foot with the meter in measuring refraction. It reported at the 4th congress in London in 1872, where Javal proposed on its behalf a unit lens with a focal length of 240 cm. Albrecht Nagel proposed instead a lens of 1 m focus as the unit. Ferdinand Monoyer, who had headed the ophthalmology clinic at the new medical school in Nancy since 1870 and had not attended the congress, objected to Javal's choice and proposed in 1872 that the unit of refraction be the power of a lens with a 100 cm focal distance, which he named the dioptrie. Some writers hold that Monoyer derived the name from Dioptrice, the title of Johannes Kepler's 1611 treatise on optics. Nagel initially considered the new name superfluous, but at the 5th International Congress in New York in September 1876 he argued for adopting the 1 m lens, "to be called the meter-lens, or if so desired Dioptrie", as the unit for all spherical and cylindrical lenses.

Eyeglass prescriptions

In a spectacle prescription, the sphere (SPH) value is the lens power needed to correct vision, written in diopters; the American Academy of Ophthalmology gives the example of -9.00 D, meaning 9 diopters of nearsightedness. A minus sign indicates nearsightedness (myopia) and a plus sign farsightedness. The cylinder (CYL) value gives the amount of astigmatism, the axis (in degrees from 1 to 180) gives its orientation, and the "add" value is extra power for near vision, often used by people over 40 who need reading correction.

These values matter for XR hardware because most built-in focus adjustments correct only the spherical part of a prescription. HTC states that diopter dials help with nearsightedness but "do not correct astigmatism", and that users with astigmatism or complex prescriptions may not benefit from them. Users with such prescriptions can use prescription lens inserts; Shiftall, for example, recommends optional add-on prescription lenses for its MeganeX 8K Mark II headset.

Focal distance of headsets

A VR headset places the display panel in front of the eye with imaging optics in between. Xiong, Hsiang, He, Zhan and Wu note in a 2021 review that conventional VR headsets use a transmissive lens with a focal length of about 4 cm, which corresponds to a power of about 25 D. Turnbull and Phillips likewise describe HMD lenses as "relatively high powered (approximately 25D)" in their 2017 study, and point out that looking away from the lens centers through such lenses induces large prismatic effects.

In most current VR modules the distance between the panel and the optics is fixed, so the whole image sits at one focal depth. The table lists published focal distances and their dioptric equivalents (calculated as 1/distance).

Device or source	Stated focal distance	Equivalent in diopters
Microsoft HoloLens (1st gen) and HoloLens 2	"approximately 2.0 m"	about 0.5 D
Windows Mixed Reality immersive headsets (general guidance)	"between 1.25m-2.5m"	about 0.4 to 0.8 D
Oculus Rift DK2 (as used by Turnbull and Phillips)	"approximately 1-meter from the headset"	about 1 D
Current VR headsets (Meta's 2023 description)	"about 1 meter or so"	about 1 D

Microsoft's design guidance treats optical infinity as starting at roughly 6 m for normal vision. It recommends placing HoloLens content between 1.25 m and 5 m, fading content out at 40 cm and clipping it at 30 cm. In dioptric terms the recommended zone runs from about 0.8 D to 0.2 D around the display's 0.5 D focal plane.

Vergence-accommodation conflict

In natural viewing, the eyes converge on an object and the eye's lens focuses (accommodates) to the same distance. In a stereoscopic headset, the eyes converge on the rendered depth of an object but must accommodate to the fixed focal distance of the display to keep the image sharp; the mismatch is the vergence-accommodation conflict. Researchers commonly state its size as the difference between vergence distance and focal distance in diopters.

Shibata and colleagues measured discomfort for conflicts of controlled dioptric size. They found that conflicts of a given dioptric value were slightly less comfortable at far viewing distances than at near ones, that content behind the screen was less comfortable at far distances, and that content in front of the screen was less comfortable at near distances. In estimating the width of a "zone of comfort", they assumed a minimum of 0.3 D, corresponding to the eye's depth of focus, and a maximum of 0.8 D, the conflict size that had caused discomfort in their second experiment. Microsoft's HoloLens guidance advises keeping content that users converge on as close to the 2.0 m focal plane as possible and limiting how often users view content that moves in depth or switch focus rapidly between near and far holograms.

Turnbull and Phillips tested 40-minute sessions in real and virtual environments with an Oculus DK2. They found no difference in binocular posture, amplitude of accommodation or stereopsis after VR use compared with the real-world sessions, but did find a significant thickening of the choroid (about 10 microns) after each VR session. The authors suggested this may mean a VR headset is not a myopia-inducing stimulus, despite the close physical viewing distances.

Diopter adjustment in headsets and glasses

A number of VR headsets and display glasses include per-eye focus dials so that nearsighted users can see a sharp image without glasses. Every published range in the table below is in the minus (myopic) direction.

Device	Type	Stated adjustment
HTC Vive Flow	VR glasses	"Adjustable diopter dials that allows easy focus adjustment for each lens"; range not stated on the spec page
HTC Vive XR Elite	Mixed reality headset	Sliding diopter dials up to approximately -6.0 diopters per eye
Shiftall MeganeX 8K Mark II	PC VR headset	0D to -7D
Rokid Max	Display glasses	0.00D to -6.00D, adjusted per eye with knobs
Viture Luma	Display glasses	Up to -6.0D
Viture Luma Pro	Display glasses	Up to -4.0D

These adjustments change only the focus of the displayed image. Rokid points out that the Rokid Max myopia correction "applies only to the content displayed via the lenses, and will not make your real-life environment look clearer." Shiftall notes that users with farsightedness or astigmatism may still have difficulty seeing clear images with the MeganeX 8K Mark II's 0 to -7D range, and that the headset's focus dial has a diopter-zero mark for users with good eyesight or add-on prescription lenses. Rokid and Shiftall both point users whose prescriptions fall outside the dial range to add-on prescription lenses.

Prescription inserts

Headsets without focus dials can be fitted with prescription inserts. Apple Vision Pro uses ZEISS Optical Inserts, including "Readers" versions for people who only need near correction. The Readers come in three strengths (+0.75 to +1.25D, +1.50 to +1.75D and +2.00 to +2.75D); after pairing, Apple says they are translated to the corresponding actual lens power and appear in Settings as +0.25D, +0.50D or +0.75D. Apple also offers Near, Default and Far presets and advises choosing Near for eye strain when viewing close items and Far for eye strain with distant content such as cinema mode.

Varifocal research

Displays that change their focal distance to follow the user's gaze aim to remove the vergence-accommodation conflict. Xiong and colleagues list multifocal and varifocal displays, holographic displays and integral imaging among the methods that produce an accommodation cue, and note that the most direct approach in AR optics is to add a tunable element such as a liquid lens or Alvarez lens.

Reality Labs Research has worked on varifocal headsets since 2015. Its Half Dome prototype, shown in 2018, used eye tracking to move the displays forward and back mechanically to change focus; Half Dome 2 (2019) used more reliable actuators, and Half Dome 3 took an approach with no moving parts. At SIGGRAPH 2023, a team including Douglas Lanman presented a retinal-resolution varifocal headset (shown by Meta as Butterscotch Varifocal) with up to 56 pixels per degree that supports eye accommodation "from 0 to 4 diopter (i.e., infinity to 25 cm)". Meta's blog described the system as moving the display closer to or farther from the eyes depending on where the user is looking.

See also

• Vergence-accommodation conflict
• Varifocal display
• Prescription lens inserts
• Accommodation (eye)
• Eye box
• Virtual image
• Interpupillary distance