Pull edge elements inward to leave margin for optical falloff

What it is

The usable picture in a headset is not a uniform rectangle. Sharpness, contrast, and chromatic aberration all fall toward the optical edge. An element drawn on the geometric boundary of the panel is seen as blur, colour fringing, and a brightness drop — not as the icon in the mock-up. An optical falloff margin pulls edge elements one ring inward so the pixels actually looked at sit where the lens is still clean. This is not the same claim as “type is hard at the edge so don’t put words there”: even a colour patch is unreliable if it sits in the ruined ring.

Why it happens

Fresnel and pancake lenses introduce field curvature, coma, and chromatic aberration at the edge of the field; modulation-transfer function falls with eccentricity. A fixed-focus headset cannot accommodate the edge sharp. Panel luminance often drops at wide angles as well. The “outermost ring of the display” and the “outermost ring where a shape can still be named” are therefore different lines, and the second sits further in. Headset model, IPD, and dirty lenses push that line further in still. Placing an icon at 0% and 100% UV on the panel hands the content to the worst ring of the optics.

Studying it

Measure identification thresholds with a resolution chart or Landolt C at several eccentricities, and draw this headset’s own usable field — not the diagonal FOV on the spec sheet.

Independent variables: eccentricity, headset model, IPD setting, glasses on or off. Dependent variables: contrast sensitivity, smallest identifiable angular size, visibility of chromatic fringe, ratings of blur / colour.

MTP or MTF photographed through the lens is instrument-side evidence, but the wearer still has to be able to read — a centred instrument is not a slipped fit.

Where it stops holding

Research helmets with a very wide, well-corrected field can run a thin margin. On a headset whose field is already narrow, another inset crushes the layout; drop content rather than jam icons into the blur. Eye position in the eye box is often off-centre, so falloff is asymmetric and a symmetric inset can leave one side still blurred. This margin is optical, not a desktop 8-pixel gutter; that gutter is nowhere near enough.

Applying it

• On the target headset, measure the eccentricity where type and icons start to mush. Keep standing elements inside that line. Do not treat the panel’s pixel border as the layout border.
• Thin lines and colour splits suffer chromatic aberration more than large fills; delete or thicken 1-pixel strokes at the rim.
• For multi-device release, set the margin from the optically worst unit, not from the one with the largest advertised field.
• How to check: push a test pattern to the outer ring of the layout and look left and right while wearing it. Obvious fringing or an unnameable icon on either side means the margin is short — pull in again.

Related

• Same group: N3.07.1 Edge content needs a head turn to be seen clearly · N3.07.2 Critical information must not sit at the edge · N3.07.3 The edge is fit for presence cues · N3.07.5 A ring layout fits the shape of the field of view better than a rectangle · N3.07.6 Content outside the binocular overlap is seen by only one eye · N3.07.7 Elements that enter and leave the view with head motion need hysteresis or they flicker
• Nearby: N1.03 Field of View · N3.04 Text Readability in 3D Space
• Search terms: optical falloff · HMD MTF · display edge inset