Vision scientists usually express the size of an image on the retina in terms of visual angle. They use this measure instead of the size of the object or square millimeters of retina covered because it is consistent regardless of the distance from the viewer or the size of the eye. , shows how size, distance, and visual angle are related.
Before continuing, calibrate your monitor with . You will need a ruler to measure 2 inches or 5 cm on the screen and to measure your viewing distance. You only need to do it once for this computer/monitor combination. The results are saved and will be used whenever needed. Many PsyCog activities use this calibration to show stimuli at the proper size.
That’s all very well, but what does 1° of visual angle actually look like? It sounds like a rather small measurement. Use , to see for yourself.
The retina, which encompasses about ±90° of visual angle, varies across its surface area. Some areas have small densely packed cones (responsible for high-resolution color vision), while others have rods and larger cones specialized for low-light vision. Thus, the monocular visual field (the area you can see with one eye) varies in its sensitivity. shows parts of the visual field, corresponding to anatomical areas of the retina. You can also test sensitivity to shape, color, and motion across the visual field. Keep in mind that, because of the eye’s lens, the left half of the visual field corresponds to the right half of the retina.
The optic disc, where blood vessels enter and axons leave the retina has no rods or cones. It corresponds to a blind spot in the retina. Measure your blind spot with , then use to explore how the brain “fills in” vision across this gap.
Many of the experiments discussed in textbooks use sinusoidal gratings to measure visual acuity, test the selectivity of cells in the visual system, or create moving patterns. The gratings are usually described in terms of the visual angle covered by one cycle of the grating. Use , to create your own sinusoidal gratings.
Use and to form and test hypotheses about the visual field.
The fovea is not actually on the optical axis of the eye, but is 4-8° to the temporal side. Most textbook illustrations ignore this, showing the fovea directly behind the lens, and I have followed that convention. For details, see this page and this one This does not affect measurement of the blind spot, which is still the correct offset from the fovea.
The Wikipedia macula entry has a good diagram of retina parts and locations and a good photograph of the retina. Wald (1967) says the central 0.12-0.13° of the retina lacks short-wavelength cones; Magnussen et al. (2001) behaviorally find the central 0.4-0.53° of the visual field to be insensitive to blue.