The Eagle Eye
An eagle's eye is about the size of yours. So how does it spot a fish from high above? Open it up in 3D, watch it focus, find its two foveae, compare its colours with yours, and find out which famous "facts" are myths.
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The look
Eagle eye and human eye, side by side
| Bald eagle | Human | |
|---|---|---|
| Eyeball length | 26.6 mm (1.05 in) | about 24 mm (0.94 in) |
| Head width at the eyes | about 6 cm (2.4 in) | about 14 cm (5.5 in) |
| Foveae per eye | 2: deep (45° to the side) and shallow (15°, forward) | 1 |
| Finest detail (best-measured eagle: wedge-tailed) | about 140 cycles per degree | 60 to 72 (sharp eyes); 30 = "20/20" |
| Cones at the fovea | over 450,000 per mm², 1.6 µm apart | about 199,000 per mm², 2.4 µm apart |
| Focusing range | 6.8 dioptres typical (9 to 10 max): to about 15 cm (6 in); cornea and lens | about 7 dioptres when young, falling with age; lens only |
| Colour cones | 4, each with a coloured oil droplet, plus double cones | 3, no oil droplets |
| Shortest wavelength seen | about 350 to 360 nm (violet edge of UV) | about 390 to 400 nm |
| Field of view | about 260° (blind about 100° behind) | about 200° |
| Blinks | about 22 per minute (third eyelid too) | about 17 per minute |
Try it: what you see and what an eagle sees
Move the object away. Both pictures are shown at the same size, so you compare the detail each eye can resolve (a sharp human eye: 60 cycles per degree; the wedge-tailed eagle: 140).
Myth or fact?
Quiz: what do you remember?
Sources and notes
- Bald eagle eye size and optics: Kuhn et al. 2015 (ultrasound biometry, keratometry); accommodation: Glasser, Murphy, Troilo & Howland 1997 (bald eagle, golden eagle, red-tailed hawk).
- Two foveae and the spiral approach: Tucker 2000, J Exp Biol 203; Tucker et al. 2000.
- Acuity and cone density: Reymond 1985 (wedge-tailed eagle); Curcio et al. 1990 (human); review: Potier et al. 2020; Mitkus et al. 2018.
- Visual fields: Martin & Katzir 1999; Potier et al. 2016. Pecten: Pettigrew, Wallman & Wildsoet 1990. Blinking: Kirsten & Kirsten 1983.
- Colour vision and UV: Lind, Mitkus, Olsson & Kelber 2013 (J Exp Biol, raptor ocular media and vole urine); Ödeen & Håstad 2003 (opsin types); bald eagle SWS1 sequence (NCBI XP_010567082.1); Govardovskii et al. 2000 (pigment template); Hart & Vorobyev 2005 (oil droplets); human cones: Dartnall et al. 1983, Stockman & Sharpe.
- Not measured in the bald eagle itself: acuity, cone density, visual field, pecten size, cone sensitivities. Where the page uses another eagle or raptor, it says so. The colour curves are a model built from the bald eagle's gene and measurements in the common buzzard and sparrowhawk.
- Photographs: Christian Sasse (SassePhoto): adult in snow (Nikon D4S, 2015), close-up (Nikon D4, 2012), young eagle (2013). 3D models: original, built from the measurements above.