How flies see
A housefly's eye is built from many small units, each looking in its own direction. Even in the sharpest part of the eye, neighboring units point about 2.5° apart, so a still scene reaches the fly as a coarse, blurry picture, far blurrier than for us. Flies see moving things better than this. The simulator shows the still-scene blur only, probably somewhat stronger than in daylight, and leaves the colors as they are, because fly color vision is not modelled here. Point your camera, or open a photo or video, and drag the line to compare.
Colors are left as you see them: how a fly sees color cannot be shown from a photo. Also blurred to 0.2 cycles per degree (one cycle is a dark and a light stripe), from the spacing of the eye's units. This is likely blurrier than a housefly sees in daylight. The long side of the picture is taken as about 70°, a phone camera's view. A phone camera and screen add their own color shifts.
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An eye made of many units
Land (1997) collected the angle between neighboring units of the eye for many insects. For the housefly, Musca domestica, the smallest angle is 2.5°. An eye can resolve at best one dark and one light stripe per two units, so 2.5° means about 0.2 cycles per degree by our arithmetic, and the simulator blurs the picture to that. The same table gives about 1.1–1.3° for a male blowfly (about 0.4 cycles per degree) and 5° for the fruit fly Drosophila (0.1). We read the table of that review, not its full text.
That blur is probably stronger than a housefly's in daylight. Blurring to 0.2 cycles per degree spreads each point over about 5° (the width at half height, by our arithmetic). How wide a patch one light-sensing cell of the housefly takes in, we found only in an old summary: McCann and MacGinitie (1965), from the turning reactions of flies in flight, put it at about 5° in a dark-adapted eye and about 40% less in bright light. So read the picture as the blurriest case. We read only the summary (abstract) of that paper.
People in a two-choice stripe test resolved 32.1 to 44.2 cycles per degree (Lind and colleagues, 2017), more than a hundred times finer than the housefly value.
Moving things look sharper than this
A blur like this is the limit for a still scene. Juusola and colleagues (2017) showed that fruit flies see moving objects better than the classic theory of their eyes predicts: their light-sensing cells respond fast and make tiny, quick movements of their own, and both sharpen the image of things in motion. That was shown in the fruit fly; we have not seen it measured in the housefly. The simulator does not model motion, so for moving things it shows a fly's view blurrier than it is.
Why the colors stay as they are
How flies see color is not modelled here: we have no measurements that we could turn into an honest picture from a photo. So the simulator keeps the colors as you see them and changes only the sharpness.
How the blur is made: the simulator follows AcuityView, the method of Caves and Johnsen (2018). Fine detail is faded along a bell-shaped curve that leaves about 3% of the contrast at the animal's acuity limit, and coarse shapes and colors stay. A picture does not say how wide the scene was, so the simulator assumes that the long side of the picture covers about 70°, the view of a typical phone's main camera (the iPhone 16 main camera is 26 mm in 35 mm terms, about 69° across). If you zoomed in or cropped, the scene was narrower and the fly would see it blurrier than shown. The blur stands for the sharpest part of the eye only. Everything runs on your device: the camera picture, photos and videos are not uploaded.
How Flies See: questions
- How well can flies see?
- A still scene, poorly: neighboring units of a housefly eye point 2.5° apart (Land, 1997), about 0.2 cycles per degree by our arithmetic, against 32.1 to 44.2 for people in a stripe test (Lind and colleagues, 2017). Moving things are another matter: fruit flies resolve moving objects better than their optics alone predict (Juusola and colleagues, 2017).
- Which fly does the simulator show?
- The housefly, Musca domestica. A male blowfly's eye units point closer together, about 1.1–1.3° apart (about 0.4 cycles per degree), and a fruit fly's farther apart, 5° (0.1), by the same table (Land, 1997).
- Is the simulator exact?
- No. It blurs the picture to the spacing of a housefly's eye units only, for the sharpest part of the eye, and assumes the picture covers about 70°. That blur is probably stronger than a housefly's in daylight. It does not model motion, which flies see better than a still blur suggests, and it does not model fly color vision.
Sources
- Land M. F. (1997). Visual acuity in insects. Annual Review of Entomology 42, 147–177. Read in Table 1 only (inter-ommatidial angles). DOI
- McCann G. D., MacGinitie G. F. (1965). Optomotor response studies of insect vision. Proceedings of the Royal Society B 163(992), 369–401. Read in the abstract only. DOI
- Juusola M., Dau A., Song Z., Solanki N., Rien D., Jaciuch D., Dongre S. A., Blanchard F., de Polavieja G. G., Hardie R. C., Takalo J. (2017). Microsaccadic sampling of moving image information provides Drosophila hyperacute vision. eLife 6, e26117. Read in the abstract only. DOI
- Lind O., Milton I., Andersson E., Jensen P., Roth L. S. V. (2017). High visual acuity revealed in dogs. PLoS ONE 12(12), e0188557. DOI
- Caves E. M., Johnsen S. (2018). AcuityView: an R package for portraying the effects of visual acuity on scenes observed by an animal. Methods in Ecology and Evolution 9(3), 793–797. Method read in the package source (R/AcuityView.R), not in the paper. DOI
- Apple. iPhone 16 technical specifications: 48MP Fusion main camera, 26 mm (35 mm-equivalent focal length), which is about 69° across the long side of a 3:2 frame. Link