A reader writes:
I know that when you look up at a clear sky, the dark things you see floating around inside your eyeballs are called... "floaters". Someone worked hard on that name, huh.
But what are the much smaller pale scooty things? You know what I'm talking about, right? I'm not the only one who sees them, am I? Are they bacteria or something? Oh god, they're bacteria, aren't they?
Presuming I'm not about to die of pale-scooty-eye-thing-itis, why do I only see these things when I'm looking at the sky?
The pale scooty things are white blood cells.
The blood vessels that feed your retina are located, thanks either to the blind forces of evolution or to the carelessness of a really incompetent intelligent designer, on top of the retina, so light has to pass through the blood vessels before it can get to the retina, and the blood vessels can cast shadows on the retina.
A really big red blood cell has about the same diameter as a really small white blood cell, and white cells are roughly spherical blobs instead of the doughnut-ish shape of the red cells. The result of this is that red cells zipping through the blood vessels over your retina are invisible, but the much less numerous white cells do show up, as little pale scooty things.
(Because floaters really are floating in the goop inside your eye, they move around even when you don't move your eyes. Blood cells are - with any luck - constrained to the vessels over your retina, so the paths of the little scooty things track precisely with your eyes, no matter where you look.)
There are two reasons why you see these things when you're looking at a blue sky, or daytime fog, or a white area on an over-bright computer monitor, for that matter.
The minor reason is that floater-shadows on your retina are pretty low in contrast, and white blood cell shadows are even lower, so they're hard to see if you're looking at something rich in detail. For the same reason, it's hard to notice minor dirt-spots on a computer monitor unless the image being displayed is pretty uniform.
The major reason why floaters and zippy leucocytes show up when you look at the daytime sky, though, is that the sky is bright.
Update: As Bernard points out below, the explanation I originally had here was wrong.
Bright light causes your pupils to contract, and contracted pupils give the eye a higher f-number, and a deeper depth of field. Floaters and blood cells are far from the eye's focal distance no matter what you're focussing on, but with a smaller aperture, they become sharp enough to be noticeable, against a uniform background.
The contracted pupil, as Bernard says, is closer to a point source of light inside the eye, and casts sharper shadows of whatever's in there onto the retina.
Many photographers are familiar with this effect. There can be all sorts of dust and crud on and even in a lens, and dust on the sensor too if you've got an interchangeable-lens camera, without any obvious problems for large-aperture pictures. Faint fuzzy circles may be visible if you look really closely, especially, again, in areas of uniform colour, but even things that you'd think would be totally obvious, like raindrops on the lens, can have surprisingly little effect on a large-aperture photo.
Stop your lens down, though, and the crud-shadows will be much closer to sharply focussed, and much easier to see and obsess over.