The rule from physical optics is that the closer the floater gets to the retina, the more focused it looks. Stuff in the eye that is farther toward the lens is less in focus; junk in the lens itself (as in cataracts) is so out of focus that it is visible only as a general loss of contrast. But individual red blood cells, or spirochetes, are so small that even if right on top of the retina, they'd block less than a single light receptor cell, and thus could hardly be visible. What would be visible would be conglomerations of red blood cells, as in clotted blood.

Thanks Norman

I was at a loss as how to proceed, you have given me clues.

Within the human eye, the cones which are the colour sensitive visual elements responsible for vision in high light intensity (photopic) are roughly 6 to 7 um in diameter while the rods, responsible for vision under low light intensity (scotopic) are roughly 2 um in diameter.

Red blood cells (eryrhrocytes) are in the range of 6 to 8 um in healthy individuals and are in the same size range as cones but much larger than rods . Spirochetes of Borellia burgdorferi are in the size range of 20 to 30 um in length but only 0.2 to 0.3 um wide. They are notoriously difficult to resolve by normal microscopy and are only clearly visible by dark-field illumination.

It is inconceivable that within the vitreous humour of the posterior chamber of the eye, given the laws of optics,  objects in the same order of magnitude as the receptors , should be resolved by the retina.

This leaves the posibility of objects within the aqueous humour of the anterior chamber (in front of the lens)  may be resolved or that objects on or within the cornea may be visible. As many of these floaters move but have a limited range of positions, it seems that they must be within the aqueous humour.

All I need now is for some skilled optical calculator to work out the physics and maths of the problem. Because we can see floaters, there must be a way of derermining the size and position of them.

Just found heaps of references to ray tracing which would give the answer but no free articles.  

R 

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R

In front of the lens, things aren't in focus either, until you get out of the eye and to the distance at which the actual objects are that you are looking at. (Although I've been writing of 'the lens', the majority of the refracting power of the eye comes from the curvature of the cornea; the lens is just the variable part.) You can't focus on things on the cornea; they have to be at least several centimeters away.

The eye isn't a multi-lens system with intermediate points of focus; it's just a single-lens system, except with a lens that's a bit complicated. There are just two points where things are in focus: the object being focused on, and the retina. Anything at any other distance is more or less blurry.