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Why do we have to strictly focus on something to really see what it looks like? Is everything else around blurred, or is our brain trained only to see in the center of the image projected on the retina?

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Basically, the retina contains two different kinds of receptors: rods and cones. Cones are concentrated in the fovea and activate ganglion cells more discretely than rods. Rods are more interconnected by horizontal cells (if I'm not mistaken...), so multiple rods can often activate the same ganglion cell, whereas each cone is more likely to have its own ganglion cell. Ganglion cells transmit visual sensory input to the brain, so information that is consolidated into one before transmission can't really be discriminated by the brain as coming from one rod or another if both rods connect to the same ganglion cell. Since cones have more independent ganglion cells, they transmit info to the brain more individually. Where cones are concentrated (the focal area of the retina), the resolution of visual perception is more fine-grained.

Here's a somewhat simplistic illustration:
Note the single ganglion cell connected to all three rods, and the three separate cells for each cone.

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