When an object is close to us, it takes up a large percentage of our field of view. As an extreme example, if we hold a business card up right in front of our eye, all we can see is the business card. If we hold it so that we can see the entire card and nothing else, we can say that it takes up 100% of our field of view or it is covering 100% of our retina. This also means that every one of the millions of photoreceptors is transmitting light impulses to the brain from this object Monocular Telescope.

If we now create a 5X5 sheet of business cards and move it back until the sheet covers our entire field of vision, we would say that each card takes up only 1/25th of our field of vision. This means that, on average, each card only has 4% of our photoreceptors trained upon it. Therefore we are receiving, on average, only 4% of the information from each card as we received when the card was close.

Thus, the farther away an object is - the smaller amount of our field of view it takes up. As an object becomes smaller in our field of view, it impacts fewer and fewer photoreceptors. The fewer photoreceptors affected, the less total optical information collected and the less clear the image becomes. At some point, the incoming light will affect such a low number of photoreceptors that the object will become invisible to us.

In the many telescopes for sale, this is where the eyepiece comes into play. The large objective lens (in refracting telescopes) or primary mirror (in reflecting telescopes) collects incoming light. This light is then focused through the eyepiece lens in order to magnify it. This magnification process causes the image to get larger, and therefore, cover a larger percentage of your field of view. This, of course causes the image to impact more photoreceptors in your eye - and thus - the image becomes clearer.
 

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