When looking with the unaided eye, light from a single point object reaches all parts of the lens and is focused back onto a single point on the retina.
Light from all points on the lens reaches all point on the retina.
Point defects on the lens affect the quality of the entire image rather than specific parts of it.
Occlusions on or near the lens are equivalunt to using a smaller lens. They predominatly make the image on the retina dimmer and fuzzier.
When optical instruments are used they do most of the focusing.
The light reaching your lenses is already prefocused.
The light going to a single point of your retina then only goes throgh a small part of the lens.
Point defects on the lens only affect small parts of the image
and occlusions on the lens actually block corresponding parts of the image.
This can be worked out usig ray diagrams
A Wolfram interactive demonstration of microscope ray diagrams can be found here.
There is also a youtube video
This post imported from StackExchange Physics at 2015-05-15 23:44 (UTC), posted by SE-user Daniel Mahler