Chromatic aberration is always present when a single lens is used.  The problem is that the edges of a lens resemble a prism and as the light is bent at slightly different angles separating into different colors.  An object viewed through a single lens appears ringed with color.

Chromatic aberration is the variation in focal length of lens with wavelengths of light.

Why is the greater part of the photograph out of focus but the image on the phone is in focus?

Both the dog and the dog bed in the iPhone are a virtual image of the actual dog and the dog bed.  They are "objects" for the lens of the camera that took this photograph.  Since the objects are at a different distance from the camera lens, their respective images are at different distances with respect to the film in the camera.  Only one can be brought into focus.

The same is true of your eyes. You cannot focus on near and far objects at the same time.

A human eye is similar to a camera.

The amount of light that enters the eye is regulated by the iris, the colored part of the eye.  The iris surrounds the opening called the pupil.  Light enters through the transparent membrane covering called the cornea.

When light enters the eye it is focused on a layer of tissue at the back of the eye, the retina.  The retina is extremely sensitive to light.  There is a small region in the center of our field of view where we have the most distinct vision.  This is called the fovea.  There is also a spot in the retina where the nerves carrying all the information leave the eye in a narrow bundle.  This is your blind spot.  Your right eye compensates for your left eye's blind spot and your left eye compensates for your right eye's blink spot.

Muscles can change the shape of the lens, thereby changing its focal length.  When the muscles are relaxed, you can see far away objects.  When the muscles contract, the focal length of the lens is shortened, and this allows you to see images of objects 25 cm or closer.

A nearsighted person can see close objects clearly, but does not see distant objects clearly.  Their eye ball is too long.  Light rays converge in front of the retina.

The eyes of a farsighted person form images behind the retina.  Their eyeball is too short.  Farsighted people have to hold things more than 25 cm away to be able to focus on them.

According to the American Optometric Association, most people have some degree of astigmatism. Astigmatism is an irregular curvature of the eye’s cornea or lens that disrupts the refraction of light rays. A healthy eye should have a round shape similar to a basketball, while an eye with astigmatism has a shape more like a football.  With astigmatism, light focuses on several points of the retina rather just one point.”

Binoculars, like telescopes, produce magnified images of faraway objects.  Each side of the binoculars is like a small telescope:  light enters a convex objective lens, which inverts the image.

The light then travels through two prisms that use total internal reflection to invert the image again, so that the viewer sees an image that is upright compared to the object.

Like a telescope, a microscope has both an objective convex lens and a convex eyepiece.  However, microscopes are used to view small objects.  The figure shows the optical system used in a simple microscope.

A compound microscope enlarges an already enlarged image.

A camera consists of a lens and sensitive film mounted in a lightweight box.  In many cameras, the lens is mounted so that is can be moved back and forth to adjust the distance between the lens and the film.  The lens forms a real inverted image on the film/screen.

A shutter and a diaphragm regulate the amount of light that gets to the film/screen in the camera.  The shutter controls the length of time that the film/screen is exposed to light.  The diaphragm controls the opening that light passes through to reach the film.

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