How does the combination of lens create a sharper image? There's a line in a book which states that the combination of lens helps create a sharper image, but I don't understand how. Does more magnification mean sharper image?
 A: It's hard to answer without knowing the context of the statement.  But generally, multiple lenses can reduce aberrations.   Real lenses aren't perfect, and images suffer because of that.  Rays originating at a single point hit the lens at different places and at different angles and they do not converge on a single point.  The image of the point is blurred. It's impossible to design a single lens that does not suffer from these aberrations, although some of them can be greatly reduced by figuring the surface in a profile other than spherical.  Additional lens elements can correct for these defects to a degree, often at the expense of something such as brightness of the image, size of the lens, weight of the lens, or larger-than-desired depth of field. The resulting compound lens will produce much sharper images.  Photographic and cinemagraphic lenses are developed with much effort in design, with high-quality and carefully selected glass types, and with very tight manufacturing tolerances.  So they are expensive. The Leica Noctilux lens, which mates to an "ordinary" (in the sense that you wear it around your neck with a strap while on vacation.  That's the only sense in which a Leica camera is ordinary) photographic camera is expensive.
Magnification generally makes things worse, not better.
A: The distance between nerve endings in the retina of the eye places a limit on the sharpness of an image that you can observe.  A good lens system can bring the image closer and larger.  This can cause the sharpness observed to be limited by other (smaller) factors.
A: I'd like to try explain this in the context of information. Images that have the highest resolution contain the most accurate information per unit area. In optics, this image information is carried by photons. The more photons collected from an image, the more accurately the image can be reproduced. In an ideal lens system, photons are concentrated from a larger "surface area" to a smaller "surface area" they pass through the lens system without alteration. This  increases the information density which increases resolution. In a non-ideal lens system, aberrations distort some of the information carried by the original photons as they pass through the lens system. This decreases the amount of accurate imformation per unit area and decreases overall resolution. In a well designed lens system constructed from good optical materials, these abberations can be considered a deviation from the ideal lens system.
