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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?

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  • $\begingroup$ TLDR: In compound lenses (i.e., lenses made from more than one lens element,) one element can somewhat compensate for the defects that are inherent in other elements. Lens elements with simple, spherical surfaces are inherently defective, but until recently, that was the only kind of lens element that anybody could afford to manufacture. $\endgroup$ May 14 at 14:23

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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.

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  • $\begingroup$ Thanks, I think I get it now. That was actually given in the geometric optics lesson, in the combination of lenses subtopic. So, basically multiple lenses reduces the aberrations. $\endgroup$
    – Shyam
    May 15 at 4:29
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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.

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  • $\begingroup$ It's not clear if the OP meant sharpness of a photograph, or sharpness of vision. I had taken the question to be about photography. But what you say is true in either case if you swap between "distance between nerve endings" (receptors?) and "pixel pitch". $\endgroup$
    – garyp
    May 14 at 16:27
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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.

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  • $\begingroup$ Pixel per square inch is the spatial resolution of the detector array, not the image resolution. The two are related in cases where the point spread function is on the order of a pixel. Practical photography cameras (as distinct from scientific cameras) have the point spread function rather larger than a pixel so that the effect of the pixel size on image resolution is negligible. Increasing the ppi does little to improve resolution. For "casual" photographers it is usually of no advantage to increase pixel pitch all other things being equal. $\endgroup$
    – garyp
    May 14 at 16:20
  • $\begingroup$ Would a better description be in the context of information? Each photon carries information about an image. The image that has the highest resolution has the largest amount of information per given area. In this way, a lens system increases image resolution. $\endgroup$ May 14 at 16:38
  • $\begingroup$ I've edited my original post based on your comments. Thanks for the input. $\endgroup$ May 14 at 17:37

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