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Here are the images, depicting the aperture stops of the objective and eyepiece, taken out from Carson microsopce. Though the lens diameter is large, there is a black disc with a hole used as an aperture stop.

enter image description here

I would like to know how are the aperture stops determined for an optical system such as the microscope mentioned? And why should one need aperture stop? What will be effect of not having one?

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Lenses have the potential to gather dust motes, and may have small inclusions or bubbles which would be distracting to see. So, microscope glass elements are all arranged outside the field of sharp focus of the optics. One literally CANNOT focus on a dust speck on the lens, or any bubble in the glass, in a well-designed microsope.

Thin-lens approximations ensure that the central field of view obeys simple design rules, and is easily optimized by the designer. Outside that central field, though, the image will be distorted and useless. So, a blockage of the useless light improves the image. In the field of sharp focus, it is beneficial to put a black, light-blocking plate with a hole just the right size for the useful field-of-view. That is the aperture stop.

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  • $\begingroup$ My question is how is the "right size" for the useful field of view calculated? How to calculate the useful field of view? $\endgroup$ – karthikeyan Feb 10 '17 at 12:38
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    $\begingroup$ Lens designers use complex computer models (and before that, enormous sheets of paper with very sharp pencils and geometric constructions) to ray-trace through a series of lens elements, and come up with useful ways to form an image. The deviations from 'good' image are quantifiable as chromatic aberrations, coma, distortion,'circle of confusion', etc. A microscope may have a dozen refracting surfaces, there's no simple formula. $\endgroup$ – Whit3rd Feb 10 '17 at 21:05
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An aperture stop changes the focal ratio and reduces spherical aberration.

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  • $\begingroup$ Well that's the purpose of aperture stop. But how to estimate the size of it? $\endgroup$ – karthikeyan Feb 10 '17 at 16:03
  • $\begingroup$ The size is determined by the user's desired outcome. Resolution, contrast, total light gathering ability, magnification limits are all factors that will be affected by the aperture stop. To estimate the size of the stop one must first determine what you want the stop to achieve. $\endgroup$ – Lambda Feb 10 '17 at 16:41

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