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Numerical Aperture of a Lens

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  1. Aug 29, 2014 #1
    Hello Forum,

    The numerical aperture NA of a lens is usually a parameter (adimensional) that indicates the lens ability to collect light. The concept of NA is usually introduced in fiber optics where we are concern with launching light into the fiber. But NA is also discussed in microscopy when dealing with resolution.

    Let's consider a simple spherical lens of diameter D and focal length f and place an object at a predetermined distance from the lens. The NA of that lens is a measure of its ability to gather light and resolve fine details in the object.

    So the larger NA the higher the resolution and the brightness of the image, correct?
    Higher values of numerical aperture allow increasingly oblique rays to enter the front lens and produce a highly resolved image.

    So image resolution depends on the diameter D of the lens, the material the lens is made of (its index of refraction) and shape.

    Two spherical lenses made out of the same material, with the same size (i.e. the same diameter D) can have different focal lengths f hence different NAs correct? The focal length of lens is determined by the lens surfaces shapes and material....

    If a bundle of parallel rays enters two spherical converging lenses with the same diameter D but different NA, what happens to the bundle of rays? Do they get focused at different distances? In the case of parallel input rays there is no need to discuss the light gathering capability of the lenses....

    Thanks,
    fog37
     
  2. jcsd
  3. Aug 29, 2014 #2

    davenn

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    Science Advisor
    Gold Member

    have a look at this site
    http://coinimaging.com/photo_articles.html

    in particular these 3 sections :smile:

    19: The aperture and resolution
    20: The aperture and resolution - Graphical
    23: Numerical aperture

    cheers
    Dave
     
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