What is Spherical Aberration?

Spherical Aberration is an optical problem that occurs when all incoming light rays end up focusing at different points after passing through a spherical surface. Light rays passing through a lens near its horizontal axis are refracted less than rays closer to the edge or “periphery” of the lens and as a result, end up in different spots across the optical axis. In other words, the parallel light rays of incoming light do not converge at the same point after passing through the lens. Because of this, Spherical Aberration can affect resolution and clarity, making it hard to obtain sharp images. Here is an illustration that shows Spherical Aberration:

Spherical Aberration

As shown above, light rays refract or change their angle when passing through the lens. The ones closer to the top and the bottom of the illustration end up converging at a shorter distance along the optical axis (black/red dotted line), while the ones closer to the optical axis converge at a longer distance, creating different focal points along the same axis. The point of best focus with the “circle of least confusion” is illustrated as the thick green line. Spherical Aberration is not just caused by lens design, but also by the quality of the lens material. Lenses made of poor quality material and large bubbles can drastically impact light refraction.

A perfect lens would have all light rays converge in a single focal point, as illustrated below:

Corrected Spherical Aberration

The best focus point with the circle of least confusion is therefore located right on this focal point. A normal spherical lens design would not allow the above to happen though, so specialized precise methods by manufacturers have been developed over the years to reduce the effect of spherical aberration.

Ways to Reduce Spherical Aberration

Modern lenses employ different techniques to dramatically reduce spherical aberration. One of the methods employs using a specialized apsherical (meaning non-spherical) lens surface, which is curved outwards on one side for the sole purpose of converging light rays into a single focal point, as illustrated below:

Aspherical Lens

Spherical aberration is most pronounced when the diaphragm of the lens is wide open (maximum aperture). Stopping down the lens even by a single stop dramatically reduces spherical aberration, because aperture blades block the outer edges of spherical lenses. A clear example of this can be found in the focus shift article.

Coming soon: Chromatic Aberration, Distortion, Coma, Vignetting, Flare/Ghosts, Diffraction and more.

Comments

  1. 1
    ) Peng
    October 14, 2011 at 10:23 pm

    Good job Nasim! Thanks for sharing.

  2. 2
    ) Sam
    October 14, 2011 at 10:26 pm

    Good tip. Thanks, Nasim.

    Have a good weekend.

  3. 3
    ) Peter
    October 15, 2011 at 6:36 am

    VERY interesting and very clearly explained, along with the consequences.
    I’m anxious to see the other analysis.

    • October 15, 2011 at 9:47 am

      Peter, I will do my best to explain some complex topics and terminology in a simple way, to then fully explain how aperture and other lens features work in detail. These articles are obviously for more advanced photographers – I will create a separate section on the site for these.

  4. 4
    ) Kapitancho
    October 15, 2011 at 6:40 am

    It’s not a bad idea to add a few sentences about focus shift as it is a consequence of spherical aberation.

    • October 15, 2011 at 9:44 am

      Coming soon in a separate article :) I realized yesterday that I missed a couple of things in this article. Will finish it up when I get back from Phoenix…

      • 9
        ) princessfaiza
        March 8, 2012 at 7:57 pm

        is this have a function in our daily life?

  5. October 15, 2011 at 10:39 pm

    das ist gutt Mr Nasim :)

    • 8
      ) peter
      October 18, 2011 at 3:25 pm

      That’s about the only German I know, other than ‘Das ist alles’ which I told my German butcher at the end of an order. He taught me that phrase.

      Es lebe die gro├če Nation Deutschland!

  6. 10
    ) De REFORMER
    July 3, 2012 at 3:31 am

    thanks; good work and keep it up

  7. 11
    ) Spectacular
    September 23, 2012 at 9:34 am

    Thanks for dis wonderful info. Am glad 2 say it was very helpful indeed. Keep up wit d gud work

  8. 12
    ) Jean Bosco
    October 25, 2012 at 2:11 am

    Helpful explanation. Thanks Mr.Nasim

  9. February 4, 2013 at 9:01 am

    After reading your article I wasn’t sure how to apply the concept until the end. The idea of stopping down to reduce the amount of aberrations is brilliant. I have heard that lenses stopped down a couple stops from the maximum aperture are sharper. Know I now the science behind the principle. Thanks for the tip.

  10. 14
    ) EricD
    February 23, 2013 at 5:56 pm

    Might your aspheric lens work better the other way round ?
    I’m thinking of the work of Descartes,
    and fossil Trilobyte eyes !
    http://ars.els-cdn.com/content/image/1-s2.0-S0030399208002454-gr8.jpg

    • October 2, 2013 at 11:11 am

      Eric, just noticed your message that you left in February of this year, as I was reviewing some of the old articles. You are absolutely right – my image was flipped and I don’t know how I let that one slip, LOL :) I have just fixed it and uploaded the correct version!

      Thank you so much for your feedback.

  11. 15
    ) chetu jm
    September 29, 2013 at 4:44 am

    thank u sir today its really helped me thanks a lot

  12. 17
    ) yaghoob
    December 20, 2013 at 7:07 am

    thanks a lot
    i dont know this subject very very thank you

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