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:
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:
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:
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.
Good job Nasim! Thanks for sharing.
Good tip. Thanks, Nasim.
Have a good weekend.
VERY interesting and very clearly explained, along with the consequences.
I’m anxious to see the other analysis.
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.
thanks sir,i got it very well.
It’s not a bad idea to add a few sentences about focus shift as it is a consequence of spherical aberation.
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…
is this have a function in our daily life?
das ist gutt Mr Nasim :)
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 große Nation Deutschland!
thanks; good work and keep it up
Thanks for dis wonderful info. Am glad 2 say it was very helpful indeed. Keep up wit d gud work
Helpful explanation. Thanks Mr.Nasim
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.
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
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.
thank u sir today its really helped me thanks a lot
thanks a lot
i dont know this subject very very thank you
Very nice topic .
Nice and concise explanation, I knew it when I saw it but didn’t know what it was, but now I know.
Nice and concise explanation, I knew it when I saw it but didn’t know what it was, but now I know.