Nazetaro's Room

What is Aspheric Lens?

• Naze Taro
  Let’s learn about aspheric lens today.
• Matt
  Aspheric lens? What is it?
• Naze Taro
  Let’s learn about it step by step. Matt, could you describe the shape of lenses that you have in your mind?
• Matt
  The surface of the lens is round, and the whole shape is like a half cutted ball.
• Naze Taro
  Good. As you said, conventional lens has a front surface of that is spherical, so it is called spherical lens. Aspheric lens is designed to have a flatter curved surface. Look at the picture below that shows some types of aspheric lenses. The center lens would be easier to see the shape. The most unique geometric feature of aspheric lenses is that the radius of curvature changes with distance from the optical axis, unlike a sphere.
  
• Matt
  The shape of asperic lens looks like a half rugby ball, curved outwards on one side.
• Naze Taro
  That’s right. That’s why these lenses are called “aspheric” lenses. Aspheric lenses are mainly used for camera lenses.
• Matt
  Why is that? Spherical lenses have any disadvantages compared to aspherical ones?
• Naze Taro
  Yes. Have you seen a photo that its center remains more in focus than the edges of the image and the intensity of the edges falls relative to that of the center?
• Matt
  Yes, I have. When I was taken a photograph and stood at end, my body looked expanded in picture.
• Naze Taro
  That happens because of 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.
  
• Matt
  That sounds a little difficult but since light rays that refract when passing through the spherical lens focus at different points, and that makes a photo a shaper image on objects in the center, but blurred at either end.
• Naze Taro
  Exactly. And the less aberrations, the better the quality of lens is.
  Spheric lens causes aberrations, but aspheric lens is calculeted and designed to minimize aberration by adjusting the curved suface of the lens. That makes aspheric lens has more complex surface curvature than spheric one.
• Matt
  Interesting. Before aspheric lens was develped, how have you adjusted aberrations caused by spheric lens?
• Naze Taro
  Good point. Conventionally, spheric lenses were piled up to minimize aberrations. Even this cannot totally reduce aberrations, though.
• Matt
  Wow, that must have made a camera bigger, heavier and expensive!
• Naze Taro
  That’s true. Unlike conventional spheric lenses, aspheric lenses can correct problems such as image distortion and focal point misalignment with one lens. With this, miniaturization, weight reduction and cost reduction are attained.
• Matt
  Aspheric lenses enabled reduction of aberration, improved function, and make a camera more compact and lighter. That sounds great.
• Naze Taro
  Now you know why aspheric lenses become popular.
• Matt
  Despite the advantages, why aspheric lenses took long time to come out?
• Naze Taro
  Let’s learn about it next time.

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