Camera Technology: Aspherical Camera Lenses

ePhotozine today has a very brief snippet on Tamron hybrid aspherical lenses. 


Basic lenses are spherical in nature.  They can be envisioned as pieces cut from a glass sphere.  The effect varies based on the diameter of the sphere they are "cut" from.
 

Two lenses "Cut" from spheres with different diameters.

The diameter of the sphere the lens is "cut" from determines the strength of the lens.  A lens with a smaller spherical diameter is stronger, meaning it will bend light more than one with a larger spherical diameter.

Lens with larger spherical diameter

Lens with smaller spherical diameter

Spherical lenses have an inherent problem: the edges of the lens band light more than the middle.  Here's an image to illustrate why that can be a problem:

The colored lines indicate light originating from the same point.  A focused image requires all those sources being focused at the same point on the film or image sensor.  Instead, the blue line meet at the sensor, the red slightly in front of the sensor and the green slightly in front of that point.

This results in an out of focus image.

An aspherical lens is one approach used to address this problem.  The lens shape is altered so that the light passing through the lens hits the image sensor at the same point.

This results in a focused image.

(The alternative requires the use of one or more additional lens elements to refocus the light properly.)

Note: These are very simplified illustrations and are only meant to illustrate the difference between spherical and aspherical lenses.


Aspherical lenses are more complex to manufacture than spherical lenses, but the cost may be offset by the ability to utilize fewer pieces of glass.


Hybrid aspherical lenses are the easiest type to manufacture.  These consist of glass lenses that have been combined with an optical plastic.  Hybrid aspherical lenses are easy to manufacture and don't require expensive materials.  The plastic component makes them unsuited for use in an area where the lens will be exposed to the environment.  The plastic is far easier to scratch than glass and the lens would quickly become unusable if exposed to dust particles.

Molded aspherical lenses are the next step up from hybrid lenses.  Optical glass is heated until it is soft.  The softened glass is placed into a two-piece mold.  The mold is pressed together to shape the glass.  These types of lenses can be used in areas of the lens assembly that are exposed to the environment.  The glass used to manufacture these type of lenses has to soften at a suitable temperature, which prevents some types of optical glass from being used.

Ground aspherical lenses are the most expensive and/or time consuming to manufacture.  These lenses are formed by grinding optical glass using a grindstone to shape the glass.  This process can be used with glass not suited for other manufacturing methods.  The cost or time-consuming nature of this method makes it ill-suited for mass production.