The Trouble With Lenses. What ’ s Important and What ’ s Not. Blake Ziegler. Light, usually traveling in a straight line, is diverted in its direction by transmission through a transparent medium.
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Light, usually traveling in a straight line, is diverted in its direction by transmission through a transparent medium.
This diversion is known as refraction. We can see refraction occur when we put a pencil into water. The pencil itself has not bent but the image is modified due to refraction caused by the change in medium.
The same phenomenon occurs when light passes through glass or any other transparent substance.
This diversion occurs only at the boundaries where the medium changes. In the illustration, you can note that there are actually two diversions, one at each change of medium, i.e.; air to glass and glass to air.
Without refraction, there would be no eyeglasses, microscopes, telescopes, cameras or any other optical device.
The term glass is used to represent many substances. Glass has even become slang for the entire lens, as in “I shoot Nikon glass.”
“Glass”, as used in optical products, is manufactured in more than 250 types. Each type is made to solve some optical problem such as dispersion, amount of refraction, weight, etc.
There are also non-glass substances such as plastic and even natural substances, like quartz and fluorite, suitable for lens making.
The lens designer typically purchases glass on the open market to fit his design idea. There are four major manufacturers of optical glass in the world:
Ohara - Japan
Schott, Carl Zeiss - Germany
Hoya - Japan
Bausch & Lomb - USA
Note that nowhere on that list are any of the camera manufacturers. Quite possibly, this is a big surprise to most. Yes, Canon and Nikon make their own glass, however, the quantities are small. (On the order of 10%.)
An example: Leica produces 10%, or less, of their requirement for glass. Ninety percent of the legendary Leica glass comes from Zeiss, through the Schott Glassworks.
Lenses made for today’s photographers are wonders of compromise. Glass makers constantly struggle to find new glass types and other materials that can do more with less.
Lens designs, today are balanced between weight and performance on the one side and cost on the other. Cost will always be the heavy thumb on the scale for lens designers.
Today, a lens designer sits at his computer and enters a few parameters, such as:
This parameter tells the computer what the flange to sensor distance is, maximum rear element diameter, mount type, etc.
Tell the computer the focal length, initial f/stop, etc.,
Photographic lenses are composed of multiple pieces of glass, each doing its job to control the flow of photons through the package of optics.
Often each element is composed of a different formulation of glass to solve specific problems as the light passes.
Sometimes the lens elements at the rear of the package are introduced to cancel out aberrations that occur in the front half of the lens design. This reversing happens in almost all “normal” lenses.
In each lens there is an iris, much like the iris in your eye that adjusts its size according to the amount of light available.
The iris is calibrated with a numerical sequence based on 1.4142135 (the square root of 2).
Beginning at 1, multiply by 1.4 and you get 1.4 as the next f/stop. Multiply 1.4 x 1.4 and you get 2 and so on.
Each f/stop changes the amount of light passing through the iris by double or half depending upon the direction you are traveling through the scale.