Chapter 02: Basics

Objectives

Open your eyes! Lenses are everywhere …

Golden Gate  bridge seen through a window on a rainy day
Golden Gate Bridge in Rain Drops

In the picture above((C) : Wikipedia : http://commons.wikimedia.org/wiki/File:GGB_reflection_in_raindrops.jpg) you can see the blurred Golden Gate Bridge and in the foreground you see various rain drops.

What can we learn from this picture?

  • As a human being with adjustable eyes, we often have the impression to see everything sharp. Here, however, some areas of the picture are clearly in focus, others do not.
  • The shape of the droplet has impact on the resulting image.
  • The images are smaller than the blurred background.
  • There are several (!) Images of the bridge section.
    Intuitively, we assume that the light of an object point takes the direct route into our eyes.
    Light from the bridge obviously reaches our eyes on different “trajectories”.
  • The images are upside down.

Since there definitely are lenses that can produce images of “desired” shape, there must be a strategy to predict the effect of lenses.

A lens model is needed !

Lens Model

Light is everywhere around us. We orient ourselves with our eyes. Light is matter of course for us. During the daytime we see colors, we feel the warmth of the sun, at night we can watch the stars on a clear day. We even see light where there is  none at all when we looked too long at the sun and this lingers on with our eyes closed.
Everyone is familiar with phenomena such as water drops, rainbow, snow, the shimmering surfaces of CDs, a magnifying glass or mirrors of the sun on the surface of water.

So of course these things are for us, there is no single model of light, which explains all these phenomena at the same time!

The simplest model is about of light “rays” . One can explain many optical phenomena with the help of the model. Although the model is not exact, but sufficiently good for the explanation of many phenomena.

Comparison entocentric – telecentric – pericentric

The vast majority of lenses work as we know from our own eyes: things closer to seem bigger.
 Such lenses are called “entozentric”.

But there are lenses that keep the size of objects constant (“telecentric“), and .. surprise .. even lenses that show near things small and more distant objects larger (“perizentric”, “hypercentric“)!
This reverses our everyday experience about “vision”.

There are tens of thousands of lenses – but why?

Lenses have been around since about 3000 years (see eg engl. Wikipedia under “Nimrud lens”)
Google for terms like “objective” or “lens”, and you get some 100 million hits.
So if you’re really looking for a lens, that’s ideally suited for a task, you are faced with a “small” problem.
If we look at this plethora of hits above that apparently exist for some reason, we notice  not one sole  brand or type lenses have become established on the market, but that there is a huge variety.
 The reason is for this diversity as follows:

Money Reasons

Long before there was industrial image processing, there was amateur and professional photography.
Cameras can indeed be produced for quite low cost with a built-in lens, but, hey,  it is much more exciting 
 for customers – and also for the manufacturers – if you can change the lenses.
Of course you then can sell more lenses than cameras for hobby and professional use. So lenses are an additional way for camera manufacturers to make money …at least, if you can sort of bind the customer.
And how to achieve that better (and easier) than by developing a dedicated mounting option (“mounts”) for lenses on cameras.
A “small” Overview of (only) about 50 (!) Types can be found about [here] …
Some camera manufacturers even define several different mounts. The actual lenses are then identical in terms of glass components, and can be sold several times with different mechanical interfaces. They call it “customer loyalty” 😉

Depending on project constraints

Depending on the tasks you maybe are interested to focus an area as large or as small as possiblelike in surveillance or in microscopy. In a project, you may need a “perfect picture – whatever the cost” (for example in manufacturing of integrated circuits), the next moment a lens is needed for a game whose optical quality is not as that important, but the super low price is. The next objective must weigh less than e.g. 40 g or must not exceed a length of, say,  15mm.

Technical development

Over the decades image capture turned from analog film to digital sensors.
The sensors are cheaper to produce when they are small – but are more demanding in terms of lens quality. New lenses are called for in order to keep pace. Firstly, the picture elements (“pixels”) shrank for a long time, then the number of pixels went larger and larger. In the professional area, there is (since 2008) digital cameras with 40-50 million pixels. In the consumer sector, we notice (the end of 2008, finally) that more pixels (given the appearance and given sensor size) certainly does not promise better pictures. So the sensor sizes had to turn larger again to give room for the higher pixel count.

 

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