Thursday, March 26, 2009

The Basics of the Lens

A DSLR camera lens is not very different from a refracting telescope or even a microscope.We need the lens to gather light and focus it while transmitting the light to the image sensor.

Here is an example of a simple lens:
It gathers light to a focus. Sort of. There are some inherent problems with a simple lens.Lets first talk about Chromatic aberration.

Because the lens is thicker in the middle than at the edges all colors of light do not focus in the same place.

There are 2 other undesirable properties of a simple lens:
Spherical Aberration, illustrated in the diagram here on the left, and Coma which is illustrated just underneath the spherical aberration example.

All of these inherent properties of glass can all be corrected by adding more pieces of glass after the front one. The lens in front is called an objective or crown lens when there are specialised correction lenses behind it.

Look at the illustration showing an Achromatic doublet being used to bring Chromatic aberration under a measure of control.

Lens designers use a variety of correcting glass so their lenses produce clear, sharp images.

There are a couple of more problems a lens designer has to be aware of.

Some of the incoming light is lost in each glass element in the lens. The designer needs to keep the number of glass elements to a minimum. The other problem is air gaps between glass elements. Each air gap can be a source of internal reflections and flare.

Over the years lens designers have discovered various chemical coatings that mitigate these problems.


For photography we need a way to regulate how much of our image is in focus. The property we want to control is called Depth of Field or DOF. We control DOF with the lens aperture.

DOF is a major part of a photographers creative control. With very shallow DOF we can have a very out of focus (OOF) background so the viewers eye is not drawn away from the subject of our image. A head and shoulder portrait is a good example.

On the other hand, for a landscape having virtually everything, very deep DOF, how we would see the landscape if we were standing where the camera has been located when the image was made.

Large lens openings like in #1, will produce a shallower DOF than small lens openings like in #2.

Notice also that the aperture mechanism is made of several 'leaves'. Ultimately the number of leaves and their shape can have an impact on the look of your images. We'll cover those aspects in a later edition of the blog.

In the next installment of the blog we'll discuss the triumvirate of exposure (shutter speed, aperture, ISO) and how they relate to each other.

See you then,

All images courtesy of Wikimedia Commons.

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