Telephoto lenses and backgrounds.

Tiger lily; same flower but shot from slightly different positions.

Focal length relates to the power or magnification of the lens.  A "normal" lens makes the foreground and background appear to be the same relative sizes.  A wide angle lens distorts the foreground and makes it look larger relative to the background than it does to your eyes (aka normally).  A telephoto lens makes the background larger relative to the foreground; it "compresses" the scene and makes it appear that the background is closer than what it actually is. 

It is this compression that I am talking about today.  As you zoom your lens in to magnify a subject you will notice that the background becomes progressively smaller.  You are enlarging both the subject and the background.  If you magnify the image by a factor of 2x the background decreases in its area by a factor of 4x (square of the magnification).  This had the advantage of being able to zoom in as much as you want and to control how much of the background you see.  More zooming means less background.  The tradeoff here though is that you have to be farther away from your subject because zooming in increases its size.

The beauty of this relationship is that, with increased magnification, you have greater control of what is behind your subject.  In the above photos of a tiger lily, I was back quite a distance and was using a relative focal length of 600 mm to magnify the subject.  That focal length also minimized the size of the background.  The nice thing here is that it was easy to pick what I wanted it to look like.  A small movement on my part produced a great relative movement of the background and allowed me to go from a mixed white and green background (right) to a fully green one (left). 

A property of zooming in has to do with depth of field; more focal length means less depth of field at any given aperture.  In a situation where an out of focus background is desired a longer focal length will be preferred.  If you want lots of background and depth of field stay with a wide angle setting on your camera.  You will not be able to affect the nature of the background though without major changes in position.  If you want a small area in the background that is blurred zoom in.  Small changes in your position will produce a major change to what is happening behind the subject.

Thanks for reading.   www.ericspix.com