Understanding Focal Length
This distance is usually measured in millimeters. From a practical point of view, focal length can be thought of as the amount of magnification of the lens.
The longer the focal length, the more the lens magnifies the scene. In addition to magnification, the focal length determines the perspective and compression of the scene.
The focal length tells us the angle of view how much of the scene will be captured and the magnification how large individual elements will be.
The longer the focal length, the narrower the angle of view and the higher the magnification. The shorter the focal length, the wider the angle of view and the lower the magnification.
From the picture above, it's mean that focal length is determine by the type of lenses . Every lens has a different focal length or lengths expressed in millimetres.
A scientific explanation of how this is measured needs more space than is available here, but what you really need to know is how focal length affects your angle-of-view.
For example, a 50mm lens gives roughly the same magnification as the human eye, so it’s called a standard lens.
Any lens with a focal length smaller than 50mm makes objects appear smaller and gives a wider angle-of-view. A focal length longer than 50mm magnifies objects and gives a narrower view.
Wide ( C )
A shorter focal length would give an even wider view, but the beach waves would become much smaller in the frame.
Standart ( B )
The magnification of a 50mm lens is similar to your eyes, hence the name ‘standard’. It’s not a common focal length, although it’s available in the mid-range of most standard zoom lenses.
Tele ( A )
The extra magnification of a telephoto lens is ideal for picking out small parts of the scene. The view indicated is the image you’d get if you used a 200mm lens to shoot this scene.
The control of perspective is the ability to use your camera and lenses to control the relationship between the background and foreground of your pictures.
When you’re using a wide-angle lens, the background appears much farther away from your subject than it actually is.
With a standard lens, the background appears the same distance away as it does with the naked eye. When using a telephoto lens, the background appears closer.
It should also be mentioned that on a wide-angle lens your angle of acceptance is much greater than on a standard or telephoto lens where your angle of acceptance is much less.
This means that with a wide-angle lens, you can sometimes surprise yourself with how much you’ve included in your picture.
On the other hand, using a telephoto lens, your subject is more prominent and less of the background is included within your frame.
So, by varying your lenses, and therefore your perspective, you can use the background to complement your pictures or isolate your subject as you require.