What is focal length?

The focal length is the distance between the lens and the camera's sensor (or film if you're using a non-digital camera).

At first glance this seems confusing because books and blogs talk about varying the focal length and my first thought was: "Isn't the distance between the lens and the camera's sensor a constant, unvariable thing?" Well, turns out the distance between the lens and the sensor is indeed something that can be modified.

How can you change the focal length? 

1.) Zooming in and out change the focal length

One very clear example of modifying the distance between the lens and the sensor is when you use the zoom. As the camera zooms in, it projects the lens outwards, effectively increasing the distance between the lens and the sensor, and thus increasing the focal length.

Usually if you look at your camera on the bit around the lens, it should give a number in milimeters. On my Canono PowerShot Pro1 for example it says 7.2-50.8mm on it for example. This number gives your camera's focal length. i.e. On my camera, the distance between the sensor and the lens at maximal zoom is a little over 5cm, and 7.2mm when no zoom is on. This is revealing of my camera's zooming capabilities: 7.2 x zoom capabilities = 50.8 ; so my zoom capabilities are about a x7 zoom.

2.) Changing the lens on your SLR or DSLR to one with a different focal length

One of the benefits of SLRs and DSLRs is that unlike point and shoot cameras, you can change the lenses. If you want, you can fit a lens with a different focal length on it to automatically get a different focal length.

What is the significance of a short or long focal length? 

In simplest terms, increasing the focal length increases the magnification.

In more complicated terms, this is how it works: The shorter the focal length, the less room there is for the light rays to travel before hitting the sensor, therefore they have to bend at a steeper angle in order to hit the sensor than at a longer focal length and this means that the image that it captures is over a larger area (ie you can get a large chunk of a landscape in shot).

With a longer focal length, the light rays can bend more gently to reach the same spot on the sensor, which means that the area over which the picture is coming in from is narrower, and you can get far less of the surroundings into your shot. ie out of a whole landscape you might just be able to capture a little bit of it.

The more you zoom in and when it starts to use digital zoom, there comes a point when you exceed the camera's capabilities and any zoom beyond this point starts to appear less sharp. For this reason you really should avoid using the digital zoom if you want super-crisp and sharp photos.

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