If you were to read through the specifications of a camera or even a lens, one of the first things you would notice is Focal Length. Engineers in the audience will immediately be taken back to their Physics class in Optics, and sometimes even they will not be able to clearly remember what this term means. Trust me, I’m an engineer :)
Very simply put, Focal length of a lens is the distance between the lens and the image sensor, while focussing on the subject. The image sensor is where the photographic image is recorded inside the camera.
Let’s look at the picture below:
There are three lens positions in this diagram. Position 1 or X, is the closest the lens can be placed next to the sensor. In camera terms, this is the base focal length. Notice that when the lens is in this position, more of the subject is captured on the sensor due to the wide angle of viewing.
Position 2 or 2X, is about 2 times the base focal length. This causes a smaller angle of viewing, thus capturing a smaller portion of the subject to fill the whole image sensor. As a result, that particular portion of the subject appears magnified or zoomed.
Position 3 or 4X is about 4 times the base focal length. This position narrows down the viewing angle even more, causing more magnification of the subject.
Hence the position of the lens determines how much of the subject is captured on the area of the image sensor. Farther away from the sensor means less of the subject is captured onto the whole sensor, giving the effect of magnification.
The most common way a camera manufacturer can introduce a variation in focal length in a camera is by installing a telescopic lens mechanism like the one shown below:
This mechanism telescopes out to change the distance between the lens at the front of the telescope and the mage sensor housed inside the camera body. The result of this movement can be seen on the LCD screen behind as a magnification of the subject.
While looking for a camera, it is best to buy one with a good range of focal lengths. This would allow you to zoom closer to the subject without physically moving close. A range of 3X to 6X is good in a compact point and shoot camera.
We shall dwell on the matter of zoom in another post.
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