Digital Photography

Zoom vs. Focal Length

Before you start shooting, it may be useful to know the how zooming and focal length work on your camera. Zoom is a term used to describe the ability of your camera to change the field of view. This is done by changing the focal length from your cameras sensor. Longer focal lengths have the effect of narrowing the field of view, making your subject larger (magnifying it). Shorter focal lengths have the effect of increasing the field of view, making your subjects smaller. When you zoom in you are increasing the focal length to get a closer picture of your subject and zooming out does just the opposite.

So what is focal length?
Have you ever looked through a magnifying glass? Remember that you have to hold it a certain distance from your eye for it to be in focus. This is because the magnifying glass has a set focal length. It must be a certain distance from your eye to focus in on your subject. This distance is referred to as the focal length. If your magnifying glass could focus at multiple focal lengths the subject would be larger if you moved in further from your eye.

Take a look at the example shown above. The top camera sensor in this example is a longer focal length. As you can see the focal point is further from the sensor and it gives a narrower field of view and because the image is the same size on the sensor it gives a magnified subject. As you can also see from the lower example uses a shorter focal length does just the opposite. A focal point closer to the sensor brings more area into the field of view which means more to fit on the sensor. In order for all that to fit on the same size sensor your subjects must be smaller.

If you use a DSLR or SLR camera with an adjustable focal length lens you will notice that it has markings on it showing what focal length you are using. If you use a point and shoot/compact camera, those cameras don't usually have markings telling you what you focal length is or what even what you are zoomed in at.

Point and shoot/compact cameras are usually rated somewhere between 2X zoom and 5X zoom. If you use a DSLR-like camera it may have up to a 10X zoom. Digital cameras use varying sizes of sensors which means that a 5X zoom on one camera may not have the same magnifying effect as another camera with a 5X zoom.

How Zoom is calculated
Your zoom X factor is simply the largest focal length divided by the smallest focal length. For example, my old CoolPix L1 camera adjusted from 6.3 to 31.4mm focal lengths. If you divide these numbers, you get 4.98 or a zoom factor of 5X (31.4/6.3 = 5).

If you want to know more on this you can read these articles:
http://www.cambridgeincolour.com/tutorials/camera-lenses.htm
http://www.cambridgeincolour.com/tutorials/digital-camera-sensor-size.htm

Focal Length vs. Shutter Speed

The longer of a focal length you use on your camera the more your unsteadiness will show up in your photo. Most people can hold a camera steady for a 1/60 of a second at a focal length of 60 mm. As a general rule of thumb you can use the 1/focal length = your shutter speed when planning your photo. This means that you can hold steady if your shutter speed is the inverse of your focal length. Example: you are using a focal length of 200 mm. so you should use a shutter speed no slower then 1/200th of a second. Or if you are using a 300 mm, focal length you should use no longer then a 1/300th of a second.