Lens basics:

What is "Focal Length"?

The technical answer is that focal length is the distance from the lens to the point at which light passing through the lens is focused, measured in millimeters. The focal length tells you the magnifying power of the lens. For example, a small focal length of 28-35mm is considered a wide angle lens. Focal lengths between 35mm and 70mm are considered normal because this range approximates what the human eye sees. Anything beyond 80mm begins to be considered as a "telephoto" lens. Another name for a single focal length or NON-telephoto lens is PRIME.

A prime lens is any lens that only has a single focal length. A zoom lens has range of focal lengths, such as 70-300mm, or 18-200mm, etc.

Zoom lenses are more convenient to use, but there are trade-offs if you use a lens that can move through a wide range of focal lengths. Prime lenses perform better and they are less expensive.

Serious photographers carry a few prime lenses in common focal ranges, but the majority of us use one or two zoom lenses that cover the whole range.

What are F stops or F numbers?

All camera lenses have a maximum aperture setting or how big of an opening the lens can be to admit light. The smaller the number, the larger the opening. Zoom lenses can't open as wide as prime lenses. What this means is that fast-moving subjects will blur unless you increase the ISO setting of the camera or shoot my pictures when there is more sunlight. SO speed affects the shutter speed / aperture combinations you can use to obtain correct exposure.

Suppose the meter in your digital cameras warns you there is not enough light to correctly expose a scene. You could use the built in flash, but let's suppose you are in a situation that prohibits use of the flash...like your grandmother's funeral, or your cousin's harp recital. In order to take the picture without a flash, you would need to use a higher ISO. This approximates the "film speed" we associate with film cameras. The higher the ISO the higher the "film speed". Set your digital on "ISO Auto" mode, your digital camera will automatically select a higher ISO. Otherwise, you can manually select the next higher ISO and see if the increase allows you to get a correct exposure. The advantage, here, of using digital cameras is that you can preview the picture you just took in a second.

If you find the camera is using a shutter speed that is too slow (1/60 sec. and slower) and you hand hold the camera, you may not be steady enough to prevent blurred pictures. Let's say you cannot set your F-Stop any higher to open the aperture any more, and you do not have a tripod. Under these conditions, you might select the next higher ISO which will then allow you to select a faster shutter speed. Experiment with this and compare your pictures. There is a downside to using a higher ISO setting. The trade off is an increase in "noise" or random, colored specks on your image. For digital cameras, darker regions will contain more noise than the brighter regions; with film the inverse is true.

ISO Speed & Noise

There is a price to pay with your image appearing more noisy. When you boost the sensitivity of your camera's image sensor by selecting a higher ISO, the image sensor is now able to record a fainter light signal. It is also true now that it will record fainter noise, where noise is any signal that is not attributed to the light from your subject. Read about NOISE, here: http://www.cambridgeincolour.com/tutorials/noise.htm  Keep in mind that there are plug in filters of PhotoShop designed to reduce noise. Neat Image, for example, is a filter designed to reduce visible noise and grain in photographic images produced by digital cameras and scanners. Michael Almond has a number of programs that he tested. The are listed on his website at: http://www.michaelalmond.com/Articles/noise.htm


Depth of Field and F Stop

The smallest f-number setting you can use is always desirable, as it will offer the greatest depth-of field (items from close to further away will ALL be in focus), but a "faster" lens (one that has a SMALLER F stop setting, will always cost more.
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Jess sends this along for our members.  A guide to all of the blank DVD media out there.