What is Focal Length and Lens Speed in Digital Photography?

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Choosing a Lens

For those that are outside of the semi-professional world of digital photography the idea of using lenses can be very difficult, but is almost required on digital SLR cameras. When you are trying to select the appropriate lens for your specific situation you have quite a choice on your hands, but there are two basic factors that are going to need to be negotiated when making the decision that is best for your photography. Here is a look at these two main lens qualities: focal length and lens speed.

Focal Length

In simple terms, the focal length refers to overall size of the lens. This lens size is usually listed in millimeters, as with a 50mm lens. As the focal length, or size of the lens, gets larger you will find that the angle of view gets smaller than before. As the focal length gets smaller you will find that you have a much shorter lens and the angle gets a lot wider.

There are a couple basic types of lenses that will determine the overall range of focal length that you can have. If you have Single Focal Length Lens you are going to notice that you the only way you can alter the appearance of the image is to move your camera as you have only one focal length, as the name implies. If you have a zoom lens this means that the focal length can change over the range that is listed on the lens.

If a camera lens is a 50mm, you will find that it is somewhat in middle, meaning it will have a focal length that is not incredibly wide but also not near to being narrow. If your lens is 130mm, however, it will have a smaller angle of view. If your lens lists itself as a 26mm-160mm then it is a zoom lens that has the ability to go from an incredibly wide angle of view to a very narrow one as it goes up in focal length.

Lens Speed

The lens speed is another aspect of the lens you choose, but instead of reflecting the focal length it indicates the overall range of different aperture settings that will be allowed. If you see that your lens is listed as a “slow lens” then it is going to be a much more open aperture that allows for a lot of light to be let in. A slow lens is going to be best for low light scenarios, indoors, and at night. If you have a fast lens then this means that you will have a much higher F-Stop and will let in much less light. You are going to want to utilize a fast lens in daylight where it is bright out or you are using high quality artificial lights, such as HMIs.

If you find a lens listed as 50mm at f/1.0 you are going to have a very slow lens, but if it is higher at somewhere around f/22 then this is a fast lens used only in very high light situations.