Introduction to Cameras for Nature PhotographyPart 2: Lens Focal Length

Osprey

This image of an osprey was captured with an equivalent focal length 750mm lens. The bird was at the top of a tree that was 100 feet away.

In the first posting in this series, I briefly explained two of the three most important lens variables: aperture and focus. The third major variable is focal length. The length of the lens, measured in millimeters (mm),  relative to the size of the film or digital sensor, determines the angle of view that is captured by the lens. A short lens will cover a wider area than a longer one. This means that if you were standing in a field and you wanted to take a photo of the field you would want to use a short or wide angle lens. If you wanted to capture an image of a bird perched on a bush on the other side of the field, you would need a long or telephoto lens.

Focal length is relatively easy to understand, but like many things in photography, there are complicating factors. One of these that you need to understand is “equivalent” focal length. Remember that the focal length is relative to the size of the film or digital sensor and that this ratio determines the angle of view. Not all digital cameras have the same size sensor. Because of this, a focal length of 35mm on a point and shoot camera is a powerful telephoto lens, but a 35mm lens on a full size digital camera would be a moderate wide angle. To reduce the confusion (and math), camera manufacturers frequently state the focal length of a lens as the actual length
as well as the length it would be to produce the same angle of view on a full size camera. This is the equivalent focal length.

Lenses can be grouped into five general categories based on equivalent focal length:

Wide Angle. These lenses typically range in focal length from about 15mm up to 35mm. They are the lens of choice for landscape photographers. In addition to a wider field of view, these lenses typically have a deeper area of focus and are more forgiving
of slight movement of the camera.

Normal Lens. This is the lens that used to be sold with most 35mm cameras. It has an angle of view that is closer to that of the human eye. This lens has a length that is very close to the diagonal distance across the digital sensor. Equivalent lengths are from about 40mm to about 70mm. Normal lenses are good for general photography.

Telephoto. The telephoto lens is the main stay of wildlife photographers. The photographer can capture a full image of the animal with little or no disturbance.  These lenses range in size from 80mm to 800mm or more. They can be very expensive. For example, a Nikon 600mm lens will run $10,000 and the wait for delivery six months or longer.

Zoom Lenses. Lenses can be a fixed focal length, called a prime lens, or a variable length which is known as a zoom lens.  A very
popular choice, you can change the focal length of a zoom lens by turning a ring or pushing a button. There are many combinations available in these lenses from wide to wider, long to longer, and even wide to long. A very popular zoom range is from about 20mm to about 200mm. This is considered by many to be a great “walk around lens” because it can cover a wide range of situations without having lug around a several different lenses.

Zoom lenses are popular because of the convenience, but many are considered a compromise in quality to a fixed length or prime lens. That used to be true, but many zooms now rival the optical quality of primes. Also do not be fooled by “digital zoom”. Digital zoom, as opposed to optical zoom, just means that the image is cropped inside the camera and as a result is lower in quality.

Specialty Lenses. There are several special purpose lenses such as a fish eye and perspective , however only one is really mportant to new nature photographers.  A macro lens is a special lens that can be focused very close to the subject. This is the lens that is used to capture images of spiders and other little creatures to show every hair  on their body. Macro lens are available in focal lengths from 50mm to more than 200mm. I recommend the longer focal lengths, because they allow you to capture the details without having to be so close that you disturb the subject or block your light.

In the next post I will illustrate the importance of understanding  aperture, focus and focal length before you buy your next camera.

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3 comments so far

  1. Al Alborn on

    Ernie,

    What kind of lens (brand) did you use for the Osprey shot? I’m “lens shopping” for a lens for my Nikon.

    Al

  2. Ernie Sears on

    Al,
    The image was made with a Nikon D300 with a Sigma 150 – 500 DG 5-6.3 APO HSM lens at 500mm (equivalent to 750mm on a cropped sensor camera.). I have a love/hate relationship with this lens. It costs a lot less than the Nikon 500mm or 600mm, but it does not always give me the sharpness that I want. It is considered a very good bird lens, but you have to learn how to use it to get the best results. Good luck in your search.
    Ernie

  3. Digitalsreviews.Com on

    Digitalsreviews.Com

    Introduction to Cameras for Nature PhotographyPart 2: Lens Focal Length | Your Piece of the Planet


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