Saturday, September 3, 2011

Lens Basics – Focal Length and Angle of View

This article is a very short excerpt from a book I am writing called, Moving Beyond Point-and-Shoot Photography: The Next Step – Learning to Use a DSLR or Interchangeable Lens Camera, due for release by Rocky Nook in the spring of 2012 in print and most eBook formats.

Angle of View

Angle of view simply means how much of a scene the camera's lens sees. In a horizontal and vertical direction, each lens can only take in so much. Each focal length (how long the lens is) has a different angle of view. To make it simple, let's examine three angles of view: wide-angle, normal, and telephoto.

The easiest way to determine whether a lens is wide-angle, normal, or telephoto is the size of the subject seen in the viewfinder compared to what the human eye sees.  If you look through the viewfinder with both eyes open and the eye looking in the viewfinder sees a subject that is smaller than what your other eye sees directly, the lens is probably a wide-angle lens. If both eyes see a subject of approximately the same size, the lens is probably a normal lens. If the viewfinder eye sees a subject larger than the other eye sees, the lens may be a telephoto lens.

Wide-Angle Lens: The size of the subject will be smaller than your eye sees it when you look away from the camera’s viewfinder. You will have used a wide-angle lens (or wide zoom lens setting) when you took a picture of a group of people or a landscape shot of a beautiful area. Wide-angle lenses allow you to capture more of the scene than other lens types. Wide-angle lenses are considered to have a short focal length.

Normal Lens: A normal lens creates a view that looks similar in size to what you would see with your normal eyes were you standing the same distance away as the lens. A normal lens (or medium zoom lens setting) is not especially wide, nor does it magnify the scene. It is a lens in the middle range and provides a normal view (not larger or smaller) than what your eyes normally see. Normal lenses are considered to have a medium focal length.

Telephoto Lens: The telephoto lens works like using a magnifying glass on a subject that is far away. It magnifies the subject in your camera’s viewfinder so that small objects become larger. A picture of a bird in a tree taken with a wide-angle or normal lens would be rather tiny and hard to see. However if you used a telephoto lens (or a zoomed-out telephoto lens setting) the bird will appear larger in the picture. Telephoto lenses are considered to have a long focal length. Let’s talk about focal length.

Focal Length Changes Angle of View

The simplest possible way to describe what focal length means is to say that focal length is the length of the lens on your camera (how long or short it is). Generally speaking, that is true. Longer lenses often have longer focal lengths and shorter lenses often have shorter focal lengths.

However, viewing the term focal length in this way is a little misleading because today’s lenses, made with computer-assisted design techniques, can manipulate (bend) light in ways that older lenses simply could not. A telephoto lens from today is often significantly shorter and lighter than a telephoto lens from years ago.

Focal length does not really mean the actual physical length of the lens, although that’s the way most photographers think about it. Technically speaking, focal length simply means the distance from a point, often in the middle of the lens—called the nodal point—to the imaging sensor surface (point of focus). If the distance from the nodal point to the imaging sensor is 50mm, you have a 50mm lens; if 200mm the lens is a 200mm lens. Before your eyes glaze over and you stop reading, I'll say, “Don’t worry about it!” That’s as technical as we need to get.

You don’t have to worry about nodal points and distances to imaging sensors. All you have to do is learn to recognize how a certain focal length lens (or zoom setting) performs on your camera. For those who want to know more about nodal points, there are plenty of easy to read articles on the web. Here is an article I found somewhat useful (although quite technical):

Imaging Sensor Size Affects Angle Of View

One important fact you must know when discussing focal length is that the size of the imaging sensor affects the angle of view. For instance, the 35mm focal length provides a normal angle of view on an APS-C (DX) size sensor and a slightly wide angle of view on a full-frame (FX) imaging sensor (same size as a frame of 35mm film). You will have to read the camera’s manual to determine what is considered wide-angle, normal, and telephoto focal lengths for your camera. In general, the following chart of focal lengths is close to being accurate for most of us:

  • 6 to 12 mm is an extreme wide-angle lens
  • 16 to 25 mm is a wide-angle lens 
  • 30 to 50 mm is a normal lens
  • 60 to 150 mm is a short telephoto lens
  • 200 to 1200 mm is a long telephoto lens

You will notice that there are gaps in my chart. That is because a particular focal length’s angle of view is hard to classify for different camera designs and I want you to understand that this is a rough chart. Each imaging sensor size has a specific range of how they display the angle of view from each focal length.

An easier way to remember this concept is: the smaller the camera’s imaging sensor the narrower the angle of view will be (less wide angle). With a smaller micro four thirds sensor a 50mm lens is soundly in the short telephoto range. On a larger full-frame sensor a 50mm lens is a normal lens. The smaller the camera’s sensor, the harder it is to find an extreme wide angle lens, but the easier it is to find a long telephoto lens.

Likewise, the smaller a camera’s sensor, the greater the telephoto effect (narrower angle of view or greater the magnification) will be from a particular focal length. A 200mm lens is a long telephoto lens on a small micro four thirds sensor, but only a medium telephoto lens on a large full-frame sensor. The larger the camera’s sensor, the easier it is to find an extreme wide angle lens, but the harder (and more expensive) it is to find a long telephoto lens.

Telephoto lenses are usually much more expensive than wide-angle lenses, so there are some benefits to having a smaller sensor for people who like to shoot wildlife pictures. A camera with a smaller sensor can use a shorter focal length than one with a full-frame sensor to get a similar field of view.  For instance, an DX sensor at 400mm has a similar angle of view to an FX sensor using a 600mm focal length. A 400mm lens costs a lot less than a 600mm lens!

Basically, you will have to determine what range of lenses best work with and are available for your camera. That’s why it is so important to buy a camera with a nice system of lenses and accessories available. Camera manufacturers will usually offer wide-angle, normal, and telephoto lenses for your camera body. Its your job to determine which lenses you’ll need and what angles of view you want to have covered. Most of us end up with at least four lenses. A basic three-lens zoom lens kit covering a wide range of focal lengths, and a macro lens for extreme close ups.

In Summary: Focal length and angle of view work together to let you capture varying views of your picture’s subject. You can go wide angle for a big sweeping view, or zoom in to telephoto for a narrow, up-close view. The imaging sensor size in your camera modifies what the angle of view for each focal length looks like compared to other cameras with different size sensors. The best way you can learn about focal lengths and angles of view for your camera is by experimentation. Following is an assignment that will let you experiment with your current lens or lenses and see what range you can capture. It is an interesting experiment. Give it a try and you will learn a lot!

Assignment: Take a series of pictures using your lenses at successive focal length setting positions (e.g., 18mm, 24mm, 35mm, 50mm, etc.). Start with the shortest focal length (wide-angle setting) and zoom out after each shot until you have reached your longest focal length (telephoto setting). Choose a subject that allows wide-angle to telephoto shot distances (e.g., a cityscape, in a park, or at the lake) and photograph the same subject in each picture. Look at each image on your computer and see how the perspective changes with each focal length.  See how much more a wide-angle lens captures and how the angle of view narrows as the focal length increases with each successive picture. Use a tripod for best results!

Keep on capturing time...
Darrell Young

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