A Lens’ Field Of View

A Lens’ Field Of View

by William Lulow

As a follow up to the last article about lenses and zoom lenses in particular, here is another, shorter piece on lenses in general and how they behave. The following are examples using several different lens settings on a typical zoom, or variable focus lens. What is important here is to notice how much of the scene is rendered by each focal length and what happens to the overall sharpness with each setting:

The first image was made with the lens racked IN to its shortest focal length (20mm). Note that all of the scene is included and is fairly well in focus. Now look what happens with the second image which was made with the lens set to its 60mm focal length. Note that you don’t see everything that you see with the wide angle setting, but you do see a good portion of the subject. The third image was made with the lens on its 85mm focal length. Now, we are beginning to see the background go out of focus while the field-of-view is now restricted so that we don’t see the full subject any more. The last image is with the lens set to its 135mm setting. Now, the subject is difficult to make out because we only see a very small part of it. Also note how out of focus the background is.

Here is a chart that will give you relative fields-of-view of several focal length lenses:

In this chart, the 1.5 times the normal lens (normal lenses for 35mm, full-frame cameras are in the 50mm range) would be around 75mm and the 3.0 times the normal lens would be around 150mm. Just interpolating these measurements for cropped sensor cameras means that the “normal” lens would be around 80mm to obtain the results in the chart and the telephoto would be around 120mm. A wide angle lens on a cropped sensor camera would have to be around 20mm and on a full-frame camera it would be roughly 32mm-35mm. This actually corresponds fairly well to what we used to use before digital cameras. My old 35mm camera kit included a 24mm, 35mm 50mm, 85mm and 200mm lens to cover most assignments I had at the time. I also had cameras that were larger format. For instance, my view cameras that shot 4×5″ film sheets had wide-angle lenses in the 90mm size. The “normal” lens for that size film was 135mm and the telephoto was more like 210mm. Also, back in those days, the focal length of many lenses was measured in inches, not millimeters. So the telephoto was an “8-inch lens,” and the wide-angle was a “3-inch lens.” The photographers I worked for in the early days would expect us to know this information.

You can’t forget that when you use a longer lens, not only is the field of view less, but the depth-of-field is less as well. These days, photographers use this information to obtain images that  purposely have the backgrounds out of focus. It is sometimes referred to as “bokeh,” which means that the background is soft, but it makes the sharp parts of the image really stand out. Here is an example:

Notice how the foreground as well as the background is out of focus and how it concentrates the viewer’s eye to the main subject. Here is another example:

The take away here is that each lens (or lens setting on a zoom lens) has its own particular characteristics. If you are going to use a variable focus (zoom) lens you need to understand what each of the focal length settings mean and what kind of an image will result.