The image on the left was photographed at 70mm, while the image on the right was photographed at 200mm (both at f/4).
Notice how the background appears to have moved “closer” to the subject with the longer focal length.
One reason for using a telephoto lens (such as 100mm or 200mm) is to keep some distance between you and your subject. This can be great for portraits of people, as well as nature photography (so you don’t scare away any animals). However, longer focal lengths affect more in your images than just bringing your field of view closer to your subject or focus point. Some of the biggest changes you will see will be in regards to depth of field and compression of the background or foreground of your images.
Take a look at the following two images, both photographed with the same lens (Canon 70-200mm f/4L IS ) on a full-frame body (Canon 5D Mark III) at different focal lengths (70mm and 200mm). The first image was photographed at 70mm, which is a great focal length for minimal distortion in your subject. Then, I stepped back and zoomed the lens out to 200mm, doing my best to keep the seagull framed the same way as it was in the first image. For the most part, the seagull looks the same, but the background has significantly changed. Using a long focal length (in this case, 200mm) compressed the background which gave it the perception of being closer to my subject. It also blurred the background much more than with the shorter focal length, even though the aperture setting remained constant at f/4.
There are other factors at play here, so the results you see will vary depending on your scene. Here are some general things to consider and some “guidelines” to follow when attempting to blur your background:
- In order to have a blurry background, you need to have distance between your subject and its background. The more distance between your subject and its background, the more blurred the background will be.
- The longer the focal length, the more shallow your depth of field will be (in other words, things will fall out of focus much more “quickly” with a longer lens, such as photographing a portrait and only one eye is in focus).
- A wide lens (such as 24mm) with a large aperture (f/2.8, etc.) will blur your background, but it will also add distortion to your subject if your subject is the largest portion of your frame. (Longer focal lengths add little to no distortion, which is why they are typically preferred for portraits.)
- Longer lenses can create lens compression, which makes items in your background appear larger than when using a wider lens.