Loosely defined, focal length is the zoom setting on the lens. It is indicated by a number, measured in millimeters, indicating the distance of the lens from the sensor. Smaller numbers, meaning the lens is closer to the sensor, produce a wider field of view. Conversely, larger numbers, meaning the lens is further from the sensor, produce more magnification with a narrower field of view.
Besides the obvious size of the main object, there are two other effects caused at the extremes of focal length spectrum. The first happens at short focal lengths and is called foreground expansion. The second happens at long focal lengths and is background compression.
Background compression happens at larger focal lengths with a narrower field of view and is where perspective causes the background to pull in closer to the subject. Here are several shots at progressively longer focal lengths: 18 mm, 55 mm, 100 mm, 205 mm and 300mm. The camera position was moved back so the main subject is the same size in all the shots. With the narrower field of view and increased magnification, even though the subject is the same size, the background is significantly changed. This can be seen best by paying particular attention to the fence and trees.
Using zoom can make things more convenient by changing the size of the main subject from a fixed position. But next time your out shooting, remember that by moving around it can also be used to change the composition and overall feel of the image.
3 comments:
Great article!
Thank you for the article. It is amazing that as many times as you read a concept all it takes is one person explaining it slightly different for it to click. I now actually understand the relationship between background and the focal length.
Thanks for the feedback. Glad it helped!
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