Why does the background look larger and blurrier with a 25mm lens than a 20mm lens when the subject stays the same size?
Asked 10/29/2016
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I compared a Panasonic 25mm f/1.8 and a 20mm f/1.7 on an Olympus OM-D E-M10 Mark II. I framed the same lamp so it stayed about the same size in both shots, but with the 25mm lens the background looks more magnified and more blurred than with the 20mm. Since the apertures are similar, is this mainly caused by changing camera-to-subject distance and perspective rather than the lens itself?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
2 Answers
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The easiest way to think of this is with an image. When you use a wider lens, you have to be closer to your subject, which emphasizes the distance between the subject and background by making the background smaller.
In contrast, if the camera is far away from the subject, you'll have to zoom much farther to get the same size of subject relative to the frame. Think of it as narrowing your frame of view. What this does, however, is show a smaller part of the background. That in turn makes the background appear larger (or closer) relative to the subject.
The difference technically is explained quite well by the accepted answer to the question, Is there a difference between taking a far shot on a 50mm lens and a close shot on a 35mm lens?
Originally by user38072. Source · Licensed CC BY-SA 4.0
user38072
9y ago
0
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Yes. The main difference is camera position and perspective, not just the small aperture difference.
To keep the lamp the same size in the frame, you had to stand farther back with the 25mm than with the 20mm. Changing camera distance changes perspective: when you move farther away, the background appears relatively larger and closer to the subject. When you move closer with the wider 20mm, the subject is emphasized and the background appears smaller and farther away.
That’s why the 25mm shot shows a “bigger” background.
The blurrier background is also expected. A longer focal length and greater subject magnification at the same framing generally make the background look more out of focus, even if the f-numbers are close. The difference between f/1.8 and f/1.7 is tiny; the bigger visual change comes from focal length and shooting distance.
If both lenses were used from the exact same camera position, the perspective would match, and the wider lens image could be cropped to look similar to the longer lens shot.
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