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40mm vs 50mm portrait comparison
Interactive side-by-side simulation at the same subject size: see how 40mm and 50mm change camera distance, perspective, depth of field and background blur.
What this comparison shows
Both images show the same portrait at the same size. For the same head size, the 50mm camera stands about 1.3 times farther back than the 40mm camera. From closer in, 40mm shows more of the room around the person. 50mm has a narrower field of view, so the background looks larger and closer and is easier to simplify.
Start at f/2 on the left and f/2 on the right. Then try f/2 vs f/2, or open both to f/2 and f/1.2. Match depth of field sets the right-hand aperture so the background is as blurred as on the left. Both cameras use the same sensor, so equal f-numbers give a similar depth of field.
At the face framing, 40mm stands 65 cm from the subject. At f/2 the sharp zone runs from 63 cm to 67 cm. 50mm stands 80 cm away, and at f/2 the sharp zone runs from 78 cm to 83 cm.
Questions
How far from the subject do you stand with 40mm and 50mm?
In the face framing, 40mm stands 65 cm from the subject and 50mm stands 80 cm. For the same head size, the 50mm camera stands about 1.3 times farther back than the 40mm camera.
Which blurs the background more, 40mm or 50mm?
At the same framing, a distant background blurs in proportion to the aperture opening, focal length divided by f-number. 40mm at f/2 opens 20.0 mm and 50mm at f/2 opens 25.0 mm, so 50mm blurs it about 1.3 times as much. Close behind the person, the focal length matters less and the f-number more.
Does 50mm compress the background compared with 40mm?
Compression comes from camera distance, not from the focal length itself. For the same framing, 50mm stands about 1.3 times farther back than 40mm, so the background looks larger and closer behind the person and less of it fits in the frame.
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Also see: depth of field, bokeh, crop factor, lens compression.