How does sensor size affect background blur when you keep the same framing?
Asked 1/30/2011
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If I photograph a subject on full frame with a 50mm lens at f/3.5, then switch to an APS-C camera with the same lens and aperture, I would need to step back to keep the subject framed similarly. I understand that depth of field increases when I do this, but what happens to the background blur itself, especially for a distant background like trees far away? Since the physical aperture diameter stays the same, does that mean the blur should stay the same, or does the smaller sensor change the result?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
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Background blur depends on your depth of field. Depth of field (DOF) is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image (wikipedia). Thin depth of field allows you to isolate your subject: the subject is in focus, and the background is blurred. Depth of field depends on several factors:
- Lens focal length (35mm, 200mm, 50mm)
- Lens Aperture (f1.8, f5.6, f8)
- Sensor size (APS-C, 35mm, medium format, large format)
- Subject distance and the ratio of subject distance to background distance
With (1), the longer the focal length, the thinner the DOF. With (2), the larger the aperture (smaller number) the thinner the DOF With (3), the larger the sensor, the thinner the DOF.*** With (4), the closer the subject, the thinner the DOF.
Example: If you have a 200mm lens, at say, f2.8, on a 35mm full frame sensor, and the subject is near you (2-3m), you can blur the background quite a lot.
Inversely, if you have a 35mm lens, at f8, on a cropped dslr (APS-C), and the subject is 6m from you, the background won't really be blurred out.
*** I am not absolutely sure if this is correct in theory, but in practice, with the same setup on APS-C sensor and Full Frame, the FF picture has shallower DOF.
Read more: http://en.wikipedia.org/wiki/Depth_of_field
Originally by user3462. Source · Licensed CC BY-SA 4.0
user3462
15y ago
0
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If you use the same lens and f-number, then the physical aperture diameter is the same on both cameras. For a very distant background, that means the background blur is theoretically about the same in relative terms when framing is matched.
The catch is that to keep the same framing on APS-C, you must move farther from the subject. That increases depth of field, and it also makes any non-infinite background closer in relative distance to the focus point, so it will usually look a bit less blurred on APS-C.
So the practical answer is:
- background at true infinity: essentially the same blur
- background merely far away: APS-C will usually show slightly less blur when you step back to match framing
This matches the common rule of thumb that it’s harder to get strong background separation on a smaller sensor when you keep the same composition, because the increased camera-to-subject distance increases depth of field.
In short: same aperture diameter helps, but once you back up to match the field of view, APS-C generally gives a little less background blur unless the background is extremely far away.
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