Why is the background still fairly sharp at 18mm and f/3.5?
Asked 3/1/2013
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I photographed my son indoors with a Canon T3 at 18mm, f/3.5, 1/125s, ISO 100. I was close enough that his head filled most of the frame, and the wall and tables behind him were about 20 feet farther back. He was in focus, but the background was also still quite recognizable instead of strongly blurred. Why would the background stay fairly sharp even at a low f-number, and what settings would create more blur?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
13y ago
2 Answers
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The small degree of defocus in the background is due to the focal length used being very short (18mm).
The amount of background blur depends on the size of the entrance pupil, not the f-number. The entrance pupil size is the focal length divided by f number, so in this case it would be about 5mm. This is quite small. A 100mm lens at f/3.5 would have an entrance pupil of size 29mm.
For this reason you will get more out of focus backgrounds at the other end of the kit zoom range at f/5.6, at 55mm the entrance pupil will be twice the size at 10mm.
Originally by user1375. Source · Licensed CC BY-SA 4.0
user1375
13y ago
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Because 18mm is a very short focal length, it naturally gives a lot of depth of field. An f/3.5 aperture sounds “wide,” but at 18mm the actual entrance pupil is still small, so background blur is limited.
For the same subject framing, stronger background blur generally comes from:
- a wider aperture
- a longer focal length
- more distance between subject and background
So with your kit lens, you’ll usually get more blur by zooming in toward 55mm, even if the maximum aperture becomes f/5.6. In practice, 55mm f/5.6 can blur a distant background more than 18mm f/3.5 for a portrait.
Also, when focusing relatively far away compared with such a short focal length, much of the scene can remain within acceptable focus, especially indoors with background elements 20 feet behind the subject.
If you want a blurrier background, try:
- zooming to the long end of the lens
- keeping the aperture as wide as possible
- moving the subject farther from the background
- moving closer to the subject (while maintaining framing with focal length)
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