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Recently I took a photograph at an aperture of f/1.8, but there was very little bokeh. The shot was of a wave with a few boats in the background. Everything seems to be clear even at f/1.8.

Can someone explain this ?

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Depth of field depends not only on aperture but also on distance to the subject. Depth of field increases as the subject gets farther away. If the wave and boats were all relatively far from the camera, but not terribly far from each other, then it's not surprising that they were all reasonably sharp.

You can use DOFMaster to run the numbers yourself. Some examples using a 50mm lens at f/1.8 on a full frame camera:

  • at 15m (50 feet) to the subject, you get a total of 11m (36 feet) of depth of field

  • at 30.5m (100 feet) to the subject, you get 68.5m (225 feet) of depth of field

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To achieve that effect of a blur gackground or enhance the bokeh effect we needs to take in consideration three parameters: aperture size, distance from the lens, and the focal length of the lens. From your question it seems that you was pretty far from the objects; the farther away an object is and focused on, the deeper the DOF is.

Here is a great post about the topic: Focusing Basics: Understanding Depth of Field in Photography. It is visually explained. Hope you like it.

My experience has been, that in addition to not having the proper configuration of the lens, the distance was not the necessary to create the effect as Caleb mentioned in his answer.

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  • \$\begingroup\$ You're not really answering the question, though. He's trying to understand how background blur works, so "experiment with it" isn't really the answer. Not trying to be hard on you, but the vote pattern would indicate that you're off target right now. \$\endgroup\$
    – Joanne C
    Dec 4, 2013 at 20:46
  • \$\begingroup\$ Agree. I'm just learning and I like it. \$\endgroup\$
    – James
    Dec 4, 2013 at 20:50
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Distant background blur for a 50mm/F1.8 setting exhibits a blur diameter of 50mm/1.8 as measured in the focus plane. What is "distant"? Well, at double the focus plane distance, you already have half of that diameter.

So assuming your wave was in focus, how large would a disk of 27mm diameter swimming in your wave appear in the image? That's the diameter of background blurring (and the diameter of highlight circles from distant lights) you can expect.

Focus closer, and such a disk would appear larger.

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