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I am new here. I have a Nikon D5300 body and a Nikon 50mm f1.8 AF-D lens(not AF-S).

I want to do this:

Two persons sitting in front of a table facing the camera. I want the table and both persons in focus, and the background blurred.

Please advise.

  • There MUST be many existing answers on SE already and there are countless 1000's on the wbe. You can find calulators and formulae that give you depth of focus in given cases, but.| Set lens to f/1.8 for best result. Aperture priority or Manual mode allow this. (Auto modes will if you play with overall settings). Make distance tobackground : distance to subject ratio as great as you can (or accept what is available). Focus on subject. Go. | For abs best result focus short of subject as much as possible while having subject acceptably sharp. Usually DOF is too shallow to have any choice. – Russell McMahon May 11 '15 at 2:57
  • Try browsing the depth-of-field tag for useful questions/answers. – damned truths May 11 '15 at 7:21
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You can calculate what you need to do as follows. First we need to look up what the pixel size r is for your camera sensor. For your camera we can find here that r = 3.8*10^(-6) m. Then the so-called "hyperfocal distance" H is given in terms of the focal length f and F-number F as:

H = f^2/(F r)

The hyperfocal distance is defined as the minimum distance you must focus at so that all object farther away than that distance till infinity are in focus. While we're not interested in doing that here, many formulas simplify when expressed in terms of H rather than in terms of r.

Then the depth of field when you are focusing on objects that are distance d away is given by:

DoF = d^2/H

This formula is valid when d << H. If we now substitute the values for r and f in the formulas we get:

H = 658 meters/F ---->

DoF = d^2 F/(658 meters) = 0.15 cm (d/meter)^2 F

In your case, a depth of field of 40 centimeters could be enough. If you focus 20 centimeters in front of the persons then everything from 20 centimeters behind the persons till 60 centimeters in front of the persons will be in focus. This will bring a normal sized table and the persons in focus. If we equate the depth of field to 40 centimeters, we get the equation:

0.15 (d/meter)^2 F = 40 ----->

d = 16 meters/sqrt(F)

If you take the F-number to be around 7 then this means that the distance to the focus point must be taken to be about 6 meters away.

Now the 40 centimeters here is the distance in front and after the focus point that is in perfect focus, 40 centimeters behind that you have a region that will get a blur 2 pixels wide, and then behind that a region of 40 centimeters will have 3 pixels blur etc. You may want to have a far steeper increase in the blur as a function of distance. But this can only be done by decreasing the DoF.

Suppose you compromise on the entire table being perfectly sharp, and you aim for a DoF of 10 centimeters to get the persons perfectly sharp. Then the tables may not be perfectly sharp but the blur of points at the front of the table will just be a few pixels wide. Then taking the aperture to be F/5, the focus point must be put 3.6 meters away. Objects that are 3 meters away from the focus point will have a 30 pixels wide blur, which should be clearly visible. So, you now get a much better blurring effect of the background.

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