Depth of filed depends on three factors

It is inversely proportional to APERTURE and FOCAL LENGTH

and dierctly proportional to DISTANCE

It is inversely proportional to aperture. 

If the aperture is opened up, the background is more out of focus The background becomes relatively sharper as the aperture is closed down.


It is inversely proportional to Lens focal length


If the focal length is more the depth of field is less. A telephoto lens has less depth of field compared to a wide angle lens. If the aperture is open the depth of field will be shallow as compared to a closed down aperture. It is in fact a combination of all the three factors which influences the depth of field.

If we are taking a picture with a telephoto lens (wildlife, sports or fashion) we are starting off with a shallow depth of field as the focal length is not in favour of DOF. To get more DOF we have to relatively close down the aperture much more as compared to if we were shooting with a wideangle lens ( landscape, reportage etc)

The following photographs have almost the same composition and frame but different focal lengths and apertures. The one with the closed aperture at a shorted focal length has more depth of field.

24mm  f 16  1/125
105mm f 9  1/200th sec

It is directly proportional to the distance between the camera and the subject.

when the camera is closer, the background is more out of focus.
By stepping back, while more of the subject is included in the frame, the background also is relatively more sharp compared to when the camera was closer.

Depth of field reduces as the photographer goes closer to the subject either physically (distance) or optically ( focal length )

DOF is not equally distributed. It is more behind the subject than in front of it.

The DOF is the region where the size of the circle of confusion is less than the resolution of the human eye. Circles with a diameter less than the circle of confusion will appear to be in focus.


How to achieve shallow depth of field

The best way to achieve a shallow depth of field is to use a tele lens or a relatively longer focal length. secondly fill up the subject as much as you can, i.e reduce the distance between the lens and the subject or in other words be somewhere close to the minimum focusing distance of your lens. Further keep the aperture as open as possible keeping in mind the aperture-shutter speed exposure combination. Keeping the aperture wide open may some time lead to too shallow a depth of field that you may not get all portions of your subject covered within the shallow depth of field This may be desired at times, which is why it important to always focus on the eyes as the eyes are are the most important area in a photograph and we can't afford to have them not in focus. To counter this, you may arrange your subject in a manner so that there is maximum possible difference between the subject and the background. Now you may closed down the aperture a little bit more as the increase in depth of field will only influence the subject and not the background.



As the focal length is increased notice that the background is becoming less sharp. Depth of field decreases as the focal length is increased. Also, if the focal length was kept the same and the photographer moved closer to the subject, it can be concluded that the depth of field decreases as the distance between the subject and the camera decreases. Notice that as the camera is moved closer or as the focal length is increased the background becomes more out of focus. There is more blur in the last picture compared to the first. DOF is inversely proportional to focal length and directly proportional to distance. 


Factor Effect of DOF on photograph

The distance between the camera and the subject 

The closer your subject is to the camera, the greater the tendency for the objects in front or behind the subject to blur in the photo. 

The size of the lens opening (the f-stop or aperture) used to take the picture 

Larger f-stops (smaller numbers), such as f/2 or f/4, produce less depth-of-field than smaller f-stops (larger numbers), such as f/11 or f/16. Remember: The size of the numbers are reversed because apertures are actually the denominators of fractions, so 1⁄2 and 1/4 are larger than 1/11 or 1/16

The magnification (or focal length) of the lens 

The shorter the focal length of the lens (say, 18mm or 20mm), the more depth-of-field is present. When the focal length grows longer (say, to 70mm or 100mm), depth-of-field shrinks. 

Focus is on the chain with the aperture closed down.

The focus is still on the chain with the aperture opened up. The car is quite a lot out of focus.

with an opened aperture the focus is shifted to the car. The depth of field which is anyways less in front of the subject is so shallow that the chain is hardly visible.