Sunday, June 10, 2012

How to get that beautiful background blur

The beautiful background blur is one of the biggest attraction for new and budding photographs towards an DSLR. Shallow depth of field (DOF) as it called or more technically put the background blur has been an aid to photographers for ages. The main reason it works is because it follows a photographic rule of negative space. Basically a shallow DOF isolates the subject and blur away the distraction into a beautiful soft out of focus background. (The quality of the blur is usually referred to bokeh)

DOF simply put is the area which is in focus before and after the point of focus or Depth of field. If more of the area before and after is in focus that is more depth of field and if less area is in focus before and after the focus point it is shallow depth of field. If you look at the image below. The camera is at the same place. For the black line the filed of view is lesser or shallow depth of field. For the blue line has more depth of field is more, more area is in focus.

Here is the tricky part how do you get that beautiful DOF. Well now we get into the technical aspect of photography. Here we go again with physics. There are three things which mainly affect DOF. The first is the lens choice, the second the aperture and the third is the distance from the subject.

Let's take all of these together

Choice of Lens
Wide angle lens have inherently more depth of field than a telephoto lens(any lens which is less than a 50mm lens is referred to a wide angle lens and greater than 50mm is a telephoto. 50mm is a normal lens because the field of view of a 50mm is close to human vision [All in 35mm sensor size equivalents]. More on this on another post )

So if you photograph a subject with a wide angle lens and telephoto lens standing at the exact same location the with the same aperture you will have more depth of field with a wider lens than a telephoto lens. Take a look at the two image below. The one on top has more DOF than the one below though the aperture shutter speed and ISO is constant

28mm f5.6 1/80 ISO 320

55mm f5.6 1/80 ISO 320

Aperture and Depth of field
If you stand at the same point, with the same lens and focus on the subject with a wide aperture (say f 2.8) compared to a narrow aperture (f 16) you should have more DOF. Take a look at the images below. In the image below the one with a wider aperture f2.8 has less DOF compared to the one below at f13 

35mm f2.8 1/400 ISO 1000
35mm f 13 1/20 ISO 1000
Distance from Subject
This is a very important factor for DOF. If you have the same aperture and if you subject is closer you will have a shallower DOF and if the distance is more the DOF will be more.This is something very important. A lot of photographers chasing that beautiful blur buy really expensive lenses and when they shoot at f2.8 and the DOF is really large they wonder why are they not getting the blur. The reason distance from subject. Take a look at the two images below. They both have been shot with the exact same lens, aperture ISO and shutter speed. But the one below has less DOF because I stepped in closer and reduced my distance from subject.
35mm f2.8 1/320 ISO 320

35mm f2.8 1/320 ISO 320

There is another factor which affects DOF and that is crop factor. Let's save that for another time. Meanwhile do check out the following links on DOF. They are really useful

Depth-of-Field: Wide Angle Lenses