Tuesday, August 24, 2010

Lesson 03 - Depth Of Focus (DOF)

I haven't had much chance to get out and about photographing 'stuff', so I thought it was about time to post another official 'lesson'.  Today, I'm going to talk about something you'll often see referred to as DOF.  DOF stands for Depth Of Focus (it also stands for Depth Of Field - there is a technical difference that I won't bore you with, but they mean the same thing as far as your pictures are concerned...)  DOF is about how much of your picture is in focus - from the closest things to the furthest things.  Let me show you an example...

In this picture I took while wandering around Hyde Park, it's obvious what the picture is of... isn't it?

Of course it is - Poppies!  Lots of them... all different colours, close-up, middle distance, far away... Poppies, Poppies, Poppies!

In fact, there are probably a few too many Poppies.  When you first look at the picture, your eye may be drawn to the white one in the middle, or the orange one at the bottom, but then what..?  Your eye wanders around trying to find other things to focus on and look at...  hmm - that's an awful lot of Poppies - just excuse me for a few minutes while I check out each one...

This is where we can use the concept of DOF to help.  Roll down to the next picture of the same scene.

Now, see the difference?

Immediately, the eye is drawn to the cluster of Poppies at the bottom of the picture (ok - this is purely for demonstration purposes - don't start flaming me for the really lousy composition). Your eye may then briefly check out the rest of the image - discover there is nothing much to report, and return to the real subject and check it out in more detail.

So used in this way, DOF is being used to isolate a particular part of the image by keeping it IN focus, and making as much as possible of the rest OUT of focus.

DOF to isolate a subject like this works best with something close, and a reasonably distant background, such as the scene you see here, or portait close-ups for example.  The closer you can get to the thing you want in focus, the 'shallower' your depth of focus will be - check out a few macro shots of bugs, that are taken from just a few inches away - even in the width or length of the bug, you'll see how one part is in focus, and the rest (just a fraction of an inch away) is out of focus. 

So what did I do differently in the two pictures?  Change the aperture - that's all (ok, you got me - when I changed the aperture, the camera changed the shutter speed accordingly, to keep the same amount of light coming in to the sensor).  The first shot was taken with a very small aperture of f/32 (remember - big number means small hole).  This renders a lot of the picture in front of and behind your subject, in focus.  The second shot was taken with a wide aperture of f/5.6 - this reduces the front and back focus spread considerably.

Having mentioned macro photography, this introduces the other side of the DOF coin.  In this case, you want to increase the DOF as far as you can to get more of that bug into focus.  How?  Well, the easy part obviously, is to use a very small aperture - as small as you can get.  However, since the idea of macro work is to get really close, the other option of distance is out of the question - it's no good having your bug 6 feet away to ensure the whole thing is in focus...  Working with a small aperture will mean longer exposure times, and this may lead to needing extra lighting, in the form of flash, studio lights, or a light box/tent, or perhaps a ring flash for macro work (just be careful not to cook your bug) - however, before I get too carried away, lets stop there, and accept that (assuming you have enough light) you can increase your DOF by using a small aperture.

So, in summary...
  • DOF (Depth of Focus or Depth of Field) is an indication of how much in front and behind your subject is in focus. 
  • Use a wide aperture to get a 'shallow' DOF, and isolate your subject against a blurry backdrop. 
  • The closer you can get to the subject, the greater the isolating effect of DOF is.
  • Use small aperture (and possibly more light) to get a deeper DOF, and have more of the foreground/background in focus.
Until next time
happy snapping.