Depth of Field - the Third Dimension

If you have done only Point and shoot photography so far, then most probably you are not familiar with the term Depth of Field.  Why ? I guess for the same reason why you may not know much about Tax laws - you didn't have any control over it! :-)

Take a look at the picture below.




Everything from the face of the angel figure to the basket-ball net is captured with good to reasonable sharpness. This is because, everything in the frame is within a range of distance where camera can capture it with sharpness acceptable to human eye. The actual point of the focus of the camera is somewhere in-between this range. This range is called the "Depth of Field".
 
Depth of Field is a the range of distance from camera defined such that all the objects at a distance from camera within this range are captured with acceptable sharpness. Objects outside this zone appear blurred.
 
What is acceptable sharpness may depend on what is the desired magnification of the final media where the image will be reproduced. But there are no standard boundaries demarking the depth of field.
 
The reason why we see most of the things clearly captured in point and shoot camera is because their depth of field is very large - typically starting from 5ft and upto infinite distance. Moreover, this depth of field is fixed for point and shoot camera. 

A Prosumer camera is different. It gives you power of changing the Depth of Field, you can move it closer or farther and you can also change its thickness. First let's take a look at the effect of manipulating the Depth of Field. 


F8.0 , 1/13 sec

In the first picture, the angel is sharp, the background is out of focus yet objects are recognizable.  Now compare it with the picture below.

F2.8 , 1/100 sec (rollover the image to see the difference)

In the second picture, the angel looks almost same but the background is completely blurred! In fact it makes the angel stand out a lot more than the picture before. Everything else is pretty much same.
 
In both the pictures, the point of focus is anchored at the same place - the angel - the subject, where it should be. What is different is the depth of field (DOF).


With proper framing you are deciding what you want to capture in terms of width and height of the field in front of you. But with Depth of Field you are deciding the range of distance from the camera that you want to capture clearly. In that sense, it's the Third Dimension of your Frame. Anything that is acceptably sharp is "in-frame" and anything that is blurred is "out-of-frame" in the sense of Third Dimension of Frame.
 
You play with this third dimension with two parameters to get the desired result. Aperture and Focus 
  
Focus: The focus distance or point of focus is sort of an Anchor Point for the Depth of Field. Everything at this distance appears with crisp sharpness. Your Subject should be at this distance. Other objects closer than this point or farther than this point will appear with varying degree of sharpness depending on your chosen DOF. What are the boundary points of the depth of field is a highly debated topic because there are no hard boundaries. It all depends on what is acceptable in the final media. 
 
Aperture: Once you have anchored the focus, you can manipulate the depth of field by changing the Aperture Value - the size of opening that lets in the light on your CCD. The Aperture is measured in F numbers. The main difference between the two pictures above is the Aperture. The first one is captured with an Aperture measured as F8.0 (smallest opening in G5) while the second is with F2.8 (largest opening allowed with full zoom in G5). It sounds a bit confusing that larger number represents smaller aperture, but more on that later. 

• If you choose a large Aperture (small F number), depth of Field will be very narrow. This causes most of the objects in front of the focus point and behind the focus point to be blurred out. This is very suitable for Portrait Photography, as it makes the subject stand out very well. In the drawing below, a darkness of shade represents sharpness at that distance. Darker the shade, more sharp the object will be at that distance.

Narrow depth of field 

Another example of very narrow Depth of Field is the macro shots of a wine glass I posted earlier. You can see that even the front part of the base of the glass is blurred out as it is too close.

• If you choose a small Aperture (large F number), depth of Field will be very wide. This is suitable for landscape photography on your site seeing vacation. Everything from the nearest tree to the farthest mountain will be sharp.

Wide depth of field

Below is another example of wide depth of field. As you can see everything from nearest flower to the farthest mountain is captured with acceptable sharpness.


F8.0, 1/200 sec

• The easiest way to play with depth of field is to use your camera in Aperture Priority Mode. In this mode, as you modify the Aperture value, the camera will take care changing the shutter speed in such a way that your CCD gets right amount of exposure (Read Earlier Post : Cooking and Photography. Also, note the shutter speed in the examples in this post)
  

The side effects: So what else you have to do to make sure you don't mess up anything else in your picture?

• If you are using large depth of field = low aperture = small opening = large F number, make sure that your picture gets enough light for proper exposure. If u are using Aperture Priority Mode, Camera take care of this. If you are using manual mode, you will have to decrease shutter speed or increase ISO sensitivity.
• Similarly if you are using a narrow depth of field = large aperture = large opening = small F number, you have to worry about overexposure. In Manual Mode, increase Shutter Speed, or decrease ISO Sensitivity. Sometimes this may not be enough, e.g. If you are trying to take a picture of a white flower in bright sunlight with very narrow depth of Field. In that case, you will have to use an external Neutral Density Filter, to reduce the intensity of the light. I use G5's built-in ND filer feature.
• If camera selected the shutter speed for you, you still need to take a note of it and decide if you will be able to hold the camera still for that long. Anything below 1/60 or 1/80 seconds should be shot with a tripod or other support.

So far, I have only played with the extremes of depth of field for portrait and landscape photography. However, in Macro photography, a proper selection of depth of field becomes more important as the distance differences between various objects are very small and with macro lenses and filters, the depth of field becomes very narrow.