Depth of Field - Aperture

To understand the basics of photography you need to understand exposure.  Exposure is the amount of light collected by the sensor when taking a single picture.  If the shot is under exposed then the image will appear too dark, whereas if it's over exposed then it will appear too bright or washed out. 

http://en.wikipedia.org/wiki/File:Aperures.jpg

There are three adjustable elements of camera setting that allows you to control exposure - these are Aperture (the size of the lens opening - that  lets light in to the camera, also known as the diaphragm) , ISO (the cameras sensitivity to light) and Shutter Speed (the length of time the sensor is exposed to light) 

For now I will be looking at aperture.  The aperture setting allows you to control depth of field - the part of the image actually in focus - or the distance between the nearest and farthest object in focus.  Aperture setting is indicated by f-numbers - each f-number represents a 'stop' of light. 

A small f-number such as 2.8 will allow for a huge amount of light to pass through the lens.  This is  known as a large aperture and results in images with shallow depth of field - this is when the subject in the foreground is clearly in focus and the rest of the background is out of focus.  At a higher f-stop such as f22 a very small amount of light will pass through the lens due to the small opening of the diaphragm, even with a longer shutter speed. 

Shallow depth of field is commonly used in macro photography - so flowers, insects, fruits and vegetables etc. It can also be used when doing close up portraiture - this helps to blur out the distracting background and puts the focus or attention on the subject being photographed.

We did some practise shallow depth of field photography in class but being a perfectionist I wasn't very happy with them so decided to do my own at home.  Below you can see the results -

I decided to photograph the tomatoes as they're very colourful and look very aesthetically pleasing. They were also very good for demonstrating shallow and narrow depth of field. 

In all the images I kept the white balance to sunny as it was a very bright day and also kept the focal length to 55mm and the ISO to 100.  I only changed the aperture and the shutter speed. 

  

f 2.8, 1/100th, ISO 100, 55mm

This image clearly demonstrates shallow depth of field as the only tomato in focus is the one in front.

f 8, 1/30th, ISO 100, 55mm

As I changed the aperture to f8 more of the tomatoes started to come in focus. 



f 22, 1/5th, ISO 100, 55mm

At f22 all the tomatoes are now in focus.  Higher f-numbers are usually used when photographing landscape as you want everything in focus.

The two images below demonstrate how shallow DOF can look pretty amazing when doing macro photography.  I took these images when I was in St Anne's and used my 70-300mm at full focal lenght. 

f8, 1/250th, ISO 100, 300mm

f8, 1/200th, ISO 100, 300mm

DOF can be used to great effect and many photographers have done so in their images.  I will discuss this further in my next blog.