Whenever people find out that I am a photographer, usually their first question is "What camera should I buy?". I'm not going to answer that question today, but rather I'll try to shed some light on two of the other more popular questions "How do I make my shot's background all blurry?" and "How do I get everything in my shot in focus?".
One simple answer to both questions has to do with Depth of Field (DOF), and we can control the DOF with the camera's aperture setting. The aperture is a diaphragm (think of it as a hole) within the lens that opens and closes to various sizes to let more or less light through to the camera's sensor. Because of some light physics that I am not going to explain here, the aperture also affects the DOF in that a smaller aperture (hole) gives you more DOF (more in focus) and a larger aperture gives you less DOF (more blur).
Lens aperture values are denoted by a "f/" followed by a number (eg. f/2.8). You may have noticed this in my Exif info that I post for each photo of the day. In a slightly strange backwards kind of way, a large aperture (large hole) has a small f/ number, and the number gets bigger as you get a smaller aperture (smaller hole). You can see some examples of this in the figure above. Using the maximum aperture on your lens is called shooting wide open. That is when you have your aperture fully open to let in the most light it can, and it will also produce the least DOF (blurry background). Closing the aperture on the lens is known as stopping down the lens and it will increase the DOF but at a cost of reducing the amount of light.
As you can see in my example Hippo shots above, when you shoot wide open with an aperture of f/1.2 (first shot) you get a very shallow DOF where only the blue hippo's nose is in focus and even his eyes are slightly out of focus. All the other hippos are a complete blur! As you increase the aperture value (f/3.5, second shot) you are decreasing the size of the hole it creates, and you are increasing the DOF to now include the entire blue hippo, but the other hippos are still blurry. Increasing the aperture even more to f/7.1 (third shot) starts to bring the yellow hippo into focus, but it takes a very small hole (f/16.0, fourth shot) to really bring almost all of the hippos into focus.
There are of course other factors that will affect this such as focal length (zoom) and how close you are to the subject. To keep this brief I'll just say this: The longer the focal length (more zoom) the smaller your DOF will be, the wider your focal length (wide angle) the more DOF; The closer you are to the subject the smaller your DOF, and the further from the subject the greater the DOF.
So the next time you want a blurry background just remember to use your maximum aperture (smallest f/ number), and when you want everything in focus use a smaller aperture (largest f/ number).
Do you have any comments or questions about this? Feel free to ask us using the comments section below or by emailing us directly using our contact page.
Exif Data
Camera: Canon 5D mkII
Aperture: f/1.2, 3.5, 7.1, 16.0
Exposure: 1/200
Focal Length: 85mm
Flash: Fired Off Camera E-TTL
ISO: 800
Flash Cord
Tripods
Post processing included conversion from RAW, and cropped
Aperture Diagram borrowed from: http://petphotographytips.wordpress.com/2011/05/24/what-is-an-fstop/
Nice write up on this. I like that diagram.
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