* Aperture controls the depth of focus of a lens *
The depth of focus is the distance in front of and behind your photo's main subject that is also kept sharply focused. It's also referred to as 'Depth Of Field (DOF)'.
The lens aperture is calculated in a series of f-stops, or f-numbers, which are fractions of the focal length of the lens. For example:
If your lens is a 50mm F2 lens, it means that the maximum aperture is HALF the focal length of the lens. So, in this case, 25mm (50 divided by 2). F-stops are fractions, so the larger the f-number, the smaller the aperture, and equally, the smaller the f-number, the larger the aperture.
Clear as mud? Good :)
The term to 'stop down' is often used to mean the closing down of the aperture to a smaller diameter. To 'open up' means to increase the aperture to a larger diameter.
Increasing the aperture by 1 stop (this is done by using the aperture ring on the lens), doubles the volume of light entering the lens (eg. F5.6 to F8). Decreasing by 1 stop, halves the volume of light allowed to enter (eg. F5.6 to F4).
This used to confuse me and I still have to think about it every time I take a photo!
This is what an Aperture ring looks like - many aperture rings also have half-stop positions, so for example, a stop between F5.6 and F8:
It's important not to snap away at your subject without considering the Depth Of Field or focus.
I shot this image at F2.8 with a shutter speed of 1/60 sec. The aim was to ensure the background and foreground was blurry (shallow depth of field), and that the main subject/model remained sharply in focus. If I'd wanted a sharper subject, I could've increased the shutter speed to 1/250 sec.
To increase/widen your DOF (everything in sharp focus, lots of detail): try stopping down and using a slow shutter speed.
To decrease your DOF (shallow): open up the aperture and use a quicker shutter speed.
Enjoy experimenting! More pics to follow ....
