What is aperture?
The aperture of a lens represent its iris. This is measure in F stop’s. I love to compare a photography lens with the human eye. When dark, the human eye can open it’s iris up to receive a much greater amount of light. You can do the same thing with a lens. The maximum aperture of your lens is written on the barrel or front of your lens. Using a lens at maximum aperture requires skill to properly achieve focus. You will also noticed that the quality in definition and sharpness will increase to achieve it’s best at around f5.6-f8 depending on the lens.
Tip: test your lenses and write the f-stop you find most usable on a sticker inside your lens cap.
There is 16X times more light available at f1.4 then at f5.6
The Aperture also gives you a creative control called Depth of Field (DOF). The depth of field is the area in which your subject is defined in your image. Meaning the point A to B where the image will be sharp. You want to pay attention to this effect since you can control your audiences eye through your image with the DOF.
For portrait a shallow depth of field (small f stop) is usually used to isolate your subject from is surroundings and the distractions around him. In landscape photography a more extended DOF is often use to show the whole surroundings.
The aperture of a lens is an important factor towards Autofocus performance. The more light a lens captures, the more light the Autofocus system will see. A bigger aperture will also allow you to see more light in your viewfinder since a digital camera also shows you the scene at maximum aperture, hence the DOF preview button on most DSLR. This is also why sports shooters will use f2.8 lenses to capture sports images. The bigger aperture will help them achieve a higher shutter speed, a more pronounced subject isolation, see more light in there viewfinder, achieve faster AF performance to chase their subjects.