Many newbies confuse the term fast lens to mean that these lenses allow you to shoot fast (frame rate). But that is not the case, Fast lenses are so called because they allow you to use faster shutter speeds for any given ISO setting. So put simply the larger the maximum aperture of the lens the faster the lens is.
Advantages of fast lenses
- Faster shutter speeds
- Better Bokeh
- Lower ISO’s
- Shooting in low light
- Shallower depth of field
- Brighter view finder
- Faster focusing
- Can use extenders and tele converters
- Better lens construction
In general lenses that have maximum aperture of f/4 or more (f/2.8, f/1.8, f/1.4, f/1.2) are called fast lenses.
Having a fast lens in your kit is advantageous in many ways. First and foremost they allow you to create those shallow depth of field pictures in which the subject really stand out from the rest of the elements and background elements are rendered in a smooth blur. This is something that cannot be done (in most cases) with slower lenses.
Another advantage a fast lens has (in majority cases) is the smooth rounded shape of the bokeh it creates which is a very pleasing effect.
In situations where you are shooting handheld and light levels are low having a fast lens or not could mean either you get your shot or not. You can keep you ISO within manageable limits to control noise.
You get a brighter view through the viewfinder and the extra light coming in through the lens helps the camera’s auto focus mechanism perform better, there by achieving faster focusing speeds and better focusing accuracy.
Fast lenses are also advantageous when you need to use extenders or tele converters with your lens. For example if you have a 300mm f/2.8 lens you can fit it with a 2x tele converter and still have autofocus enabled on all focus points. The lens now behaves like a 600mm f/5.6. Remember with lenses that has maximum aperture of f/8 only the center auto focus point works and for even slower lenses auto focus does not work at all.
Last but not least, faster lenses in general (although costly) are better to their slower siblings in both lens quality and construction quality.
Next: Depth of Field