lens speed

[Perry Wilson]

How do I know if a lens is fast and what does a fast lens do in comparison to one that isn't...

[cjwolff]

In general terms, a fast lens has a large aperture which makes shallow dof possible.

[Perry Wilson]

so is the speed of the lens in the name like Tokina 11-16mm f/2.8 AT-X 116 Pro DX Autofocus Lens for Select Canon EOS Digital

[tony240sx]

Yes. The speed is the f stop. So the speed of that lens would be 2.8. It determines how much light is needed for proper exposure. The faster the lens the less light needed.

[Perry Wilson]

so the smaller the f/ number the faster the lens?

[J Davis]

Before video came along a lens was described as 'fast' because you can use 'fast' shutter speeds

ie. it lets a lot of light through because the glass has a good coating and the aperture opens wide

Example: a nikon 50mm f1.4 is a faster lens than a nikon 50mm f1.8
(f1.4 being a wider hole to let light thru than f1.8)

http://upload.wikimedia.org/wikipedia/commons/thumb/8/87/Aperture_diagram.svg/462px-Aperture_diagram.svg.png

[DanDOF]

Yes, so a fast lens (i.e. a lens with a small f-stop like 1.4, 1.8, 2.0, 2.8) is a bright lens.

Imagine a f/1.4 lens is like having your eyes wide open.
A f/2.8 lens might be like having your eyes normal.
And... an F/4 lens would be like squinting your eyes a bit.

Most lenses can be adjusted at least to be as dark (or slow as) f/16.
F/16 might be like squinting your eyes as hard as you can, but still seeing some light.

[Perry Wilson]

Yes, so a fast lens (i.e. a lens with a small f-stop like 1.4, 1.8, 2.0, 2.8) is a bright lens.

Imagine a f/1.4 lens is like having your eyes wide open.
A f/2.8 lens might be like having your eyes normal.
And... an F/4 lens would be like squinting your eyes a bit.

Most lenses can be adjusted at least to be as dark (or slow as) f/16.
F/16 might be like squinting your eyes as hard as you can, but still seeing some light.

lol you guys are the greatest