What is a “Fast” Lens? ~ Photography Tip

What is a “Fast” Lens? ~ Photography Tip

Examples of Nikon Fast Lenses

Nikon AF-S Nikkor 35mm f/1.8G DX Lens

Nikon 10.5mm f/2.8G ED DX Fisheye Lens

Nikon Zoom Super Wide Angle AF 17-55mm f/2.8G ED-IF AF-S DX Zoom-Nikkor Autofocus Lens

Nikon AF-S Nikkor 70-200mm f/2.8G ED VR II Lens

Nikon Zoom Telephoto AF Zoom Nikkor 200-400mm f/4 G-AFS ED-IF VR (Vibration Reduction) Autofocus Lens

Examples of Canon Fast Lenses

Canon Normal EF 50mm f/1.4 USM Autofocus Lens

Canon Telephoto EF 85mm f/1.8 USM Autofocus Lens

Canon Zoom Super Wide Angle EF 16-35mm f/2.8L II USM Autofocus Lens

Canon EF 70-200mm f/2.8L IS II USM Lens

Canon EF 500mm f/4L IS (Image Stabilizer) USM Lens

Fast lens, also called fast glass, are lenses that have a lower f-stop rating. Let’s start out with the standard zoom lens which comes with most cameras when purchased. They are usually rated something like f/3.5-5.6. What this means is in the widest setting you can set your f-stop down to f/3.5, but as you zoom in to the tightest zoom of the lens, the lowest you can set the f-stop will be f/5.6. In low light settings like a school gym, f/5.6 isn’t going to do so well.

A fast lens will have a lower f-stop rating, and it’s usually fixed for all zoom points of the lens.  A good example is a 17-35mm f/2.8 zoom lens. This lens can be set down to f/2.8 zoomed wide at 17mm all the way through to 35mm. f/2.8 is a wider opening, than f5.6 which will allow more light into the camera and will make shutter times faster – thus the name “fast glass”. This was always confusing for me before too. If you look at a list of lenses and you see, for example a 70-200mm f/2.8 and an 80-210mm f/4-5.6, they look like they should be similar, except the f/2.8 is always about 5x more expensive. This is because it’s bigger, has much more optical glass, and is a true professional lens. The faster lenses, with lower aperture settings will allow you to shoot a shallower depth of field and in lower light situations. And because they allow more light in, most camera will focus faster as well.

Now the fast glass zoom lenses are pretty expensive, but you can get the advantage of fast glass a little cheaper with a prime lens. A prime lens doesn’t zoom, it’s simply one fix length (i.e. 35mm, 50mm, or 85mm, etc.) The cool thing with this is, if you know a lens focal length you like (I like the 55mm for doing portraits), it’s light weight, relatively less expensive, and you can get very low f-stops (wider aperture equals more light quicker). Some prime lens come in f/1.4 which is like a cat’s eye, and can almost see in the dark.

Regular Zoom lenses

• usually variable minimum aperture
• not as quick to focus in low light
• limited depth of field range
• slower shutter speeds in low light
• less expensive
• light weight
• good for travel

Fast Glass Zoom and Prime Lenses

• fix minimum low aperture
• shallowest depth of field = soft blurred background
• good shutter speeds in low light
• quicker to focus
• a true professional lens

Special Thanks to @Jewelzdezine on Twitter for the question that led to this post.

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