What is fast glass?

If you’ve read my digital camera buying guide and started researching and shopping for a new camera, you may have come across the term lens speed or heard a sales person say a certain lens was fast.

This post will explain what lens speed is all about. Lenses all have a maximum aperture (also called f/stop). The maximum aperture is the widest opening the lens can have. The wider the lens opening the more light it will let in to the camera sensor (or film). Here is a graphic representing apertures:

The relationship from one aperture to the next is that each aperture value lets in twice as much or half as much light as one next to it.

For example: f/2.8 lets in half as much light as f/2, and twice as much light as f/4.

So what does that have to do with speed?

I thought you’d never ask! Let’s compare these two lenses:

Lens 1 has a maximum aperture of f/2.8

Lens 2 has a maximum aperture of f/4

If you’re shooting a sports event and you want to freeze the action, you need to use the fastest shutter speed possible.

  • If you’re using the f/2.8 lens (at maximum aperture) let’s say the corresponding shutter speed is 1/2000 second.
  • For the f/4 lens in the same lighting conditions, you would only be able to get a shutter speed of 1/1000 second.
  • That makes the f/2.8 lens faster, because you can get a faster shutter speed when it’s wide open.
  • Here’s another real life example:

You are in a low light situation, perhaps indoors. You don’t want to use flash, so you open up your lens to it’s widest aperture.

  • At f/2.8 your meter gives you a corresponding shutter speed of 1/60 second.
  • If your lens only went to f/4.0 you would need a shutter speed of 1/30 second, and that may cause blurring in your image, because typically you can’t handhold your camera steady if your shutter speed is slower than 1/60 second.

So why don’t we all just use fast lenses?

Fast zoom lenses usually cost a lot more than slower ones.

Typically your entry level dSLR camera comes with something like an 18-55mm zoom lens that only opens up to about f/3.5. That’s close to f/4. So not very fast. By the way even really good zoom lenses typically don’t go wider than f/2.8.

Prime lenses, the ones that have just one focal length can go as wide as f/1.2, and some even f/1.0, so really fast!

If you want to work with a faster lens you can usually pick up a plain ole 50mm prime lens with a maximum aperture of f/1.8 for around $100. Sometimes you can even get them used for cheaper. If you have an old film camera kicking around, it probably has a 50mm f/1.8 lens attached to it. That was the old standard. So, have a look at your camera and lens now.

What is your maximum f-stop? Go look for that.

For more information on choosing your next lens, check out this post too.

One last thing of note: If you have an 18-55mm kit lens, you might see something like f/3.5-5.6 printed on the front of the lens. That means at the widest focal length (18mm) you have a maximum aperture of f/3.5 and when the focal length is set at 55mm, your lens only opens up to f/5.6. So it lets in even less light. These are called variable f-stop zoom lenses and they tend to be less expensive than lenses with just one maximum aperture setting. Basically what happens is as you increase your focal length the light reaching the sensor diminishes.

Learning depth of field is next…

Depth of field and bokeh are also controlled by f/stop. Read this post to learn what other things control depth of field.

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