Today’s Question: The term “fast glass” was common decades before autofocus lenses came on the market. Fast glass allowed faster shutter speeds for use in sports photography, to better stop the action of fast-moving subjects.
Tim’s Quick Answer: While my previous answer related to a “fast lens” or “fast glass” emphasized autofocus performance, a lens with a larger maximum aperture size would indeed enable faster shutter speeds, all other things being equal.
More Detail: Every now and then an installment of my Ask Tim Grey eNewsletter leads to a surprising number of responses from photographers, and the subject of “fast lenses” led to an especially large number of replies. It is certainly fair that a lens with a larger maximum aperture size would enable a faster shutter speed, but I would consider that a secondary factor in the context of today’s technology.
As many photographers reminded me, the term “fast glass” for lenses predates autofocus. I suppose that wasn’t as strong in my perspective because while I started out using only manual focus lenses, for most of my time as a photographer I’ve had the luxury of autofocus.
In addition, with the common availability of lens stabilization technology in lenses and camera bodies, it has become less important to choose a fast lens for purposes of achieving a faster shutter speed. For example, when a lens is available in models that feature a maximum aperture size of f/2.8 versus f/4, having image stabilization technology will provide a greater advantage than the additional stop of light via the aperture.
So, while I agree that the term “fast lens” or “fast glass” was in use before autofocus was available, I do think that autofocus performance is a more important reason to consider opting for a lens that has a larger maximum aperture size compared to the ability to achieve faster shutter speeds.
And, of course, as a photographer who tends to favor capturing photos with narrow depth of field, a wider aperture can enable you to create images with less of the scene in focus as well.
I very much appreciate the photographers who replied to my original email on the subject of fast lenses. One of the things I have always enjoyed about publishing the Ask Tim Grey eNewsletter is that readers are quick to respond if they find something I’ve left out, that I’ve stated incorrectly, or that they simply disagree with me about. The feedback is appreciated.