Why Buy Fast Glass?
I've had a number of questions over recent times regarding the purchase of fast glass. The question usually revolves around the question "should I buy a fast, fixed lens?" Well, it all depends. Here's what I think.
What Does Fast Glass Mean?
Just as the camera's Shutter Speed determines the amount of time, usually in fractions of a second, that light is allowed to reach the sensor or film it is the Aperture that determines how much light the lens allows through. And in photography speak an italics f is used to define a particular aperture (e.g. f4).
An aperture of, for example, f22 is referred to as a narrow aperture as it's relatively small and lets very little light through. Conversely f2 is a relatively wide aperture that lets significantly more light through.
The vast majority of lenses available today offer the ability to set one of numerous apertures from, for example, f4 to f22. Basically the term fast glass relates to the ability of the lens to let more light through and onto the sensor or film than would be the case with a lens with a narrower maximum aperture. So a lens with a maximum aperture of f2 provides the ability to let 2 stops (4 times) more light through than would be the case with a lens with a maximum aperture of f4. In this case both lenses have an aperture of f4, but only the f2 lens has the ability to open the aperture wider than f4. In this case as wide as f2.
Do You Need Fast Glass?
You buy a fast lens for its ability to produce exceptionally shallow DOF and to be able to work, hand-held, under low light conditions. What's more faster lenses are generally more expensive which means they're generally of a higher quality. And by that I mean they promise better image quality and are more robust. The better versions will likely be made of glass and metal, rather than plastic and plastic, and may incorporate weather and/or dust seals.
One of Many Compromises
As zoom lenses are, in effect, a variety of different focal lengths within the one physical lens they incorporate a number of individual glass elements, some of which move as the lens focal length is altered (by zooming in or out). These factors tend to reduce the amount of light getting through to the sensor. As a result zoom lenses with maximum apertures wider than f2.8 are very rare. This is particularly true for ultra wide-angle and the most powerful telephoto (as in telescopic) zoom lenses.
The same is not true for fixed or prime lenses. Depending on the focal length it's possible to acquire a fixed lens with a maximum aperture of f1.4 for a number of wide-angle, standard and short telephoto lenses. As a way of explanation f1.4 allows 8 times more light to reach the sensor than f4. That has the affect of increasing the camera's shutter speed from, for example, 1/8 second up to 1/60 second minimizing the chance of blur resulting from camera and/or subject movement.
An aperture of f1.4 also has the capacity of producing a substantially shallower depth of field (DOF) than would be the case with the same image made at an aperture of f4.
Improvements to Framing and Composition
There's no doubt that a zoom lens offers a great deal of flexibility when it comes to framing an image. If your subject is too far away simply zoom in to allow them to fill more of the frame. Likewise zooming out will allow you to fit more of the surrounding scene into the frame.
The problem is that zoom affect more than just the size of the subject within the frame. Zooming changes focal length which alters perspective and the angle of view. As a consequence the relationship between the subject and its surrounds is altered, and not always for the best.
One advantage for the serious photographer is that, where and when you have the ability to do so, moving closer, further away or around your subject will likely produce far more interesting and compelling compositions than would likely be the case simply by zooming in.
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Glenn Guy, Travel Photography Guru