A Word About Lenses

A Word About Lenses

by William Lulow

The following will serve as a greatly simplified lesson in optics for digital photography.

Lenses are curved pieces of high-quality glass that serve to focus light rays as they enter the camera on a spot behind the lens called the focal plane. The better lenses can focus the light, the sharper the image will be. One of the characteristics of all lenses is that they are made up of a number of elements of glass all spaced in such a way as to make the focusing of light accurate. The further apart these elements are, the more the lens is able to focus on a small part of the scene and magnify it or make it appear larger. The closer together these elements are, the more the lens is able to take in more of the scene. If the elements are able to move within the lens (as in a variable focus or zoom lens), the more they will be able to focus on near objects and distant objects both. Movable elements introduce a factor that often makes it difficult for the lens to achieve really sharp results. That is not to say that acceptable results can’t be achieved with zoom lenses, but as a rule, they are not as sharp as fixed focus lenses with no movable elements.

As a lens is used at its largest aperture, more of the edges of the glass are used to focus the light rays. Because of this, the larger the aperture, the less the lens will need to be used at the largest aperture to make a truly sharp image.  If the lens can be stopped down one or two f/stops, it will be using more of the center of the lens and less of the edges. This will yield sharper images. In other words, a lens with an aperture of f/1.8 has a larger piece of glass in order to be able to admit a correspondingly larger amount of light. Therefore, this lens, if used at an f/stop of f/2.8 will yield a sharper image than a lens whose largest aperture is f/2.8, because it will be stopped down one or two f/stops, thus using more of the center of the glass, as mentioned. As a lens uses more of its center to focus light rays, its ability to render sharp focus increases up to a certain point. (Within the lens, the shutter blades themselves can cause some refraction of light. So, if a lens is stopped down to its minimum aperture (say f/16 or f/22) some refraction may take place that could alter the sharpness of the image.

So, the point of all of this is that if a good lens is used a couple of stops down from wide open and/or a couple of stops from its minimum aperture, it will probably achieve its maximum sharpness. For photographers intent on producing really sharp images, this means using lenses that are fast enough (have a wide enough maximum aperture) for their purpose and having enough light in the scene so that they can be used somewhere in the middle of their aperture range. If you are shooting in mostly low light situations where you have to shoot wide open, you probably won’t be able to achieve really sharp images. One way around this is to increase the camera’s ISO settings to allow for shooting one or two stops down from wide open. I do this quite often when I am shooting musicians in concert or at a small club where the light is not very bright. Most of my concert photographs are shot with an ISO of 2000 and lens settings somewhere around f/5.6 at 1/100^th of a second.

Here is a shot of the singer/songwriter Paula Cole during a performance. Notice how sharp all the details are.

Another shot of singer, Judy Collins. Most of my performance images are made with either a Canon 24mm f/1.4 or an 85mm f/1.8 lens. Occasionally I use my Canon 20mm f/2.8 lens as well.

The main take-away here is to buy the fastest lens your budget will allow. Also, if you want to use variable focus lenses (zooms), make sure the lens will carry its widest aperture throughout its entire range of f/stops. If a lens says something like “f/2.8-f/5.6” it means that when the lens is used at it’s maximum telephoto setting, its maximum aperture will only be f/5.6. This lens will probably not be good to use under any low light conditions.