Digital Lenses & Image Sharpness

Note: I continue to get questions about how to obtain really sharp images from digital lenses, so I like to publish this little primer from time to time in case anyone interested hasn’t already read it.

Digital Lenses & Image Sharpness

by William Lulow

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

Photographic optical 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, depending on the lens setting. 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 variable focus or 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 an acceptable exposure. 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. (As an additional note though, I have made many images at the smaller apertures, even f/32 on my 60mm macro lens, with no ascertainable loss of sharpness). It is for this reason that it can be said that if you are looking for truly sharp images, buy the fastest lens you can afford. Naturally, the truly fast lenses are always the most expensive.

This is a 24mm, f/1.4 Canon lens. It is the fastest wide-angle lens in the Canon line up. It is capable of producing extremely sharp images in very low light situations. It is also priced at around $1,600 brand new, just for the lens!

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.

This image of singer/songwriter Paula Cole was made in a very low light situation in a club. Exposure was f/5.6 @ 1/100th of a second and an ISO setting of 2000 with a Canon 85mm f/1.8 lens. Notice how everything in the image is perfectly sharp.

Many digital cameras these days, come with what is referred to as a “kit lens.” This means that the lens is sold with the camera as a kind of “starter” digital “kit.” If you examine these lenses carefully, they are usually variable focus (zoom) lenses that aren’t very fast. This means that they may only have a widest aperture of only f/4. Also, they don’t carry that f/4 opening through the range of focal lengths of the lens. If you have a zoom lens with a speed rating of f/3.5-f/5.6, this means that on its wide-angle setting the speed is f/3.5 but if you use it as a telephoto, it’s speed is reduced to f/5.6. This makes a big difference when you are shooting in any other conditions but bright daylight. Other, more expensive lenses have maximum apertures of at least f/2.8. We say that these better lenses are “faster.” They are able to shoot in much less light than standard “kit” lenses. An aperture of f/2.8 is two stops faster than f/4. (It will admit 4 times the amount of light). These faster lenses are more expensive because the quality of the glass is better as is the size of the glass as well. Take a close look at them at your camera store. You will see instantly how different they are.

In addition, many camera enthusiasts love to use their kit lenses with their cameras on AUTO. This only compounds the mistakes that lead to image softness and will only result in disappointment. When you let the computer chip in your camera make exposure decisions for you, you will almost always be disappointed.

This article hopefully convinced you that sharpness in your images is obtained by carefully thinking about what kinds of lenses to use and how to use them properly.