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Friday, April 1, 2005

What Is Sharpness?

A Deeper Understanding Of Sharpness Can Help Better Control It

Most photographers think a tripod is used to prevent blurry photos, and to a degree, it is. It also ensures that the image brilliance inherent in a lens is captured. Sharpness can be lost with cheap, too-light tripods, however. They can shift enough during exposure so that while the photo stays reasonably sharp, image brilliance suffers. A quality, rigid tripod and head is critical to ensure you capture the image brilliance and sharpness of which your lenses are capable.

If you put a solid black line on a white paper, it will appear very sharp. If you use a light gray line, it won't look as sharp, and you may find yourself struggling to focus on it if your eyes are much beyond 30 years old.

Contrast has a significant affect on apparent sharpness. If your subject contrasts strongly with its surroundings, it will be seen by the viewer as sharp. This is one reason why backlit photos often look so sharp—bright contrasts appear between the light and shadow in those conditions. It also explains why foggy photos don't look as sharp—subtle grays have little contrast.

Look for contrasts in your composition wherever appropriate. You could move slightly one way or another to allow a dark area to line up with a key light part of your subject, just for the contrast. Or you could look for contrast in a scene that could be included in a composition.

Have you ever noticed how images of Western mountains and deserts often look sharper than East Coast forests? This is due in part to the light. There typically are fewer clouds in the dry areas of the West, plus there aren't as many trees blocking the sky. This allows clear, crisp light to skim rocks and other geological features, bringing out texture and form, which makes a landscape look sharper.

In addition, with fewer clouds and less humidity, the clear light has more of an edge to it, making it more specular and more contrasty (including color contrast from shadow to light) and heightening the look of sharpness. This certainly can happen anywhere with the right conditions, but haze is more common east of the Rocky Mountains, making this effect on sharpness harder to manage there.

Anytime light can be used to increase contrast, apparent sharpness will increase. While you can't control the weather, you can change your angle to the subject. Sidelight and backlight always give more contrast than light that strikes the front of the subject. Low light also tends to increase contrast, which is another reason why shooting landscapes with the sun near the horizon is such an effective technique.

Backlight is a great light for sharpness, but it has its own challenges. As light strikes the lens, flare often appears. Specular flare, the bright spots that show up in those conditions, is annoying, but has no effect on sharpness. Diffused flare is a different type of flare, insidious because it's often missed, and it diminishes sharpness.

Flare occurs because light is bouncing around inside the lens. The better baffled a lens is in its interior, the less flare can bounce around. The popular zoom lens has a lot of glass elements inside, resulting in many surfaces for light to bounce off of, making this type of lens very susceptible to flare. Multicoating helps, but it doesn't eliminate flare.

If the light spreads out across an inside lens surface, diffused flare occurs, reducing contrast and sharpness and possibly affecting color. You often can see this effect by using your hand to shield the lens, blocking any direct light to it. A lens shade is a critical accessory for sharpness to prevent diffused flare problems. You also can use your hand or a hat to shield the lens from particularly bad flaring conditions.


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