Plugged In: Sharpening the focus on image quality

By Joe Kashi, for the Redoubt Reporter

Making photos that are sharp and crisp ultimately requires good-enough lenses used correctly, and that’s our topic this week.

Your camera body and your lens both affect image quality. Last week, we considered how your camera body impacts image quality. You can review that article from our web edition at http://www.redoubtreporter.wordpress.com. It’s obviously helpful to choose a camera body with the right imaging sensor and effective image-stabilization hardware.

Once you’ve chosen your camera system, though, it’s time to use your lenses effectively. Being able to achieve sharp focus when desired is critical. Most users, though, simply rely on a digital camera’s inevitable autofocus mechanism and hope for the best. That’s not always optimum because preprogrammed autofocus hardware may not correctly determine the main subject that should be in critically sharp focus.

As a result, your camera may not autofocus where you want it to focus, and may not focus as accurately. One solution is to manually set the autofocus for a particular focus point by using the camera’s autofocus lock, by using a centered autofocus point and half depressing the shutter, by manually selecting the best focus point setting using your camera’s control buttons, or by using a touch-focus-shoot screen, like those found in some Olympus compact-system cameras. With a bit of practice, any of these four common methods of ensuring the best autofocus point becomes fast and easy.

Contrast detection autofocus hardware, used in virtually all point-and-shoot and mirrorless compact-system cameras, is inherently more accurate but often slower than the phase-detection autofocus hardware built into digital SLR cameras. Because dSLR cameras tend to focus faster while providing a brighter, clearer optical viewfinder, they’re still preferred for fast-moving subjects, such as sporting events. With photographing fast-moving subjects, like sports and wildlife, it’s usually better to use only the central autofocus points.

In order to be reliably effective, autofocus hardware requires some basic knowledge and careful usage. You’ll find an extended discussion about effectively using both manual and automatic focus in our Jan. 18, 2012, Feb. 1, 2012, and Feb. 8, 2012, issues. The direct links to those earlier articles can be found in the web version of this week’s article.

Keep depth of field in mind when composing your photo and setting your focus. You can think of depth of field as the front-to-rear portion of a subject that’s in acceptably sharp focus. If the area that must be rendered sharply is fairly deep from front to back, then focus upon the most important part of the subject and stop the lens down to a smaller aperture like f/8 or, in a pinch, f/11. Generally, there’s about twice as much depth of field behind the point of focus as in front of it, so choose your focus point accordingly.

Shorter lenses on smaller 1/1.7-inch sensor cameras, like the Canon S100 or Panasonic LX-7, inherently have greater depth of field, so it may make sense to use a small sensor camera in some situations if you have enough light to use a low ISO sensitivity. On the other hand, use a large-sensor camera and wider aperture, like f/2.8 or f/4, when you want to blur the background so that the sharp subject stands out more distinctly.

Test your lenses to find their sharpest aperture settings. Usually, that’s around f/5.6 for good, aftermarket APS-C lenses, and between f/5.6 and f/8 for most kit zoom lenses. Better compact-system camera lenses are usually sharpest in the f/4 to f/5.6 range, with kit lenses showing their best resolution around f/5.6. Compact consumer cameras are usually sharpest in the aperture range between f/2.8 and f/4. Very few lenses are acceptably sharp at their widest apertures and almost all lenses lose resolution and become softer as they’re closed to apertures smaller than f/8.

Raw sharpness isn’t everything about a lens. You’ll also need to ensure that it has good contrast, so that it can suppress lens flare and other bright lights and internal reflections, and that the lens renders colors in a pleasing manner.

Check the http://www.slrgear.com website to determine whether your lens has been tested. If it has, then open the interactive sharpness panel and adjust the lens aperture and (for zoom lenses) focal length sliders for a graphical display of how your lens performs at different settings. This is also an excellent way to identify the sharpest lenses for your particular camera and budget. Other good lens review sites include http://www.photozone.ze and http://www.lenstip.com.

Because all but the very best lenses (read: typically expensive) are sharper at the center of the frame, often noticeably so, you may consider selectively sharpening important corner and edge portions of your image when post-processing those images on your computer.

The easiest way to selectively increase the sharpeness of the outer portions of an image is to use the “adjustment brush” feature in Adobe Lightroom, increasing the adjustment brush sharpening and “clarity” settings to a positive number, usually between +25 and +50, with 100 percent feathering to smooth out the changes. Just brush the areas that you want to sharpen or, using negative settings, blur. It’s quite easy and effective with a bit of practice.

Sharpening and clarity controls only work with details that are already resolved by your lens and already present in the image file. You can’t sharpen and add detail that’s just not there. Too much sharpening actually degrades image quality, accentuating noise and resulting in halos and other noticeably artificial changes to the image file.

For best results if you have enough light, use a camera’s lowest (“base”) ISO sensitivity, where sharpness, grain, color rendition and dynamic range are all at their best. Using the base ISO setting in dim light either requires a tripod or good image stabilization hardware.

Even for 18-by-24-inch exhibit quality enlargements, a 10- to 12-megapixel sensor is usually adequate unless you’re trying to capture exceedingly fine detail, which is likely beyond the capability of most affordable lenses. Realistically, it’s unlikely that you’ll need a sensor with a resolution greater than 16 megapixels, even for exhibition-quality prints.

In fact, as you increase megapixels, quality and resolution often drop for a given sensor size, particularly when the ISO sensitivity is set higher than base ISO. As the megapixel density increases, say to 24 megapixels on a standard APS-C sensor, effective resolution often drops off quickly as ISO is increased.

Internal reflection in a lens, usually termed “lens flare” is another sharpness killer. Even when not showing obvious optical problems, like out-of-place bright rays and splotches, lens flare can so reduce the overall contrast of an image that important detail is lost. That’s why a lens’ contrast characteristics are so important — a lens with high internal contrast seems much sharper because edges are not blurred by stray light bouncing around inside the lens. Multicoated lenses (sometimes termed “nano-coating”) usually have the best lens flare properties.

Even multicoated lenses, though, are negatively affected by lens flare. For that reason, use a lens hood to block stray light from hitting the front glass of your lenses and avoid shooting directly into unshielded bright light sources where possible.

If you’re dissatisfied with the image quality produced by your current camera and lens, it makes sense to first hone your technique so that you’re consistently getting the best possible results from that hardware. If you’re still dissatisfied, then it may be time to consider upgrading your hardware, but that’s a subject for another day.

Local attorney Joe Kashi received his bachelor’s and master’s degrees from MIT and his law degree from Georgetown University. He has published many articles about computer technology, law practice and digital photography in national media since 1990. Many of his technology and photography articles can be accessed through his website, http://www.kashilaw.com.

Leave a comment

Filed under photography, Plugged in

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s