Plugged In: Photo tips exposed: Make sure light is right

By Joe Kashi, for the Redoubt Reporter

Proper exposure is the second core principle of good photography, complementing the techniques of sharp focus and crisp detail that we examined last week.

While special circumstances sometimes benefit from soft focus or nonstandard exposure, the norm for most photos remains sharp, well-exposed images.

That’s particularly true for family and travel photos and documentation. It’s worth noting at this point that a final output that appears slightly underexposed is often particularly attractive, with richer, more saturated colors and detailed highlights.

Exposure is affected by three primary factors — ISO sensitivity, lens aperture and shutter speed. Readers of last week’s article may recall that two of these factors, lens aperture and shutter speed, also affect apparent sharpness. Finding the best combination of settings to maximize overall image quality inevitably requires finding the compromise that optimizes all factors for a particular subject and lighting condition.

It’s worth noting that the “P” Program exposure mode found on most cameras will set the camera to more or less balanced settings generally applicable for many situations. Program mode exposure often works fairly well, setting your camera to what the manufacturer’s engineers believe to be good general compromises among the factors affecting proper exposure and sharpness.

Similarly, “Scene” modes often work well for specific circumstances, like brightly lit snow scenes, sunsets or photos at the beach. However, “Automatic Exposure” mode should be relied on for any important photos without enough prior testing by you under similar circumstances to warrant your confidence.

Modern large-sensor cameras often provide more leeway to later correct possible exposure problems. Here’s why: Most recent model cameras built around large Micro Four-Thirds, APS-C and full-frame sensors retain good dynamic range and lower image noise when used at higher ISO sensitivities. That allows you to either increase ISO or to “underexpose” a RAW format image file and then later use the extra dynamic range to make corrections to the final output. To take advantage of these more modern capabilities, you’ll need to save your files in an RAW image file format. JPEG files have little or no capability for later correction.

ISO sensitivity, lens aperture and shutter speed all combine to require that a specific amount of light strike the sensor. As ISO settings increase, less light is required to produce a normal-looking image.

However, it’s best to use the lowest feasible ISO setting when possible because image quality is always best when a camera is set to its lowest “base” ISO, the native sensitivity of the digital sensor. At “base” ISO, image noise is lowest, details are sharpest and dynamic range is highest.

ISO settings higher than base ISO are not true increases in sensitivity. They’re increased amplification of an otherwise too-dim sensor output. When light levels are lower, there’s less true data signal (light) relative to the sensor’s random electronic noise.

Higher than base ISO settings amplify both signal and random electronic noise, resulting in the digital “noise” or graininess that reduces the quality of high-ISO shots. It’s the same phenomenon that produces “snow” in a weak broadcast TV signal.

It’s not always possible, though, to use “base” ISO without that lower sensitivity requiring a shutter speed that’s not fast enough to avoid blurring from either camera shake or subject motion, or forcing use of a lens aperture that’s either inherently unsharp or that doesn’t provide enough depth of field.

Modern cameras using APS-C and Micro Four-Thirds sensors generally do well up to about ISO 800, with minimal loss of final image quality — nothing that can’t be reasonably corrected in post-processing. Full-frame cameras usually produce decent results up to ISO 3,200.

If you’re confronted with a situation in which “base” ISO sensitivity would result in too-slow shutters speeds or nonoptimum lens apertures, then the first, and least damaging, approach is to increase your camera’s ISO setting up to its acceptable upper limit.

In many instances, you can just rely on the “auto-ISO” feature if you set its upper limit to the highest ISO sensitivity that you’ve found to provide reliably good results.

In several recent articles, we’ve discussed how a too-slow shutter speed is one of the most significant factors degrading image quality. What’s fast enough depends on both the effectiveness of your camera’s image stabilization and whether your subject is moving quickly. With relatively static subjects and good image-stabilization hardware, a shutter speed of 1/30 second to 1/60 second is usually sufficient for all but the highest-magnification telephoto lenses.

Some cameras with advanced image stabilization, such as the new Olympus OM-D E-M5 II, avoid camera shake at much slower shutter speeds, giving you a wider range of options.

However, fast-moving subjects may require shutter speeds as fast as 1/250 second to 1/1000 second to avoid blur. Unless you’re in bright sunshine, fast subjects may require a higher ISO sensitivity setting.

Because modern lenses are typically sharp in the f/4 to f/5.6 range, that’s often a good starting point for cameras using APS-C and Micro Four-Thirds sensors, unless you need a different lens aperture to provide more or less depth of field in a particular situation.

Lenses designed for film cameras and full-frame digital cameras are often at their best between f/5.6 and f/8, and so I generally use f/6.3 as a starting point with full-frame lenses.

Unless your lighting is already perfectly matched to your subject and situation, it’s unlikely that you’ll be able to simultaneously optimize ISO, lens aperture and shutter speed settings. Using Program mode and carefully limited Auto-ISO features is a good starting point for the less experienced, but you’ll find your best photos usually result from some advance testing and knowledge that allow you to take control of the situation and personally determine the best balance of settings affecting sharpness and exposure, the two pillars of good image quality.

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

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