By Joe Kashi, for the Redoubt Reporter
Until recently, only prograde digital cameras reached the thin air of 24-megapixel resolution, but now even a few entry-level digital SLR cameras reach those rarified heights. To wring the most out of these higher-resolution sensors, more accurate exposure becomes increasingly critical.
For casual photographers, though, a Micro Four-Thirds or APS-C sensor with 16-megapixel resolution seems ideal. It’s enough to make highly detailed large prints but not so much that every technical flaw becomes glaringly obvious. If you doubt the print resolution that can be attained with 12 or 16 megapixels and a good lens, observe the very sharp Fireweed fibers in this week’s Illustration 1, a tiny, highly cropped section of a mere 10-megapixel image file.
Of course, not all megapixels are created equal. Small, high-megapixel sensors, such as those used in “superzoom” cameras, simply don’t have enough surface area per pixel to capture an adequate amount of light. As a result, such cameras tend to do comparatively poorly in dim light and high-contrast situations compared to a similarly sized sensor with fewer megapixels. The same general concerns apply to cameras using larger APS-C sensors, though to a lesser degree.
Inevitably, the size of each digital sensor pixel becomes smaller when the same sensor area contains a larger number of pixels. When individual pixels get smaller, several problems can occur.
As the size of each pixel decreases, noise levels increase, potentially throwing away the extra resolution. This occurs at a lower ISO sensitivity. Dynamic range, critical to capturing both highlight and shadow detail in high-contrast situations, decreases. Lower dynamic range also places an additional premium on correct exposure because there’s less latitude for later post-processing correction. Lower ISO sensitivities may be required for decent image quality, resulting in the need to use slower shutter speeds or wider lens openings.
Using wider lens openings makes correct focus more critical, introduces the optical aberrations that we’ve previously discussed and, in any event, is limited by the optical construction of your lens. Wider lens openings also reduce the depth of field, the front-to-back area in which focus and sharpness are acceptable. Slower shutter speeds run the risk of both camera shake and blurring due to subject motion.
Properly balancing these competing limitations is usually the best way to make good photographs. In practice, there are no hard-and-fast rules, only some basic guides to finding the optimum balance for each particular situation. Of course, most of these limitations will not be a major concern when you’re shooting routine scenic photos with bright sunshine streaming over your shoulder and set to the camera’s lowest ISO sensitivity. Rather, they’re most likely to occur in those unusual situations that often result in the most interesting photos.
Saving your photos as RAW+JPEG image files make sense. Often, out-of-the-camera JPEG images taken in good light are adequate for low-resolution requirements, such as posting on social media or newspaper illustrations. By saving your images as RAW+JPEG files, though, you’ll have instant access to JPEG when needed as well as the ability to make more serious corrections using the RAW file.
Processing an RAW image file is the best way to later tweak your images to optimize noise, sharpness and dynamic range. When you use RAW image files and a nondestructive program, like Adobe Lightroom or Apple’s Aperture, you can always go back later and fine-tune your adjustments to fit changing needs, something that’s not possible with JPEG files or when using an editing program like PhotoShop. So the first solution to high-resolution demands is to always use RAW+JPEG image files. If you don’t need the RAW image file, you can always delete it later.
With higher resolution sensors using smaller pixels, exposure needs to be more carefully controlled, particularly for high-contrast subjects and photos that contain a lot of bright highlight detail. For such subjects, I find it’s often best to bracket the image by setting your camera to automatically take three or more rapid-fire photos of the same photograph with a single press of the shutter release. One photo should be exposed at the calculated exposure, one image with 0.7EV more exposure and a third image with 0.7EV less exposure. One of these exposures is likely to be pretty close, which I define as being within 0.3EV of the best possible exposure. You can delete the unused files.
Some scenes, such as snowy landscapes and beaches, require more exposure than calculated by your camera’s auto exposure. In these situations, set the exposure compensation to provide for +0.7EV more exposure to keep bright subjects bright-looking. In very dark situations, such as when photographing dark spruce trees, use the exposure compensation feature to expose at -0.7EV, to keep dark main subjects dark.
When in doubt about a scene with very bright highlights, protect highlight detail by reducing the exposure slightly. Blown highlights with little or no detail generally look worse than very dark shadow areas. Besides, it’s usually easier to later recover some detail from overly dark shadows than from blown highlights.
With higher resolution sensors, I’m finding that it’s best to not exceed ISO 400 to ISO 800 in most instances where high image quality is critical. Previously, I would have been comfortable using ISO 1,600 in the same situations.
At these lower ISO settings, using a tripod or some other sort of camera support becomes more important, especially in dimmer light. By using a camera support, at least where the subject’s static, you’ll be able to use a slower shutter speed while still using the intermediate lens apertures where sharpness is best and depth of field adequate. This isn’t a hard and fast rule, of course. In dim light, you’ll get much better photos by using a fast-enough shutter speed to avoid blur, even though your lens isn’t pinpoint sharp at wider apertures.
Sharp optics are needed, but provide no benefit if you’re out of focus. Autofocus mechanisms tend to be somewhat unreliable, especially the “phase-detection” systems used in nearly all dSLR cameras. These are prone to misalignment and jarring. If in doubt about whether your autofocus mechanism is accurate, buy a copy of the “Lens-Align” tool to check whether your camera may be focusing its lens either in front or in back of the subject. Most better dSLR cameras allow you to incrementally adjust focus calibration through the menu system. For highly critical focus tests, use a tripod, Live View, and the high magnification central manual focus feature found in many better cameras.
- If you’re in Homer on Friday, you’re invited to stop by the main floor of Kenai Peninsula College’s Homer campus for the opening reception of my new photo show, “Homer Beach, Ebb Tide.” The reception’s open between 5 and 6:30 p.m., with free refreshments. A poetry reading and workshop follows in the same building. The show will be on display through the end of March.
Local attorney Joe Kashi received his bachelor’s and master’s degrees from MIT and his law degree from Georgetown University. Many of his technology and photography articles can be accessed through his website, http://www.kashilaw.com.