Ansel Adams used the sharpest lenses he could find for his cameras, experimenting with a number of them to discover the best ones for his work, be it a 70-year-old, 12-inch Voigtlander, the renowned 12-inch Goerz Dagor or the latest 121mm Schneider Super Angulon. Adams shot mostly large-format (4×5 inches and larger); with today’s 35mm-based DSLRs, good lenses are even more critical (35mm film was referred to as “miniature format” in Adams’ day). To get the best image, a quality lens is mandatory.
So what lenses do today’s pros use? Last year, we polled a number of our favorite landscape pros to find out. We learned that most shoot with DSLRs (many with sub-full-frame models) and use zoom lenses more often than primes. Essentially, they used top-of-the-line zooms (and primes). The results were published in “Lenses For Landscapes” in the May 2009 issue of Outdoor Photographer; you can find the article on www.outdoorphotographer.com/gear/lenses/lenses-for-landscapes.html.
Maximizing Sharpness: Lens Considerations
The lens is an essential part of the sharpness equation. A high-resolution DSLR and a sturdy tripod won’t produce sharp images if the lens isn’t sharp—and sharply focused. This is especially important in landscape work, where fine detail is crucial and prints generally will be large.
Obviously, for top-quality images you want to use top-quality lenses. These will be the manufacturers’ “pro” lenses. The high-end lenses are sharper, better corrected for aberrations and distortions, and really do produce better image quality, especially the newer ones designed for digital cameras. Naturally, the best lenses cost more. But there’s a difference, and if maximum image quality is important, you should use a good lens. Ansel Adams would.
Here are some tips to help you maximize sharpness in your lenses:
Keep the lens clean. Dust and smudges on front or rear lens elements reduce image quality.
Use a lens hood. It blocks much stray light from striking the front element of the lens and causing flare, which reduces image quality.
Use a tripod. While image stabilization is a wonderful feature, a tripod can hold your camera steadier than you can. Adams used a tripod.
Focus manually, or spot-autofocus on the point in the scene where you want focus to be. If your camera has Live View mode, use it and focus manually on a zoomed-in image on the LCD monitor, or use contrast-based spot AF to put focus exactly where you want it.
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Shoot at an intermediate aperture unless you really need extreme depth of field or very fast shutter speeds. At wide apertures, various aberrations reduce image sharpness. At small apertures, diffraction (the bending of light around the edges of the aperture) reduces sharpness. Adams and other large-format photographers often shot at ƒ/45 or ƒ/64, but large-format film images are much larger than 35mm or APS-C images. While the width of the Airy Disk (the diffraction pattern produced by a given aperture) is the same for ƒ/64 regardless of focal length, it becomes much larger relative to the size of the image (and pixels) as format decreases. Thus, at a given aperture, the degree of apparent blurring due to diffraction increases as image format decreases. That’s why compact digital cameras rarely stop down beyond ƒ/8—diffraction with their tiny sensors and pixels would render images unacceptably blurred.
Each lens has its sweet spot, where sharpness is greatest. This is usually one to three stops down from maximum aperture. Test your lens and camera to see what’s sharpest with your system.
If your camera has an “AF fine-tune” feature, use it to make sure all your lenses are being focused as precisely as possible. In film and early digital SLR days, if you got a body/lens mismatch, you were stuck with it. Today, many DSLRs let you adjust the focus plane in fine increments to compensate for a lens that’s a bit off. You can store in the camera’s memory AF corrections for a number of lenses you use most often. (Unfortunately, you can’t store different corrections for different focal lengths of a zoom lens, but the feature can still help maximize sharpness.) LensAlign from RawWorkflow.com is a great help in using AF fine-tune.
Shoot RAW, and use the RAW-processing software’s lens-correction features. You can correct chromatic aberration, distortion and peripheral light falloff (vignetting). And, of course, you can optimize sharpening for each image.
If you want really wide shots, consider stitching together a series of shots made with a longer lens, rather than using a really wide lens. There are a number of stitching software programs that make it relatively easy to do, and the results turn even a modest megapixel-count DSLR into a monster-megapixel device with excellent detail.
Designed For Digital
There’s more to it than just how sharp a lens is, however, because there are some basic differences between film and digital sensors. Film consists of three-dimensional silver-halide crystals (gains) embedded in an emulsion, while a digital sensor consists of millions of tiny photodiodes (pixels) covered by an RGB Bayer Array filter grid, a layer of micro-lenses and an anti-aliasing low-pass filter. The three-dimensional film grain can accept light rays striking them at an angle, but the pixels are like tiny “buckets” and light must strike them more nearly square-on. The low-pass filter is very flat and far more reflective than film, so lenses designed for film can suffer from reflections from the filter surface.
To take the second point first, lens manufacturers use special anti-reflective coatings, baffle designs and meniscus protective glass over the front of large-diameter lenses to minimize the effects of sensor reflections. Older lenses without these technologies are more prone to reflections when used with DSLRs, and those reflections reduce image quality.
As far as the angle at which light rays strike the film or image sensor, again, newer lenses designed for DSLRs produce better results, but high-end pro lenses also produce very good results, even if designed for film; they’re well corrected for aberrations and distortions.
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Lenses for 35mm SLRs were designed to cover a 43.2mm image circle—the diagonal dimension of a 35mm image frame. A full-frame DSLR has a sensor the same size as a 35mm image frame, so the 35mm SLR lenses cover it very well. However, problems such as vignetting (light falloff at an image’s edges) are more pronounced due to light striking the pixels at too great an angle at the edges. Vignetting (and distortions and aberrations) can be corrected when processing RAW images; some DSLRs do it automatically in-camera (for JPEG images).
With smaller-sensor DSLRs, the sensor doesn’t “see” the entire 43.2mm image circle; it just sees the center portion of it. On the one hand, this means the sensor is reading only the sharpest portion of the image produced by the lens, which is good. On the other hand, this means the lenses are bulkier than they need to be. So lens manufacturers now offer lenses designed specifically for smaller digital sensors. Besides being more compact than lenses designed for 35mm SLRs, these lenses more easily send light directly to the pixels on the smaller sensors.
DSLRs based on a 35mm-SLR form factor come in several formats, dictated by the size of their sensor. Full-frame DSLRs have sensors the same size as a 35mm film frame: 36x24mm (Nikon calls this format “FX”). APS-C DSLRs have sensors about the size of an Advanced Photo System “C”-format frame: around 23.6×15.8mm (Nikon calls this format “DX”). Canon’s APS-H DSLRs have sensors between full-frame and APS-C: 28.1×18.7mm. Four Thirds System DSLRs have sensors measuring 17.3×13.0mm.
The sensor size determines a given lens’ angle of view. A 35mm lens is a wide-angle on a full-frame camera, with a 63° angle of view. But on an APS-C camera, it’s more like a “normal” lens, with a 39° angle of view—about equivalent to a 60mm lens on a 35mm or full-frame DSLR. An APS-H sensor has a crop factor of 1.3x compared to a full-frame sensor, an APS-C sensor has a 1.5x factor (1.6x for Canon), and a Four Thirds sensor has a 2.0x factor. A 100mm lens on an APS-H camera frames like a 130mm lens on a full-frame camera (100mm x 1.3). A 100mm lens on an APS-C camera frames like a 150mm or 160mm lens on a full-frame camera (100mm x 1.5, or 1.6). And a 100mm lens on a Four Thirds System camera frames like a 200mm lens on a full-frame camera (100mm x 2).
To determine the focal length you’ll need on an APS-C camera to match the angle of view of a given lens in the 35mm format, multiply the lens’ focal length by 0.67. If you’re using an APS-C DSLR and want the field of view provided by a 24mm lens on a 35mm or full-frame DSLR, you’ll need a 16mm lens (24mm x 0.67). For an APS-H camera, multiply the focal length by 0.77. To get the field of view of a 24mm lens on a full-frame camera, you’ll need a 18.5mm lens on an APS-H camera. For a Four Thirds camera, you’ll need a lens with a focal length half that of the lens for the full-frame camera—a 12mm lens to get the field of view a 24mm lens provides on a full-frame camera.
This crop factor is great for telephoto users, as it effectively increases the focal length of the tele lenses by 1.3x to 2x. But it’s not so great for wide-angle fans. Early DSLR users had trouble getting wide-angle shots. Full-frame DSLRs were very expensive, and smaller-sensor DSLRs “crop” the image produced by any lens. Things are better today, as there are four DSLRs selling for under $2,700, including one for under $2,000. And lens manufacturers make lenses designed specifically for the smaller-sensor cameras, including some very short focal-length ones that provide true wide-angle capability, along with better optical performance with those sensors.
All DSLRs can use lenses designed for full-frame (35mm) cameras, but full-frame and 35mm cameras can’t use lenses designed for smaller sensors or vignetting will occur (in many cases, the small-sensor lenses can’t physically be mounted on full-frame cameras; with Nikon and Sony full-frame DSLRs, the camera automatically crops to APS-C format when a small-sensor lens is attached).
Canon’s small-sensor lenses are designated EF-S. Nikon’s are designated DX. Pentax’s small-sensor lenses are designated DA (all Pentax DSLRs are APS-C format). Sigma’s small-sensor lenses are designated DC. Sony’s are designated DT. Tamron’s small-sensor lenses are designated Di II. Tokina’s small-sensor lenses are designated DX. All Four Thirds System lenses were designed specifically for the Four Thirds System sensor and can be used on any Four Thirds System DSLR.
Originally Published February 9, 2010
