Landscape Lens Tech

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This cutaway illustration shows the complex optics and mechanics of a modern lens. To get a sharp, corrected image with good contrast and accurate colors, all of these precision parts must work perfectly in concert with one another. Zoom lenses like this have benefitted so much from computer-aided design that they’re now the norm for landscape pros.

Finding your perfect landscape lens is a matter of defining the priorities. Because everything in photography is a trade-off, it’s largely a matter of deciding where you can make a sacrifice in order to gain a key benefit. Let’s look at the most critical attributes for taking scenics.

Sharpness
The single most important attribute has to be sharpness. Landscape images require fine detail and are often printed large. But no matter how fine-grained your film or how many megapixels your digital camera has, if the lens can’t provide appropriate sharpness, the images will suffer. You want a landscape lens to be sharp across the frame (although soft corners/edges can be tolerated with some subject matter) and across the zoom range. Generally, this means using the higher-end lenses, as they provide better performance. (It also requires using a tripod or other steady camera support—camera movement will destroy image quality even with the sharpest of lenses.) For a zoom lens, you want one that’s sharp at all focal lengths, not just the widest one or the longest.

Distortion
After sharpness, the lens’ distortion characteristics come into play. The ideal landscape lens will have minimal distortion: Straight lines should remain straight, especially the horizon when placed high or low in the frame. Fisheye lenses curve straight lines that don’t pass directly through the center of the image; non-fisheye wide-angles should not (although most do, to some degree). You can check for distortion by nearly filling the frame with a rectangular object like a newspaper page, or by shooting scenes with the horizon high or low in the frame, or even framing with a wall-ceiling intersection high or low in the frame (in both horizontal and vertical formats).

Sensor Formats And Lens Choice

What a given focal length “sees” depends on the size of the sensor or film with which it’s used. Smaller sensors and film formats “see” a smaller portion of the image projected by a given lens than larger sensors or film formats do (see the diagram, right).

For a given format, the normal lens is considered to be one with a focal length approximately the diagonal measurement of the image format. For example, a 35mm film frame measures 36x24mm, with a diagonal of 43.2mm. Thus, a normal lens for this format is around 43mm (50mm is the most widespread normal lens for the 35mm format).

Lenses shorter than the image diagonal are wide-angle; they produce a wider angle of view than the normal lens. A 28mm lens on a 35mm (or full-frame) DSLR is wide-angle; it produces a wider angle of view than the normal 43mm or 50mm lens.

The digital APS-C format, used by many DSLRs and new mirrorless, interchangeable-lens models, measures around 23.5×15.7mm, with a diagonal of 28.9mm. Thus, a normal lens for APS-C is around 29mm; a 29mm lens on an APS-C camera will “see” what a 45mm lens “sees” on a 35mm or full-frame digital camera. If you put a 28mm lens on an APS-C camera, it “sees” like a 42mm lens on a 35mm SLR: normal, not wide-angle.

Four Thirds System sensors measure 17.3x13mm, with a diagonal of 21.6mm, half that of a 35mm or full-frame DSLR. Thus, a given focal length used on a Four Thirds System camera frames like a lens twice its focal length on a 35mm or full-frame digital SLR.

Each SLR manufacturer offers a wide range of lenses for its cameras, whatever their format. But since shorter focal lengths are required to produce a given angle of view with the smaller sensors, wide-angle lenses for these cameras have very short focal lengths; to provide the angle of view of an 18mm superwide-angle on a 35mm or full-frame digital SLR, an APS-C DSLR would require a 12mm focal length, and a Four Thirds camera, a 9mm focal length. It’s more difficult to produce a 9mm or 12mm lens that’s sharp and distortion-free than an 18mm, one reason why digital landscape photographers tend to prefer larger formats.

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Contrast And Color
A good landscape lens should produce good contrast, a function of its design and components, and the coatings applied to the surfaces of the lens elements. Each glass-to-air surface reflects some light, and some lenses—especially zooms—have many elements and thus many such surfaces. This reflected light isn’t transmitted to the image sensor or film, and the light loss can be considerable with high-element-count lenses. These reflections within the lens also cause flare and ghosting (as does flare from an uncoated front element).

Good multicoatings greatly improve light transmission and reduce flare and ghosting. Higher-end lenses have the best coatings. Canon’s SWC (Subwavelength Structure Coating) and Nikon’s NCC (Nano Crystal Coating) use nanometer-scale structures to provide very effective control of reflections, flare and ghosting. Lens manufacturers also design internal baffling to minimize internal reflections and flare. Of course, a landscape lens should produce accurate colors. This, again, is an attribute of the lens design, composition of the elements and coatings.

Zoom Or Prime?

AF-S Nikkor 14-24mm ƒ/2.8G ED

We were surprised a couple of years ago when we polled a number of pro landscape shooters to see what their favorite landscape lenses were and they came up with 16 zooms, two tilt/shifts and a full-frame fisheye—not a prime wide-angle among them.

Naturally, one’s choice of focal lengths for landscapes depends largely on how one sees landscapes, but the large number of zooms—and not just wide-angle zooms—indicates that a goodly number of pro landscape specialists see landscapes in a variety of ways.

Most people think of wide-angles when they think of landscape lenses, and many landscapes are photographed with those. But many strong landscapes are shot with normal and longer lenses as well.

AF-S Nikkor 24mm ƒ/1.4G ED

It’s a matter of framing and perspective. The focal length of the lens (along with the sensor or film format with which it’s used, and the distance from the subject/scene) determines the framing of the image. The distance determines the perspective—how large closer subjects appear relative to more distant ones, and how much space there appears to be between them. We generally move closer when using a short focal length, and this expands perspective. Conversely, we generally use longer focal lengths from farther away, and this compresses perspective. A zoom lens lets you change the framing (but not the perspective) when you can’t move closer or farther, and lets you do it without exposing your image sensor assembly to dust as changing lenses would do. The zoom also lets you move closer or farther away and then reframe as desired, even if that requires an in-between focal length not available in prime lenses.

Historically, prime (single-focal-length) lenses have been the best choices for landscapes because zoom lenses produced noticeably worse image quality. After all, it’s tough enough to correct all those aberrations for a single focal length; when you have to do it for a whole range of focal lengths, that really complicates things.

Computers and advanced ray-tracing models came along, fortunately, allowing lens designers to perform complex calculations and test designs far more quickly. And the creation of exotic lens elements became more widespread and less costly. A major result is that today’s best zoom lenses are quite capable of turning out pro image quality throughout their focal-length ranges. As noted earlier, today, many pro landscape photographers work with DSLRs and zoom lenses, mostly the higher-end pro zooms.

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Coverage
An important consideration if you use filters for your landscapes is whether they will produce vignetting with your lenses. This is especially important with wide-angle lenses. If you use filters with wide-angles, you should use the special thin ones whenever possible to avoid or minimize vignetting. If you’re shooting digitally, you can correct some vignetting using RAW conversion software. Some cameras even correct it in-camera, but it’s best to avoid vignetting in the first place whenever possible.

Speed
Lens speed isn’t really a factor for a landscape lens since the subject isn’t moving (and, hopefully, neither is your camera). Fast lenses are more costly than slower ones, and also much bulkier. Especially when hiking to less accessible vantage points, lens bulk is a consideration. If you’re working from your vehicle, the lens bulk is less important, and a number of the pro landscape shooters we polled about their favorite landscape lenses do use fast zooms. Also, bear in mind that the best pro lenses tend to be fast and bulky.

Lens Aberrations Defined

Various aberrations affect lens sharpness, either overall or at different parts of the image. Lens designers generally counter these by using combinations of special elements in their designs.

• Spherical Aberration: Parallel light rays passing through the edges of a spherical (evenly curved) lens element aren’t brought into focus at the same plane as light rays traveling through the center of the lens. This spherical aberration is most obvious in wide-angle and wide-angle zoom lenses. You can reduce it by stopping the lens down (and thus increasing depth of focus). Lens manufacturers use aspherical lens elements—elements on which the curve changes from center to edge—to reduce or nearly eliminate spherical aberration.

• Chromatic Aberration (CA): Conventional glass elements tend to focus short (blue) wavelengths closer to the lens than medium (green) wavelengths and long (red) ones. This is called axial, or longitudinal, chromatic aberration. Such elements also tend to focus blue wavelengths traveling at an angle to the lens farther from the lens axis than green or red wavelengths; this is lateral chromatic aberration. Lateral chromatic aberration is a major cause of the purple/green color fringing you often see in digital images. This can be corrected when postprocessing RAW images, to a large degree, but the best results will occur with lenses that don’t produce it (or at least minimize it) in the first place. Stopping the lens down can reduce the effects of axial CA a bit, but not lateral CA. Manufacturers greatly reduce both types of CA by using low-dispersion and fluorite elements (and Canon’s DO Diffractive Optics elements). Note that CA affects black-and-white images, too, reducing sharpness.

• Astigmatism: Just as with human eyes, an astigmatic lens produces differing degrees of focus in different parts of the image. Stopping the lens down can reduce it a bit, but better lenses are well corrected for it.

• Coma: Coma causes light rays from an off-axis point passing through the edge of a lens to refract differently than rays from the point passing through the center, resulting in comet-shaped blurs rather than points in the image. Better lenses are corrected for coma.

• Curvature of Field: A single curved lens element will focus an image on a curved plane, not a flat one, so that if the center of the image is focused, the edges will be unsharp, and vice versa. Stopping the lens down can reduce the effect; a good lens eliminates the effect through use of multiple elements.

• Bokeh: This is a term for the way-out-of-focus elements—especially those in the background—that appear in an image of a nearby subject shot with the lens wide open. Some lenses have “pleasant” bokeh, others not-so-pleasant bokeh (pleasant/unpleasant depends on the observer’s personal taste, to a degree). Part of it is due to the lens design, its aberrations and how they’re corrected, and part to the aperture diaphragm—generally diaphragms with more blades create more nearly circular openings and more pleasant bokeh. If your landscape photography involves selective-focus images, bokeh is an important consideration.

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Diffraction And ƒ-Numbers

Lens aberrations are generally strongest at wide apertures and are reduced as you stop the lens down. Stopping the lens down also increases depth of field. So you’d think that stopping way down for every shot would be a great idea. It isn’t, for at least a couple of reasons. First, sometimes you want to limit depth of field to isolate a subject from the background. Second, as you stop the lens down, diffraction reduces image quality.

Diffraction becomes a significant issue when the lens is stopped down to small apertures. We suggest you do your best to keep to ƒ/11 unless it’s absolutely necessary to go smaller.

Diffraction is the spreading of light rays as they pass through an opening. The smaller the opening, the more they spread, reducing sharpness. So, as you stop down, depth of field increases, but overall sharpness decreases. Each lens (and with a zoom, each focal length) has a sweet spot, where the balance between wide-aperture aberrations and small-aperture diffraction is optimal, and best image quality will occur at that aperture. Which aperture is it? You should test your gear to find out, but generally 1.5 to 2 stops down from wide open is pretty close.

In practice, use the aperture you need to provide the necessary depth of field, but only that; don’t stop down any farther than necessary.

The ƒ-number is the ratio between the diameter of the opening in the lens and the focal length. An 18mm lens at ƒ/16 has an effective diaphragm opening of 1/16 of 18mm, or 1.125mm. A 12mm lens at ƒ/16 has an effective diaphragm opening of 0.75mm; a 9mm lens at ƒ/16, an opening of 0.563mm. The smaller the effective opening, the more diffraction adversely affects image quality, so this is another reason why landscape photographers tend to favor larger formats. That said, however, many of our surveyed pro landscape shooters do work with smaller digital formats—using moderate apertures no smaller than ƒ/11.