Get Ready For Prime Time

You can get improved speed, sharpness and a compact size in prime lenses, but what do you give up? We look at the pros and cons of single-focal-length lenses.
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Zeiss Touit 12mm ƒ/2.8 and 32mm ƒ/1.8 lenses

Zoom lenses are especially popular because they provide a wide range of focal lengths in a single unit, you can change focal lengths with the twist of a wrist (with no need to physically change lenses and allow dust to enter the camera to settle on the sensor assembly), and a lot of entry-level zoom lenses are available at low prices. But there's a lot to be said for prime (fixed-focal-length) lenses, as well.

Lens designers have to deal with issues that can reduce a lens' optical performance—spherical aberrations, chromatic aberrations, coma, distortion, flare, ghosting, astigmatism, curvature of field and more—all the while trying to keep down cost and bulk. Each focal length has its own collection of optical challenges. It's a lot easier to correct everything for a single focal length than for a whole range of focal lengths, so prime lenses in a given price range tend to have better optical performance than similarly priced zooms. Today's better zooms are excellent and widely used by pros, but today's better prime lenses are, well, better.


Tokina AT-X 100mm ƒ/2.8 PRO D Macro

Lens Speed
One advantage prime lenses have over zooms is that they can be faster, i.e., have wider maximum apertures. The fastest zooms have maximum apertures of ƒ/2.8, while there are many prime lenses that open up to ƒ/1.8, ƒ/1.4, ƒ/1.2 and even faster (the recently introduced HandeVision IBELUX 40mm has a maximum aperture of ƒ/0.85!). Faster lenses are advantageous in dim light and when you want to really limit depth of field to focus the viewer's attention on a specific portion of a subject or scene.

Of course, fast lenses are bulkier and more costly than slower ones. The ƒ-number is the focal length divided by the diameter of the effective aperture: ƒ/4 means the aperture diameter is one-quarter the focal length of the lens, 25mm for a 100mm lens. An ƒ/1.4 100mm lens would have to be large enough to accommodate an effective aperture diameter of 100/1.4 = 71mm, nearly three inches. (That's why there aren't any 500mm ƒ/1.4 lenses; that would require an effective aperture diameter of 500/1.4, or 357mm, more than 14 inches.)


Tamron AF 90mm ƒ/2.8 Di SP A/M Macro

Larger elements cost more to produce than smaller ones, and it's harder to correct various aberrations in large-aperture lenses. So fast lenses do tend to be more costly, and sometimes not as sharp wide-open as slower lenses of equivalent focal length and level (entry-level, mid-range, pro). There are times when an ƒ/1.4 lens may be better than a slower one, and vice versa. You have to consider the types of shooting you do and choose lenses that work for situations.

Prime Thinking
One advantage of a zoom lens is that it lets you change the framing of a scene without having to move closer or farther away. This can be especially handy when you can't physically move closer or farther back, as when standing near the edge of a canyon cliff or on the sidelines at a sporting event. But moving closer or farther away produces a different effect than just changing the focal length: Moving closer or farther away changes the perspective, while just changing the focal length does not.


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Canon EF 50mm ƒ/1.2L USM

When you use a zoom lens, you know that you can easily change focal length, so you tend to stay in one spot and zoom to compose. With a prime lens (or several prime lenses), you either have to take more time and trouble to change focal lengths, or you have to move closer or farther away to change the composition. Moving close with a short-focal-length lens can produce a dramatic expanded-perspective effect, while moving far away and zeroing in on a subject with a long lens can produce a "stacked" compressed-perspective effect. Merely zooming from a single spot won't change the perspective; it just changes the framing.

You can prove this to yourself with a simple experiment. Set up your camera on a tripod and compose a scene with a wide-angle lens. Then just remove the wide lens and attach a longer one, and take another shot. (If you have a wide-range zoom lens, you can use the short and long ends of that instead of switching primes.) The long-lens shot takes in less of the scene, and everything in the frame appears larger, but the perspective doesn't change: The apparent distances between objects in the scene remain the same. To prove this, just blow up the middle of the wide shot on-screen so it covers the same area as the long-lens shot. You'll see that the perspective is identical.


TOP: Olympus M.Zuiko Digital D+ED 75mm ƒ/1.8 ABOVE: Nikon AF-S Nikkor 35mm ƒ/1.8G

If you want to change the perspective—the spatial relationships among the objects in the scene—you have to move the camera. Moving close expands perspective; moving farther away compresses it. Wide-angle expansion occurs because we generally use wide-angle lenses at close range, and telephoto compression occurs because we generally use long lenses from farther away.

The point here is that using a zoom might make you "lazy": If you want to change the framing, you'll just turn the zoom ring rather than use your feet. If you do this, you'll miss out on some dynamic possibilities. And a prime lens can also give your creative muscles a good workout: When you have just one focal length, you'll soon think and see differently than if you go out knowing all you need to do is twist your wrist to go from wide to tele view.


A representative sampling of 24mm, 28mm, 35mm & 50mm prime lenses.

Matching Lenses To Your Camera Format
Digital cameras come with a variety of sensor sizes, and this affects the field of view of a given focal length. Since DSLRs originally were adapted from 35mm SLRs, photographers generally think of DSLRs with sensors the same size as a 35mm image frame (36x24mm) as full frame. APS-C DSLRs have smaller sensors, around 23.6x15.7mm. These crop into the image produced by the lens at the camera's image plane, such that a given lens used on an APS-C camera frames like a lens 1.5X longer on a full-frame DSLR (or 35mm SLR). For example, a 100mm lens on an APS-C camera frames like a 150mm lens on a full-frame camera. Micro Four Thirds cameras have even smaller sensors—17.3x13.0mm—and a 2X crop factor: A 100mm lens on an MFT camera frames like a 200mm lens on a full-frame camera.

Makers of smaller-than-full-frame cameras offer some lenses that were designed for those formats. The advantages of these lenses are that they can be smaller than full-frame lenses (they don't have to cover as large an image area), and they can be designed to deliver light more directly to the active image area. The disadvantage is that the major DSLR makers—Canon, Nikon and Sony—don't offer many such lenses in prime (single-focal-length) form. Canon offers one APS-C prime (the EF-S 60mm ƒ/2.8 Macro), Nikon offers four (the AF DX 10.5mm ƒ/2.8G fisheye, AF-S DX 35mm ƒ/1.8G normal, AF-S DX 40mm ƒ/2.8G Micro and AF-S DX 85mm ƒ/3.5G Macro), and Sony offers three (the DT 30mm ƒ/2.8 SAM Macro, DT 35mm ƒ/1.8 SAM normal and DT 50mm ƒ/1.8 SAM portrait tele). All of their other APS-C lenses are zooms. You can use their full-frame lenses on their APS-C cameras, of course, but those tend to cost and weigh more than APS-C lenses would. Pentax offers a wider selection of APS-C primes—more than a dozen—primarily because they don't make a full-frame DSLR; all their DSLRs are APS-C.


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The widest range of focal lengths is available for full-frame DSLRs (and these lenses can, as mentioned, also be used on APS-C DSLRs from their manufacturers). Canon, Nikon, Sigma and Sony all offer prime lenses in focal lengths from fisheye to supertelephoto. (Sigma's DSLRs are all APS-C, but Sigma also makes a wide range of lenses for Canon and Nikon DSLRs, and many for Sony and even some for Pentax DSLRs.) If you use a full-frame DSLR, you have a wide choice of prime lenses available.


Available Prime Lenses As Of August 2014

Micro Four Thirds also does well in MFT prime lens selection, with 16 primes offered by camera makers Olympus and Panasonic, and more in MFT mounts from third-party manufacturers. Full-frame mirrorless currently lags in terms of AF primes. There are only three full-frame mirrorless cameras (the Sony a7, a7R and a7S) and two Sony/Zeiss primes (the 35mm ƒ/2.8 wide-angle and 55mm ƒ/1.8 normal). APS-C mirrorless has a wider selection of native AF prime lenses. Due to the short flange-back distance of mirrorless cameras, a tremendous variety of lenses designed for other cameras can also be used via adapters (see the "Lens Adapters" sidebar).

Leica's M-series rangefinder cameras are "mirrorless," and Leica offers a good selection of manual-focus prime lenses for them (18mm through 135mm focal lengths). Third-party M-mount lenses provide a wide range of less costly alternatives.


Novoflex Lens Adapter

Lens Adapters
Lens adapters let you use lenses that weren't made for your camera, greatly expanding your range of lens choices. Adapters range from simple metal rings with a camera mount on one side and a lens mount on the other to elaborate devices containing electronics, glass elements and even a phase-detection AF system.

Because of their short flange-back distances (the distance from the lens mount to the image plane in the camera body), mirrorless digital cameras can use any lens for which an adapter can be found. (DSLRs are thicker, and if you add an adapter, the attached lens may not focus out to infinity unless the adapter contains glass elements, which add cost and could affect optical performance.) Most mirrorless camera makers offer adapters to attach popular lenses to their bodies, and there are lots of third-party adapters available on eBay and the like. Do some online research, because if the adapter's front and rear mounts aren't perfectly parallel, image quality could suffer.

Adapters of special note are Sony's LA-EA4 and the Speed Booster from Metabones. The LA-EA4 lets you use Sony A-mount DSLR lenses on E-mount mirrorless cameras, and incorporates an SLT phase-detection AF system similar to the one in the SLT-A65 DSLR. The Speed Booster is sort of a reverse teleconverter. Where the latter spreads the image from the lens, making it larger and less bright, the Speed Booster concentrates the image, making it smaller and brighter. Yes, a teleconverter costs you lens speed, but the Speed Booster actually increases it. Put a 35mm ƒ/2.8 full-frame lens on an APS-C camera with a Speed Booster, and it effectively becomes a 25mm ƒ/2.

Note that simple ring-type adapters merely let you attach the lens; they don't provide autofocusing, or in most cases, even auto-diaphragm control. Live View can be useful when using them, allowing you to zoom in and focus right on the LCD monitor. A magnifying loupe (and tripod) can be very helpful here.

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