Tuesday, July 29, 2014
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.
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.
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.)
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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