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Tuesday, February 9, 2010

What Lens Would Adams Use?


To get the very best sharpness, colors, contrast and overall image quality, you need to use the best lens possible

Labels: LensesGear

This Article Features Photo Zoom

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 23.6x15.8mm (Nikon calls this format “DX”). Canon’s APS-H DSLRs have sensors between full-frame and APS-C: 28.1x18.7mm. Four Thirds System DSLRs have sensors measuring 17.3x13.0mm.

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.

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