Tuesday, December 15, 2009
DSLRs For Low Light
The combination of improved sensors, advanced processing engines and powerful algorithms makes the current lineup of pro DSLRs capable of a whole new kind of low-light photography
The 14.6-megapixel, APS-C-format K-7 is Pentax’s top DSLR and provides a normal ISO range of 100-3200, expandable to 6400. A new PRIME II imaging engine works with the sensor to improve image quality, speed operation and provide Live View and 720p HD video capability (it also can do video at 1536x1024 and 480x416 resolutions, all at 30 fps, with mono sound via a built-in microphone or stereo sound via an optional stereo mic).
Very compact at 5.1x3.8x2.9 inches and 22.9 ounces, the K-7 nonetheless makes room for a 3.0-inch, 921,000-dot LCD monitor and a 100% viewfinder. The body is rugged, made of magnesium alloy and well sealed against weather, dust and cold. Like other Pentax DSLRs, the K-7 can use all Pentax lenses, but provides best performance with the DA* series optics containing SDM focusing motors (SDM lenses are weather-sealed like the camera body).
The K-7 provides sensor-shift Shake Reduction that works with all lenses, a sensor-dust remover, multiple-exposure capability and even an intervalometer. It can shoot at up to 5.2 fps (for up to 15 RAW images or 40 best-quality JPEGs). Like its K20D predecessor, the K-7 can shoot in two RAW formats—Pentax’s proprietary PEF and Adobe’s “universal” DNG. You can convert RAW images to JPEG or TIFF in-camera, apply lens chromatic aberration and distortion corrections, and even automatically create three-shot HDR images (JPEG only).
Sony’s first full-frame DSLR, the A900 also was the highest-pixel-count, 35mm-form-factor model when it came out (Nikon’s D3X and Sony’s new DSLR-A850 have since matched the A900’s count). Normal ISO range is 200-3200, expandable to 100 and 6400 when needed.
Notable features include the 24.6-megapixel, full-frame Sony Exmor CMOS image sensor, sensor-shift image stabilization that works with all lenses, a related sensor-dust removal function, dual high-speed Sony Bionz processors, a 3.0-inch, 921,000-pixel LCD monitor, slots for both CompactFlash and Sony Memory Stick Duo media, rugged construction with good dust and moisture sealing, and more. Sony’s lens line isn’t as extensive as Canon’s and Nikon’s, but there are very good ones in it, including Zeiss and Sony G-series optics.
There are similarities among the 24-plus-megapixel CMOS image sensors in the A900 and A850 and the one in Nikon’s D3X—all deliver images measuring up to 6048x4032 pixels, although Sony calls it 24.6 megapixels and Nikon calls it 24.5. But the Sony and Nikon cameras feature different processors, processing algorithms and sensor filtration niceties, so each has its own “look” despite the same pixel count. DxOMark.com gives the Nikon D3X higher marks for low-light/ISO performance, but the D3X costs about three times what the A900 costs, and the Sony models were very good, scoring higher than any APS-C-sensor cameras in the DxO testing.
A full-frame DSLR, the A850 incorporates many of the A900’s fine features, including its 24.6-megapixel CMOS sensor and normal ISO range of 100-3200, expandable to ISO 6400. The body is the same size and weight, the AF and metering systems are the same, and both feature sensor-shift image stabilization that works with all lenses, the only full-frame sensor models to do so. The major differences—the A900 can shoot up to 5 fps vs. 3 fps for the A850, and the A900 has a 100% viewfinder vs. the A850’s 98% viewfinder. Neither model has Live View mode or video capability, although both feature Sony’s Intelligent Preview function, which displays a “test shot” and histograms on the 3.0-inch, 921,000-dot LCD monitor so you can judge the effects of adjusting exposure, white balance and the Dynamic Range Optimizer.
Like the A900, the A850 can use all Sony-mount lenses (which includes excellent Zeiss and Sony G-series optics, and legacy Minolta Maxxum lenses). If a Sony or Zeiss DT lens (designed for APS-C-sensor cameras) is used, the camera automatically crops to that format, so photographers moving up to the A900 or A850 from an APS-C-format Sony DSLR can use all their lenses.
|DxO Labs’ DxOMark Sensor Ratings|
|DxO Labs—known to most photographers for its DxO Optics Pro RAW-conversion and lens-correction software—also provides equipment and services used by the photo industry and others to assess image quality. Just over a year ago, the company put up the www.dxomark.com website, which presents results of their tests of RAW sensor image quality in three areas: Color Depth, Dynamic Range and (appropriate to this article) Low-Light ISO. Essentially, the Low-Light ISO rating is the highest ISO setting at which the camera produces a signal-to-noise ratio of 30 dB, while maintaining a dynamic range of at least 9 stops and a color bit depth of 18 bits. While there are many things to consider when choosing a camera, this website is a valuable resource, and you should check it out.|
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