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Pellicle penalty: A dark side to Sony camera design

DxO Labs quantifies how much light is lost to Sony's A33 and A55 cameras' transparent mirror. The sensors are still competitive, though.

Sony's A55 scores competitively on DxO Labs' DxOMark sensor tests.
Sony's A55 scores competitively on DxOMark sensor tests.
DxO Labs

The partially transparent mirror in Sony's SLT (single-lens translucent) cameras offers some interesting features, but what hasn't been clear is the toll it takes on image quality.

Until now.

DxO Labs released today sensor test results for Sony's SLT-A33 and A55 and found that the mirror soaks up about one-third of an F-stop's worth of light.

This means a shot that otherwise could be taken at a shutter speed of 1/200th of a second would have be slowed down to 1/160th to get the same amount of light on an SLT camera, for example--or, holding other factors unchanged, that ISO sensitivity in dim conditions would have to be increased from 1,600 to 2,000, with the commensurate increase in noise.

The DxOMark sensor tests also found that A55's new 16-megapixel sensor is very competitive, tying the Nikon D90's score of 73 on the test. The A33 scores 70.

DxO Labs made the measurements by comparing the results from the A33 with the compact NEX-5, which uses the same sensor but lacks the partially translucent mirror arrangement.

That type of mirror, also called a pellicle mirror, lets the SLT camera line perform some neat tricks--autofocus can be continuously engaged, which allows the A33 to shoot 7 frames per second and the A55 to shoot 10 frames per second. With a pellicle mirror, autofocus works when shooting video, too.

Sony is preparing a new higher-end SLT model that's set to debut within a year.

Camera sensors are a fiercely competitive arena right now as camera makers try to increase both sensitivity and pixel resolution at the same time they add video abilities and other features. It's particularly important in the SLR camera market, where customers pay a premium for higher image quality and, when they buy an extra lens or three, are less likely to shift allegiance to another camera maker. SLRs accounted for 21 percent of Canon's camera shipments in fiscal 2010, but for 65 percent of its camera revenue including lenses and accessories.

Kodak's Truesense color pattern for digital image sensors captures more detail but less color information.
Kodak's Truesense color pattern for digital image sensors captures more detail but less color information. Kodak

Even higher up the ladder than conventional SLRs are medium-format models costing even more, but even in this rarefied market, competition is getting spicy. Pentax's new 645D medium-format camera, with a 44x33mm sensor larger than that of high-end SLRs from Canon and Nikon, ranks fourth place overall in the DxOMark tests with a score of 82.

Image sensors are propagating elsewhere, too, such as in security cameras and industrial equipment used to examine products on a manufacturing line. The latter area is one market where Kodak is concentrating.

The company announced a new 29-megapixel sensor for the industrial market on Monday. It's available with a newer technology it calls Truesense, which captures more detail but less color information.

Typical image sensors use what's called a Bayer pattern to filter the light that gets to the sensor pixels; each pixel gets red, green, or blue. With Truesense, though, only half the pixels have this color filter; the other half are "panchromatic" pixels sensitive to all light. The result, Kodak says, is higher sensitivity, good for "light-starved" applications.