Sony Alpha A900

	Resolution Summary
• Best overall resolution score among tested full-frame cameras • Tested 24-105mm lens showed significant chromatic aberration in corners • Distortion levels within acceptable range • Sharpness results very good at all tested zoom levels • Dynamic range over 7 stops at ISO 200; only Canon 5D Mark II scored higher
	Noise	Page 5 of 18	White Balance

Resolution (10.64)

Merely piling on the megapixels doesn't necessarily translate into superior resolution performance, but there's no denying that the 24.6-megapixel Sony A900 spanked the competition in our resolution testing, achieving an overall score for this section nearly 25% higher than the Canon 5D Mark II, its nearest full-frame competitor.

Our resolution test is a multi-faceted challenge, factoring in both sharpness and chromatic aberration. For 2009 we enhanced our resolution testing to incorporate sharpness measurements not only at the center of the lens, but at points across the entire image, including the four corners and midpoints along the diagonal axes. When a kit lens is avaialble, that's what we use for our testing. Sony does not offer a recommended bundle, though, so we chose their SAL-24105, a 24-105mm lens with an f/3.5-4.5 aperture range which sells for $469.99. We felt this provided a practical zoom range for a primary lens, at a reasonable but not bargain price.

We photograph our test chart at three focal lengths, representing the minimum, maximum and midpoint of the zoom range. At each focal length, we shoot the minimum, maximum and mid-range aperture setting. Images are then analyzed using Imatest software for sharpness, chromatic aberration and distortion. Since distortion results are entirely lens-based, we don't factor these results into the overal resolution score, but we present the test results in the following section.

Distortion
With the lens at its widest 24mm setting, we measured 2.37% barrel distortion. By the time we zoomed to 70mm the lens switched to pincushion distortion at the 2.17 level, which rose modestly to 2.42% at the longest 105mm zoom. While these levels of distortion would be noticeable, particularly in demanding applications like architectural shooting, they are within an acceptable range for most photographic needs.

Chromatic Aberration (7.85)
Chromatic aberration refers to the misalignment of red, green and blue pixels, causing a noticeable color fringe along line edges. Shooting with the 24-105mm lens, the Sony A900 produced low levels of chromatic aberration in the middle of the zoom range from edge to edge. At the widest and longest telephoto settings, though, the corners became problematic, producing clearly visible fringing in our sample images. The difficulty wasn't symmetrical, with the most noticeable defects occurring on the horizontal plane on the right side. Of course, there's nothing unique about the trend shown here, but the Sony did underperform compared to the Nikon D90 and D700 outfitted with similarly ranged lenses.

Sharpness (11.84)
To test image sharpness, we shoot our test chart at three zoom settings (widest, longest zoom and mid-range), at three focal lengths for each distance. We found that, outfitted with the 24-105mm lens used in our testing, the Sony A900 produced a very good image sharpness result overall, signficantly higher than either of its full-frame rivals (the Canon 5D Mark II and Nikon D700). Only the Nikon D90 fared better in our sharpness testing, and not by much of a margin.

At 24mm, the widest lens setting, the image is very sharp in the center, with a maximum lw/ph measurement of 1718 horizontally and 2011 vertically, when shot at f/9. At this zoom length, performance remains strong from corner to corner.

	Image Sharpness
	_f/3.5_			_f/9_			_f/22_
24mm

In the middle of the zoom range, maximum vertical resolution falls a bit to 1768 lw/ph, but the horizontal figure stays nearly the same at 1728 lw/ph. There is some softening midway between the center and the corners, but nothing that raises alarms.

	Image Sharpness
	_f/4.5_			_f/10_			_f/29_
_70mm_

Finally, at the full 105mm zoom, as long as the lens is stopped down a bit, the lens maintains very good sharpness, and only really falls off significantly away from the center, when shot with the aperture wide open. Here again, the discrepancy is no more than we'd expect given the settings in effect.

	Image Sharpness
	_f/4.5_			_f/11_			_f/29_
105mm

When compared to the other tested cameras, the Sony A900 is the leader in overall resolution performance among the full-frame cameras, with only the 12.3-megapixel Nikon D90 posting a higher score.

Resolution Score Comparison

Picture Quality & Size Options (14.45)

The Sony A900 can shoot at three aspect ratios: the native 3:2 format, widescreen 16:9 and a reduced-resolution APS-C size capture for use when a lens designed for a smaller sensor size is attached.

JPEG images can be saved at three compression settings: Extra Fine, Fine and Standard. In addition, there are two RAW file formats available, a straight RAW file and a compressed RAW (cRAW) which will be about 60-70% the size of an uncompressed RAW file. Either RAW format can also be shot as RAW + JPEG, with a Large-size JPEG stored simultaneously.

Dynamic Range (8.29)

The Sony A900 performed very well in our dynamic range testing, bested only by the Canon 5D Mark II. All of our comparison cameras produced similar results at low ISO settings, but dynamic range inevitably declines as ISO increases, and the Sony and Canon maintained  levels above 5 stops out to ISO 1600 and  well over 4 stops at ISO 3200, which is a very strong result.

We measure dynamic range — the camera's ability to capture detail in both the brightest and darkest areas of an image — by repeatedly shooting a standard grayscale chart at multiple ISO settings, under tightly controlled conditions, then feeding the resulting files to Imatest, which combines the results to determine the overall dynamic range.

Dynamic Range

At ISO 200, the Sony A900 demonstrates an impressive 7.34-stop dynamic range, which falls off at a reasonable rate as ISO settings increase.

Dynamic Range Comparison

Only the Nikon D90 outperformed the Sony at ISO 200, as shown in the chart above. Taking results from all official ISO settings into acccount to produce an overall score, the Canon 5D Mark II is the strongest performer, but the Sony A900 comes in a respectable second.

Dynamic Range Score Comparison

Image Stabilization (4.71)

The A900 did a decent job of compensating for handshake; we found that with low levels of hand shake, images with the SteadyShot stabilization on were significantly sharper than with it off. However, this was reversed with larger amounts of hand shake; the system made the images slightly less sharp. Most SLRs try and deal with the way your hands shake by moving a small part of the lens, but the A900 takes a different approach; the image sensor is on a moving platform. When the gyroscopes in the camera detect that it is moving because your hands are shaking, this small platform moves to compensate. We test how well a camera deals with hand shake by putting it on a motion control platform, shaking the camera and taking a large number of photos with image stabilization on and off. We also test by moving the camera in both a horizontal (left to right) and vertical (up and down) direction, with two different levels of shake; one low and one high. We then analyze these photos to determine how sharp they are; the better the job the camera does of compensating for the camera movement, the sharper the photos will be. For more details on how this test works, see our How We Test article.

Sony A900 Image Stabilization: Low Shake
Horizontal: Sharpness Vs Shutter Speed Horizontal: Sharpness improvement with IS on	Vertical: Sharpness Vs Shutter Speed Vertical: Sharpness improvement with IS on

Our first test is for a low level of hand shake, such as when you are trying to hold the camera steady with two hands, or braced against a wall. In this situation, the camera only moves a small amount, but there is still more than enough shake to ruin an otherwise perfectly good picture. In this case, the SteadyShot feature of the A900 led to sharper images at all of the shutter speeds that we test at, ranging from 1/500 of a second right down to 1/8 of a second. The only exceptions here were when the camera was moving vertically (up and down); at 1/500 of a second  and 1/60 of a second, the SteadyShot system made things slightly worse. But overall, it did a very good job of compensating for the simulated hand shake in this test.

Sony A900 Image Stabilization: High Shake
Horizontal	Vertical

Our next test uses a higher level of shake; about the level you would get if you were trying to take a photo while walking, or while taking a candid shot one-handed. The A900 didn't do as well here: the larger amount of shake seemed to be more than the SteadyShot system could cope with when the camera was moving horizontally (left to right). On most of the shutter speeds the images taken with SteadyShot on were less sharp than with it turned off, as you can see in the examples below. There was some improvement in sharpness with vertical (up-down) movement, though, but we feel that the horizontal correction is more important,since handshake is more of a left-to-right movement than up-down. 

The bottom line here is that we would recommend that you leave SteadyShot turned on in any situation where hand shake is likely to be an issue, as it does a good job of making shots significantly sharper across the range of shutter speeds with small amounts of hand shake. However, like all image stabilization systems, it can't work miracles, and in situations with larger amounts of shake, it sometimes made things a little worse. So, turn it off if you are running around, but turn it on if you are standing still to take shots.

Sony A900 Image Stabilization Comparison

The graph above shows the performance of the A900 compared to the Canon 5D Mark II with the 24-105mm f/4L IS USM lens and the Nikon D700 with the Nikkor 24-120mm VR lens. As you can see, the A900 didn't have the problems that the Canon 5D Mark II had with stabilization results worse at some shutter speeds; it made improvements across the entire range. But it's no slam dunk; we found that the image stabilization of the Nikon D700  produced sharper images at the critical shutter speeds of 1/60 and 1/125 of a second. However, in overall performance, there was not that much difference between the A900 and D700.  You should remember, however, that the performance of the 5D Mark II and D700 is dependent on the lens, as the image stabilization on these cameras happens in the lens itself. This means that if you were to use either camera with a different lens, you would probably get very different results.

It is also worth remembering that the two different types of image stabilization have their pros and cons. The sensor approach used by the A900 means that the lens does not require any complex built-in mechanisms, and you'll get the benefit of image stabilization whatever type of lens you use. This could also mean that the lenses are cheaper as they are easier to make. However, the lens-based image stabilization that cameras like the Canon 5D Mark II and the D700 use means that you can upgrade the image stabilization whenever you buy a new lens, and the lens manufacturers are constantly improving the technology that they use.

Image Stabilization Score Comparison

Below are stills from some of our test photos taken with the Sony A900, showing an average image from the horizontal test. The target is a slanted line.

	Image Stabilization Comparison Table
	Low Shake, IS Off	Low Shake, IS On	High Shake, IS Off	High Shake, IS On
1/500
1/250
1/125
1/60
1/30
1/15
1/8

Noise	Page 5 of 18		White Balance
Sony A900 Digital Camera Review Navigation
Intro	Product Tour	Color	Noise
Resolution	White Balance	Sample Photos	Playback
Hardware	Controls	Design & Handling	Speed & Timing
Canon 5D Mark II Comp	Canon 50D Comparison	Nikon D90 Comparison	Nikon D700 Comparison
Conclusion	Specs & Ratings