Sony Alpha SLT-A99 In-Depth Review
Resolution Chart Comparison (JPEG and Raw)
Images on this page are of our standard resolution chart which provides for measurement of resolution up to 4000 LPH (Lines Per Picture Height). A value of 20 equates to 2000 lines per picture height. For each camera we use the relevant prime lens (the same one we use for all the other tests in a particular review). The chart is shot at a full range of apertures and the sharpest image selected. Studio light, cameras set to aperture priority (optimum aperture selected), image parameters default. Exposure compensation is set to deliver approximately 80% luminance in the white areas.
In order to eliminate any potential sources of vibration from adversely affecting the results, we illuminate the scene using flash, which provides an effective shutter speed many times faster than the camera's own shutter speed would allow.
What we want to show here is how well the camera is able to resolve the detail in our standard test chart compared to the theoretical maximum resolution of the sensor, which for the charts we shoot is easy to work out - it's simply the number of vertical pixels (the chart shows the number of single lines per picture height, the theoretical limit is 1 line per pixel). Beyond this limit (which when talking about line pairs is usually referred to as the Nyquist frequency) the sensor cannot faithfully record image detail and aliasing occurs.
This limit is rarely attained, because the majority of sensors are fitted with anti-aliasing filters. Anti-aliasing filters are designed to reduce unpleasant moiré effects, but in doing so, they also reduce resolution (the relative strength and quality of these filters varies from camera to camera). In theory though, a sensor without an AA filter, when coupled with a 'perfect' lens, will deliver resolution equal to its Nyquist limit. Therefore, even though it may be effectively unattainable with normal equipment in normal shooting situations, an understanding of a sensor's theoretical limit provides a useful benchmark for best possible performance.
On this page we're looking at both JPEG and Raw resolution. For a (more) level playing field we convert the latter using Adobe Camera Raw. Because Adobe Camera Raw applies different levels of sharpening to different cameras (this confirmed) we use the following workflow for these conversions:
- Load raw file into Adobe Camera Raw (Auto mode disabled)
- Set Sharpness to 0 (all other settings default)
- Open file to Photoshop
- Apply an Unsharp mask tuned to the camera, in this case Amount 150%, Radius 0.6, Threshold 0
- Save as a TIFF (for cropping) and as a JPEG quality 11 for download
|JPEG Extra Fine (6000 x 4000)||Raw (6000 x 4000)|
|JPEG Extra Fine 100% crop|
|Raw 100% crop|
|JPEG Extra Fine 100% crop||Raw 100% crop|
With its 24MP sensor, the SLT-A99 ranks among the highest resolution full frame DSLRs on the market, narrowly beating the 22MP Canon EOS 5D Mark III and falling shy only of the class-leading 36MP Nikon D800. The A99's JPEG performance on our resolution chart is about what you'd expect from a Bayer sensor at this resolution, with the ability to resolve detail up to about 2800LPH. Minor edge halos are present to a greater degree than on the Nikon D600, which we found very impressive in that regard, but moiré is kept to a minimum. Raw files can resolve a bit more detail here and are able to tolerate a moderate amount of sharpening without producing visible edge halos.
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