Sony SLT Alpha A55 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 set to deliver approximately 80% luminance in the white areas.
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. Nyquist is indicated in these crops with a red line.
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 a Unsharp mask tuned to the camera, usually 100%, Radius 0.6, Threshold 0
- Save as a TIFF (for cropping) and as a JPEG quality 11 for download
|JPEG (4912 x 3264) 4.0MB||RAW (4912 x 3264) 4.2MB|
The A55 makes good use of its 16 megapixel resolution and shows some detail up to approximately 3000 lp/ph which puts it in a similar ballpark as the 18MP Canon EOS 550D and a step ahead of the 12MP Nikon D90. The JPEG output is also fairly clean and free of artifacts and moiré.
A little bit more detail can be drawn out of images shot with the A55 in RAW mode but the improvement isn't that dramatic. More moiré patterning and slightly better vertical resolution are the two most obvious differences compared to the default JPEG output, but RAW files stand up to post-capture sharpening rather better.
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