Noise, resolution and sensitivity

These images show 72ppi (100% on a computer screen) sections of images of a resolution chart, captured using the lens set to its optimum f/5.6 aperture. We show the section of the resolution chart where the camera starts to fail to reproduce the lines separately. The higher the number visible in these images, the better the camera’s detail resolution at the specified sensitivity setting.

Most cameras use a Bayer pixel filter arrangement, which has its red, green and blue pixels on a single ‘layer’. Establishing the ‘true’ resolution of a X3 Foveon sensor is complicated in relation to a Bayer pixel arrangement, but in our in-depth investigation of the sensor in AP 8 October 2011, we ascertained that the equivalent resolution as we would understand it is around double the ‘spatial’ pixel count. This means that although the maximum output is 4704 x 3136 pixels and print size is approximately 15.7×10.5in, the DP2 Merrill’s ability to resolve detail is roughly equivalent to a 30-million-pixel Bayer sensor.

Without a doubt, then, the single most outstanding aspect of this camera is its image quality in good light. Images taken at ISO 100 in these conditions contain a class-leading level of detail, up to the 32 marker in our resolution charts. In fact, there is some detail all the way to the end the charts. It is moiré patterning – which suggests a weak anti-aliasing filter – that has at times a dramatic impact on the camera’s performance in our resolution charts and stops it from scoring even higher. In real-world images, detail is crisp, thanks in part to the fixed 45mm (effective) lens, and requires little sharpening post-capture.

As I mentioned in the introduction, the design of the Foveon X3 sensor compromises the degree of light it can use, and as such its ability to handle noise is, by today’s standards, poor. Images up to ISO 200 are largely unaffected, but luminance noise becomes apparent in images at ISO 400, obvious at ISO 800 and distracting above ISO 1600. Unprocessed raw files shot at ISO 3200 and 6400 show a high level of chroma noise, too. With this in mind, I would like to see optical image stabilisation included, which would allow the use of lower ISO settings in more situations.

Image: It is clear that to get the most out of the DP2 Merrill, it is essential to shoot in raw format. This is especially the case in low-light scenes like this. The detail in midtones is much patchier and prone to chroma noise in the JPEG file, while a raw file with noise reduction applied is much crisper

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