The actual sensitivity of each indicated ISO is measured using the same shots as are used to measure ISO noise levels, we simply compare the exposure for each shot to the metered light level (using a calibrated Sekonic L-358), middle gray matched. We estimate the accuracy of these results to be +/- 1/6 EV (the margin of error given in the ISO specifications). Note that these tests are based on the sRGB JPEG output of the cameras, in accordance with the Standard Output Sensitivity method defined in ISO 12232:2006, the standard used by camera manufacturers.
By our tests, the SD1's measured ISOs match the marked ISOs near-perfectly, i.e. to better than 1/6 stop accuracy.
Noise and Noise Reduction (JPEG)
Note: this page features our new interactive noise comparison widget. By default, we show you the default noise reduction settings of the camera tested, and three other models of the same class. You can select from all available NR options, and from other cameras. The 'tricolor' patches beneath the familiar gray/black/portrait images are taken from the same test chart, and show how noise impacts upon blue, green and red areas of a scene.
The SD1 produces excellent results at low ISOs, giving excellent detail with very low noise (the camera's distinctly muted colour rendition in its Standard setting is also very obvious). As ISO is raised, noise quickly becomes a problem, and the camera's processing responds with an intense assault on chroma noise while leaving luminance noise largely untouched. However by 800 noise is beginning to become quite obvious in the colour patches, although detail is still well-retained. At ISO 1600 the Foveon sensor is struggling to keep up with conventional designs, and ISO 3200 sees a combination of severe colour desaturation and intense luminance noise, giving a characteristic grainy look. Finally at ISO 6400 the image has lost almost all semblance of colour whatsoever.
The graphs expand on this story, particularly in comparison to other high-end APS-C SLRs and Sony's SLT-A77. The SD1 holds its own up to about 1600, but then noise levels shoot up dramatically. Sad to say that at the top two ISOs (3200 and 6400), the SD1 is the worst-performing APS-C camera we've seen for some time. Note also that you get no control over the camera's JPEG noise reduction settings - you have to accept what you're given.
RAW noise (Sigma Photo Pro 5.2 - noise reduction set to zero)
In this section we'd normally show raw files developed using Adobe Camera Raw, with the images brightness-matched and processed with all noise reduction options set to zero. However Adobe does not yet support the SD1, so we're making a slightly different comparison. We're using Sigma Photo Pro to develop the SD1's RAW files with noise reduction controls set to a minimum, then comparing this to our usual ACR conversions from other cameras. This isn't a perfect comparison, but the aim as usual is to provide data that's as photographically-relevent as possible.
The SD1's files are again very clean at low ISOs, although again Sigma Photo Pro produces default colour output that's just as muted as the camera's JPEGs. At 1600 the SD1 is starting to look distinctly noisy, and at the top two ISOs things start to go seriously awry. The red, green and blue patches in particular suffer from a serious loss of saturation.
What's not immediately obvious on these small crops, though, is the low-frequency green and purple chroma blotching that blights the SD1's images at ISOs of 1600 and above. We'll look into this with real-world samples later in this review.