Andy Westlake tests Sigma’s unconventional SA-mount mirrorless camera, the sd Quattro
Sigma sd Quattro review: Dynamic range, noise and resolution
Like the dp Quattro compacts we’ve already tested, the sd Quattro’s Foveon X3 sensor offers a decidedly Jekyll and Hyde approach to image quality. At low-sensitivity settings of ISO 100-400 it’s excellent, with astonishing pixel-level detail. But at high ISOs it’s awful, lagging far behind conventional Bayer sensors. In what appears to be a tacit admission of this, Sigma Photo Pro 6.4, by default, now drops the output resolution to 4.9MP at ISO 800 (2,712×1,808 pixels), 2.2MP at ISOs 1,600 and 3,200 (1,808×1,205 pixels), and just 1.2MP at ISO 6,400 (1,356×904 pixels). It’s still possible to extract full-size images if you prefer, but they’re not great.
As we’ve seen previously, our Applied Imaging dynamic range analysis software won’t sensibly handle the Foveon sensor’s X3F raw files, so we can’t reproduce our usual graph here. Empirically, though, the camera delivers reasonably decent dynamic range at ISO 100, although it can’t match the latest conventional APS-C sensors in terms of shadow detail recovery. When shooting JPEGs, the in-camera tone control helps with getting the maximum amount of shadow and highlight detail into the files, and when shooting in raw, Sigma Photo Pro’s X3F Fill Light tool does a particularly effective job, although it can easily be overdone. Above ISO 800, dynamic range is very limited with considerable levels of noise in shadow regions.
Characteristic of the Foveon sensor, at low sensitivities the sd Quattro gives excellent image quality, with attractive colours and remarkable amounts of ultra-fine detail. But even at ISO 100 some gritty luminance noise is visible when viewing images at the pixel level, along with hints of green-and-purple chroma noise, although you won’t see this in print. At just ISO 400 both are more visible in neutral midtones and colour saturation is deteriorating. At ISO 800 colour is further subdued and detail starts to smudge away.
Noise is better suppressed when using raw: above we’re showing SPP’s default pixel-binned output at higher ISOs. This has the effect of reducing the visibility of pixel-level noise, but even so I wouldn’t venture beyond ISO 1,600. This is poor for an APS-C camera, and severely limits the camera’s versatility.
In raw mode the sd Quattro resolves an impressive 3,500l/ph, which is about as high as its sensor could theoretically deliver, and with none of the false colour artefacts that Bayer sensors can display. However, Sigma’s JPEG processing reduces this to around 3,200l/ph at ISO 100. This is maintained reasonably well to ISO 800, but at ISO 1,600 JPEG resolution plummets to around 1,800l/ph owing to a change in processing mode. At higher settings noise increases markedly, but has little effect on measured resolution.