Summary: A transmission step wedge was photographed with a Sigma DP1 Merrill and dp1 Quattro. Signal mean value and signal-to-noise ratio (SNR) were estimated for each color channel from the raw (X3F) files using RawDigger. Quattro raw image signal strength is substantially more uniform across color channels. Quattro images also have higher SNR (i.e., less noise) than do Merrill raw images in the red and green channels. Quattro blue channel signal strength was substantially less, and SNR was slightly inferior to that of the Merrill, particularly in the well-exposed portions of the images. Averaged across color channels, the difference in SNR is equivalent to about 1/2 EV advantage in dynamic range for the Quattro. In practice, the Quattro may have a ≥ 1 EV superiority because, in contrast to the strongly blue-channel biased overexposure of the Merrill sensor, the Quattro blue channel is relatively resistant to overexposure; and all three color channels tend to become overexposed in concert. These differences in signal strength and SNR can be understood at least partly by reference to differences in sensor design. They suggest that the Quattro design was chosen to improve signal strength and SNR in the red and green channels, while sacrificing some signal quality in the blue channel. The net effect is substantial improvement in overall signal characteristics: in particular better balance among color channels. ISO series were made with a DP2 Merrill and a dp2 Quattro. Raw files were processed through Sigma Photo Pro, exported as TIFFs, and taken into Photoshop. As expected from the signal strength and SNR analyses, Quattro images had an approximately 1-stop advantage in high ISO image quality. That is, Quattro images exposed at ISO 1600 were similar to, or slightly better than, Merrill images at ISO 800.
Key Words: Sigma Quattro, Sigma Merrill, DP1, dp1, DP2, dp2, Foveon sensor, RawDigger, signal strength, signal-to-noise ratio, image noise, Sigma Photo Pro, sensor design, step-wedge