DSLR ISO Settings

Q: I just picked up a Canon Rebel XT and I've been impressed with its daytime capabilities, now its time to take it out at night. I'm curious about the ISO setting, there are notes throughout the manual that higher settings will cause a grainy image. As I understand it, the setting varies the analog gain between the detector chip and the ADC. Assuming the amp is a reasonably low noise device (its self noise is lower than the image noise) why would increasing the gain make an appreciable effect in the image SNR. The only thing I've come up with is that the manual comments are based on very low noise daytime images. In this case the amplifier noise floor would degrade the image SNR, but at those levels I'm not sure it would be noticeable.

Answer by Bill Keicher

You have it right, changing the ISO increases the amplifier gain by a factor of 2 (6 dB) for each doubling of the ISO. The on-chip amplifier readout noise becomes significant for single frame JPEG images when increases in the following amplifier gain reduce the camera system dynamic range to just under than 48 dB (around ISO 800 for the Olympus E-300 DSLR at room temperature). A JPEG image uses 8 bits per color (8 bits = 48dB dynamic range, 6 dB per bit). So when shooting at ISO 800, amplifier noise is just visible in E-300 JPEG images. This is why the E-300 requires selection of a special override, "ISO boost", to shoot at ISO 800 and 1600. Between ISO 100 and 400, the camera images are limited by JPEG image quantization noise.

Most DSLRs have a 12 bit A/D converter that corresponds to a maximum of 72 dB dynamic range. CCD well size and amplifier noise are the limiting factors in all of these cameras. When shooting in RAW format at ISO 100, the E-300 DSLR can produce an image with a dynamic range of about 64 dB (limited by CCD well size and readout noise electrons - equivalent to 10.66 bits per color). Increasing the gain by a factor of 2 increases the amplifier noise voltage at the A/D converter input. Since dynamic range is the ratio of maximum signal voltage to rms noise voltage, the dynamic range is reduced by 6 dB for every factor of 2 increase in ISO.

It is best to shoot at a low ISO (100 or 200 versus 800 or 1600) in RAW image format. Why? Because DSOs can have a very wide dynamic range present in the inherent illuminance of the DSO (e.g., galaxy cores versus spiral arms of the galaxy). Limiting the recorded image's dynamic range by shooting at a high ISO, or using a JPEG or TIFF format limits the image processing possibilities. I have made this mistake in standard, daylight photography.

One more thing… Dynamic range = 20*log(maximum signal voltage/rms noise voltage) dB and signal voltage (or current or photoelectrons) is directly proportional to incident optical power in the linear response region of the CCD.