Digital Camera Exposure Indices - Vision Research - #9

ISO sensitivity Method SNR (18% gray) Overexposure latitude

Saturation Noise Noise Noise Noise

1300 2500 3700 7200 10000

65 40 30 20 10

0.5 stops 1.5 stops

2 stops

3 stops 3.5 stops

Table 3: Summary of ISO numbers for a monochrome Phantom v10

the ISO 12232:1998 standard, based on the SNR plot in Figure 2.

In addition to the ISO numbers and associated SNR values, the table shows the overexposure latitude afforded by each setting. Even when light is abundant, it is not clear that the "highest quality" EI of 1300 is better than the "excellent quality" EI of 2500. The increase in SNR from 40 to 65 is hard to even notice, and the latter setting is less prone to clipping the highlights. Lower exposures could be used in low-light, high dynamic range situations.

It should be noted that in order to take advantage of the higher dynamic range at low exposures, the camera must reach it's readout noise specifica­tion while capturing the full dynamic range (gain set to minimum). This implies that the image is saved using a large enough bit depth (at least 10 bits, prefferably 12). Applying gain before the image is saved does allow an increase in EI, but at the expense of overexposure latitude.

The images on the following pages illustrate the increase in noise that ac­companies an increase in exposure index. The images were taken with the same v7.3 camera characterised in section 4. To obtain the images, the camera and lighting setup was kept fixed. Between two successive pictures, the lens was closed by one f-stop, except between the first two images, when the difference has been 1/2 stop. The images were "stretched" so they span the full output range. Unless otherwise noted, no noise reduction has been applied.