|Understanding NikonScan's Autoexposure
Function and the Effect of Light Leaks
As with a camera, one of the critical functions a scanner must perform is setting its exposure. Because a scanner is a digital device, a measure that a proper exposure setting has been achieved is that the resultant number of intensities is maximized.
For NikonScan set an initial exposure through autoexposure; the user then has the option of using analog gain to override or modify the initial settings. NikonScan's autoexposure operates by evaluating the entire frame. The crop setting has no effect on the exposure.
What Autoexposure Accomplishes
The function of autoexposure is to match the dynamic range of the image with the sensitivity of the scanner. If done properly, this results in a full representation of intensities in the scanned image.
A Properly Exposed Image
For example, assuming an image containing a full range of tones, the histogram for a properly exposed image might appear as follows:
The full frame histogram is important because that's what the autoexposure interprets and bases the exposure setting:
Within the full-frame, looking at the histogram for the intended image (yellow crop area), autoexposure has been successful because the range of tones has not been diminished:
Now suppose we've underexposed the above image. It might appear as:
What results is a raw scan with approximately half the tones in 1a:
This is what the tonal scale in fig. 1b.
The histogram for the underexposed image is similar to the properly exposed image (fig. 1b), but compacted to the left.
The intended image (as defined by the crop box), a subset of the full-frame, therefore suffers a diminishment of the number of tones:
On the other hand, suppose we've overexposed the above image. It might appear as:
The histogram for the overexposed image is similar to the properly exposed image (fig. 1b), but shifted to the right with pixels shifted up against the right margin:
What results is a raw scan with approximately as many tones as 1a, however, the line line along the right margin indicates a loss of highlight definition (some of the highlight tones in the frame have been clipped). Unfortunately this loss of definition in the highlights has occurred in the intended image (defined by the crop box), as confirmed by the presence of the histogram bar along the right margin:
This is visually evident in the washed-out areas in the table and background.
If You Want a Full Range of Tones Why Not Perform 8+Bit Scans?
The reason for taking care in setting exposure is that we want to maximize tonal definition in our image. (Remember this is the digital world, and images have a discrete number of tones.) This is accomplished by matching the scan to the sensitivity of the scanner. If we underexpose the scan, as in fig. 1e, the result will be like fig. 1k,
which results in many missing tones after the black-white points are set. Overexposure, on the other hand, results in tones lost through clipping (shifted into white, fig. 1i).
A seemingly obvious alternative of obtaining a full range of tones would be to rely on the scanner's extra bits. Nowadays, even the cheapest scanners claim extra-bit precision (10-, 12-, 14-, or even 16-bits). The interval covered by a histogram contains 256 values (0 - 255). If you have a 12-bit scanner, for example, there are 15 values between each histogram value. It would seem, therefore, so that when the distribution of tones is expanded to set the black-white points, the in-between values fill any gaps. What your scanner vendor doesn't tell you, however, is how accurately and consistently it can measure the tones. The accuracy of most consumer scanners comes far short of being able to utilize their potential precision. Without multi-sampling, your 16-bit scanner may be giving you only about 7 accurate bits; the rest are garbage. The result will noisy shadow areas with little definition. The more bits your scanner offers in precision, the more accuracy is required of the scanner to use those bits. Extra bit precision is no substitute for proper exposure.