Let me tell you about single-pass multi-scanning: some scanners (mine included) have the capability to read a line multiple times and average the result, in order to minimize the noise. As Vuescan help says:
Input | Number of samples
This option is available for scanners that support multi-sampling. As the scanner head passes over the media it makes multiple exposures for each location. The results for all samples are averaged.
This is a useful feature because any one exposure may be inaccurate, resulting in noise in the output. Noise will appear as one pixel whose color or tone is different than surrounding pixels. By taking multiple samples and averaging the results, the effect of inaccurate exposure is reduced.
This option will slow down scanning because the scanner is doing more. You should experiment with your scanner to see which balance of speed and accuracy is appropriate.
Number of samples is similar to Number of passes. Multi-sampling is preferable as the scanner head is positioned once, which ensures that the same area will be exposed for each sample. Multi-sampling is available only on a limited number of scanners.
Advanced Option: This option is displayed when the scanner is capable of multi-sampling.
So, last night i wanted to see if the improvements are to be seen with the naked eye and if the procedure worth including it in my workflow. To have an idea of what Number of samples cost me, i’ve started to scan a sample at highest optical resolution my scanner is capable (4800dpi) and i got these results:
| Number of samples | How much seconds the scan cost |
|---|---|
| 4 | 494.05 |
| 2 | 500.89 |
| 1 | 521.50 |
So… when i decrease the number of samples, the scan time grows?! Ok, maybe the option Number of samples does nothing when scanning grayscale, and the small differences come from my way of starting the clock etc. Totally understandable.
Then, let me show you the results for the same material, scanned at 2400 dpi:
| Number of samples | How much seconds the scan cost |
|---|---|
| 16 | 510.83 |
| 8 | 510.07 |
| 4 | 514.37 |
| 2 | 528.90 |
| 1 | 209.11 |
Can someone explain the last result (one sample — two minutes 49 seconds)? Again, i got some crazy timing for scanning at 1200dpi too:
| Number of samples | How much seconds the scan cost |
|---|---|
| 16 | 287.65 |
| 8 | 286.98 |
| 4 | 231.59 |
| 2 | 93.88 |
| 1 | 51.93 |
I really can’t tell what is the limiting factor. I will try to redo the scans on a USB3 port, maybe i will find an explanation.
Later edit. Ed Hamrick — the creator of Vuescan — gracefully explained the “problem”. Thank you, Ed!
The maximum number of samples is resolution-dependant. The way it’s implemented is to scan in the stepper motor direction at a higher resolution than in the CCD direction. However, at optical resolution (for instance 4800 dpi) the maximum resolution of the stepper motor is often just 2× the optical resolution (for instance 9600 dpi). This limits the number of samples to 2. However, if the resolution is 2400 dpi in this example, then the number of samples can be up to 4 since the stepper motor can scan at 9600 dpi (4 times the optical resolution).
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„The maximum number of samples is resolution-dependant. The way it’s implemented is to scan in the stepper motor direction at a higher resolution than in the CCD direction. However, at optical resolution (for instance 4800 dpi) the maximum resolution of the stepper motor is often just 2x the optical resolution (for instance 9600 dpi). This limits the number of samples to 2. However, if the resolution is 2400 dpi in this example, then the number of samples can be up to 4 since the stepper motor can scan at 9600 dpi (4 times the optical resolution).“ — Ed Hamrick