Friday, 17 April 2009

Testing LS-4000 with Stouffer Stepwedge

Now that I have a LS-4000 in my home I thought I would apply some of the same testing that I have previously used on my Epson flatbed scanners. I used the FH-3 holder to hold a Stouffer Stepwedge and scanned it in sections to see how the scanner responded to levels of denisty.

I was stunned how it ran out of grunt in ability to penetrate the dark areas. I noticed how sensitive to the 'clear' area of the wedge it was so I popped a little bit of analog gain into the scan (0.8 actually). I did this to hopefully extend the dynamic range into the dark areas and bring the white areas up to begin clipping at "film base".

Cutting straight to the results it is not only not good, its actually surprisingly not good.

This is the result of a scan with 0.8 of analog gain applied to the master gain.

I have plotted a Log10 graph of the results in red (as this is how scanner responce needs to be measured for photographic purposes).

It seems to have made a strange change to the "linearity" of the scanner with a marked step occurring at about step 13.

Its strange that both parts remain more or less straight with a deviation in the middle.

Further, the 'blooming' of the scanner is apparent in the darker areas (note the small intrusion of one of the "frame" supports in the FH-3, this shows that the depth of black is not fully reached even though the ability to discern graduations is substantially impaired).

out of interest this is the result from my Epson 3200, and no blooming is evident in this.

so ... this makes the result from the Nikon particularly scary. Feel free to compare this to the results from my Epson 4990 scanner here.

Note: this effect of blooming is also apparent with x8 oversampling, so it is not just a digital noise issue.

I also thought it was worth mentioning that not only is there substantial bloom (clearly visible here) but that altering the analog gain disturbed the levels of RGB too. Looking at the RHS of this graph you can see three peaks, this is the places where Red Green and Blue have moved up towards 255 but clearly unevenly.

So, without careful profiling using the analog gain on this scanner to 'punch into dark areas' will result in strange colour shifts. I advise use with caution.

I left this section 'zoomed in' to show just how much blooming results in the transition from dark area to light area, it is significant.


I was asked some additional quesitons about this so I have repeated the tests and included results for 0 analog gain.

NOTE: rather than wait over the weekend I've disassembled the scanner and sure enough the mirror was rather filthy. So a quick clean with windex and a cotton bud (removing the mirror of course) later and its now performing well

My method was to scan with the scale set as linear from 0 to 255, the scanner software is set to put the data into Bruce RGB, so after scanning this profile was assigned to the data to ensure that the results are correct. As the wedge is so long, I had to scan it in 3 sections. To perform this I needed to
  • insert the wedge strip into the FH-3 holder
  • disable any preview or focus on insert
  • scan each step without prescan or preview
some overlap was possible in scanning (as it is manually fed) so I can confirm that (for instance) the values of step 4 (and the clear part of the step number) were consistent from scan to scan.

The results (data first) include the stand deviation of the values in the selection swatch. Standard error is calculated as the percentage of standard deviation to the median value (not, I use median in this testing not mean).

It seems to indicate that the error is getting quite high as the values go above 13 on the stepwedge. This needs to be taken into account when assessing how effective alteration of the image with curves will effect colour accuracy or noise in shadow details. Now the graphical view

seems that the linearity step at about 13 is still there

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