Spiky histograms

[snusmumriken]

I recently had to switch to a new scanner (from Epson/Vuescan to Plustek/SilverFast) and photo editor (from Aperture to GIMP). With both systems I have scanned b/w negatives as 16-bit greyscale TIFF images. The new setup works adequately, but not as well as the old.

I don’t know very much about scanning, but I’m picking through every aspect in the hope of matching earlier form. One thing I’ve noticed is that new scan files imported to GIMP have histograms showing fine spikes within the broader peaks. Example below, with its image.

Can anyone tell me why this occurs, whether it is a problem, and if so what I can do about it?

 

[joe bosak]

Might be aperture uses broader or otherwise different histogram buckets compared to gimp. Or for whatever reason deliberately smooths the values. In which case nothing to worry about.

Might also be a similar difference in scanners in terms of how they measure, and how they record their measurements.

 

[Kino]

Spikes shouldn't be an issue. However, gaps are another story...

 

[nmp]

Normally this kind of spikes occur when the data is in 8-bits but I see you are scanning in 16 - so that wouldn't be it. In Photoshop (I don't have experience with GIMP) histograms can be spiky at times as by default they are created with 8-bits for expediency. If you want an accurate rendition using the 16 bits, you have to click on an icon above and it recalculates the correct histogram at which point the spikes disappear. Wonder if GIMP is doing something similar.

:Niranjan.

 

[reddesert]

Often, the cause of spiky/noisy histograms is that an image was created with low resolution of the levels - low bit depth, like 8-bit or many jpegs, as Niranjan said. Then resampled or otherwise adjusted, for example by tweaking the contrast or color curves. This results in re-quantizing the data onto a new low resolution representation, causing spikiness in the histograms and a posterization effect in the image.

Scanning as 16-bit TIFF ought to avoid that but you also have to check what the internal representation of the editing program is doing - if it's converting your TIFFs to a lower resolution format when they are imported, then you'll need to change that. Ideally, if the bit depth needs to be lowered (eg exporting JPEGs for web display) that should be the very last step after all other adjustments.

 

[snusmumriken]

Thanks to all for your thoughtful replies. It’s reassuring that none of you feel this is a major issue.

I believe GIMP preserves the 16-bit depth on import, but I will drill into that a bit deeper. GIMP does favour its own file format, which is a bit mysterious.

It hadn’t occurred to me that there might simply be a mismatch in bucket boundaries between the scanner and GIMP. That sounds very plausible. I haven’t yet found a way of recalculating the histogram as in PS, but expect there is a way - there generally is!

Thanks again.

 

[Alan Edward Klein]

Doesn't converting to jpeg cause this? Or if the scanner is on auto adjust? Try switching to manual scan 16 bits and create a tiff image file. Set levels (black and white points) to manual adjust, not auto.

 

[snusmumriken]

Doesn't converting to jpeg cause this? Or if the scanner is on auto adjust? Try switching to manual scan 16 bits and create a tiff image file. Set levels (black and white points) to manual adjust, not auto.

Thanks, Alan. Yes, I am already doing all those things. I also tried saving a RAW file from the scanner, but that creates a lot more work and a worse result, given my lack of skill.

I suspect the spikes arise on import to GIMP, as they are present before saving into the default .xcf format that GIMP likes to create.

 

[Alan Edward Klein]

Doesn't the Plustek scanner have it's own software? Why are you using GIMP?

 

[snusmumriken]

Doesn't the Plustek scanner have it's own software? Why are you using GIMP?

The Plustek software is rubbish. Fortunately it came bundled with SilverFast, which is quite good as far as it goes, although I don’t much like the user interface.

I use GIMP for manual correction of dust or other blemishes; to create different size versions with curve correction and sharpening for each size; and to add a simple border if required. I used to use Aperture for those purposes, which was perfect because one could create a blemish-free version and then use export presets to automate resizing etc.