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I have 4 images that I scanned and saved as TIFF using my 4 in 1 desktop scanner/printer thing.

I drew all the images myself by hand using the same pen, so the originals are quite consistent.

When I open the files in GIMP, the levels are not consistent from one image to the next. If I put them side by side, they don't look like they belong together. Some "black" lines are darker than others, and some white backgrounds have more noise. I tried adjusting the levels manually on each image and that improved the individual image quality, but there is still variation amongst the set.How can I use GIMP to (hopefully) batch adjust the levels so they are consistent with each other again?

I realise that the problem probably comes from the scanner itself, but I've looked at the settings and can't see how to fix it there, so I'm hoping to be able to fix it after the fact.

Any suggestions?

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Definetelly your scanner software is trying to run some "auto" thing by itself. You should really try to turn that off.

There is no easy way to make then consistent again but for trial and error. Becasue white and black level are not the only thing there, but How steep is the Value curve from light to dark - which affects line width directly, and also how "smooth" it looks.

I'd say "levels" is not the tool for this job - you should use GIMP's Curves tool instead (edit->curves). You can even transition from the Levels dialog, once you've got a good preset to start with to the curves dialog by pressing the corresponding button on the levels. This way you could work only on the curves' slopes to get your desired look.

Again, check if there are third party software to use your scanner - your scanner driver seems to be "too smart" and letting too little control to the user. If you are on Windows, you could even try getting some Linux (Linuxes can boot from pendrives or virtual machines, no need to mess with your setup). Since Linux is another O.S., and with little support from the hardware manufacturers, all drivers are made from the ground-up in a consistent way across devices, and don't pull control away from the user.

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    Thanks @jsbueno Curves worked well for me, and when I realised I can save a curve profile, I was able to apply it to the other images as well. Still not perfect due to the scanner factor, but it was a better result than I got by playing with the levels. RE the scanner, it's not attached to the computer directly and scans onto a USB stick. All it's drivers are internal and I have no idea how to deal with that short of finding a way to flash the printer itself and I think that is too drastic. Cheers.
    – Lucien Stals
    Aug 25, 2015 at 23:36
  • If the drawings are really important and should be very consistent (eg for use in a professional project) I'd advise you to get them scanned by a professional, such as a decent copy center or printer. Chances are they'll have some experience with this kind of thing...
    – PieBie ♦
    Aug 26, 2015 at 13:17

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