Photoshop auto-levels inconsistency

November 11, 2006 3:56 PM

Photoshop question: I have a large image which I want to color-correct. I've been testing on a shrunken version of the file, and I'm actually happy with the results of just doing auto-levels. However, if I do auto-levels on the full-size image, I get totally different results. How can I get the same results on the full-size image?

Is there a way to ensure similar results in both versions?

I have to apply this to 15 similar large files (they're maps, so they have the same color palette) and I don't want to have to futz with all of them, I'd rather set up a PS action to tear through them overnight.

posted by krunk to computers & internet (13 comments total)

Need a bit more info. What are the resolutions of both the originals and your test image? There shouldn't be any difference in the effect of Auto Levels unless the difference in resolution is HUGE.
posted by ernie at 4:04 PM on November 11, 2006

Is it possible the shrunken image was saved in a different colorspace than the full-size image?
posted by dmd at 4:11 PM on November 11, 2006

They're both 200DPI, same color space. I didn't save after resizing, I just scaled to 12.5% of the original size.

Obviously the small version will have fewer colors than the large one, but I assumed that it wouldn't make a difference to the levels.
posted by krunk at 4:18 PM on November 11, 2006

Are you comparing areas of the image at the same level of magnification? That is, if you're adjusting a thumbnail size at 100%, adjust the larger image and then compare the two by zooming in to 100% on the larger image. The color difference may only be a result of the quick resizing Photoshop is doing to show you a smaller version of the large image.
posted by odinsdream at 5:29 PM on November 11, 2006

You can easily set a curves profile based on the first image and then automate application of that to the others. Curves is an alternate way of manipulating both levels and color balance simultaneously, and therefore faster.

Try this:

open the biggun. Do a careful one-click manual levels deal (command-M, on a mac; select white point eyedropper, click brightest level you want; select blackk-point eyedropper, click darkest level you want).

Once you have an adjustment you like, a) save the file to a new file in a new directory b) close the source file.

Now, open a small file or make a new one, whatever. What's in the file is unimportant. With the file open, go to your 'actions' palette, and create a new action. Set the action to record. Now press option-command-M (I think that's alt-crtl-M on PC).

Stop recording. Close the file, without saving.

Now put the files you need to fix into a distinct directory, and open the 'batch' menu command. select the new action you just recorded. Have the batch save the files into a new directory so you can easily make corrections if thisngs don't work out.
posted by mwhybark at 5:41 PM on November 11, 2006

oh, and alt-crtl-L or alt-crtl-U should do the same thing for levels or color balance respectively.
posted by mwhybark at 5:44 PM on November 11, 2006

Autolevels can be quite different on the two pictures if the larger one has tiny bright and dark points in it. They would dissappear when the image is scaled down.
posted by StickyCarpet at 6:00 PM on November 11, 2006

They're both being compared at 100% zoom. Even if I shrink the big one down after doing auto-levels, they look different.

I've posted a sample here.

I'll likely do manual levels like mwhybark suggested, but my crap Mac mini takes eons to test adjustments on big files; it'd be nice to be able to adjust a small sample file and then apply it to the big one.

Cheers!
posted by krunk at 10:14 PM on November 11, 2006

Load the one that does auto-levels like you want. Go to Image->Levels. Click "Auto", which should end up doing the same adjustments that you like. Now either write down the values for all 3 channels (whitepoint, blackpoint, gamma; although I think gamma remains at 1 for auto-level), or save the settings to a file with the handy "Save" button.

Now go into the larger image and apply those same Levels settings, by either inputting them manually or loading the LVL file you saved.
posted by neckro23 at 11:48 PM on November 11, 2006

off-topic: nice image, what is it?
posted by londongeezer at 12:38 AM on November 12, 2006

"Mississippi Meander Belt?"

Seems to be a depth chart, based on how the river was carved in pre-historic times.
posted by cowbellemoo at 1:54 AM on November 12, 2006

You should just add an adjustment layer. Create a levels or curves adjustment layer on the small image, then open the second image and drag the adjustment layer into the new document. You've now got the changes.
posted by jeremias at 5:20 AM on November 12, 2006

Brilliant, thanks for the help!
I didn't realize that the manual levels dialog had an "auto" button. I tweaked its settings and saved it as an adjustment layer, which I applied to the large image. Worked perfectly, and was way faster than fiddling directly with the large image.

The image is a geological investigation of the Alluvial Valley of the Lower Mississippi River. I found out about it from here, and downloaded several hundred megs of high quality images. I'd like to print out all 15 and display them down a long hallway I have.

Thanks for all the help!
posted by krunk at 11:48 AM on November 12, 2006