Using RGB Auto Levels

by James Duncan Davidson

One of the standard first operations that you might perform on a photograph is to apply auto-levels. Aperture makes this easy, and it's become almost a reflexive action for me. Usually, I almost use the Luminance auto-levels tool as that fixes up levels without affecting the overall color balance of the photograph. However, there are lots of situations where you have a photograph with a color cast and want to quickly remove it. In many cases, using the RGB auto-levels tool is just the ticket.

For example, I'm going to show you an aerial photograph I shot recently on a trip. This is how it looks in Aperture on import with no adjustments made to it:



Jim N.
2007-02-10 05:47:55
James, I stick with Luminance most of the time, and thought that Apple designed these shortcuts as one-or-the-other. I never considered using them together. So do you use Luminance and *then* RGB, or one or the other? Or do you only jump to RGB when there is a tricky color cast?
James Duncan Davidson
2007-02-10 15:10:10
Indeed, the shortcuts are "one or the other". IE, when you hit the RGB version, it appears to relevel based on RGB values and doesn't take into account the previous leveling opearation. I use Luminance first and then, when there's still a cast that I want to get rid of, I'll try the RGB one. There have even been times where I've flipped back and forth trying to make up my mind before moving onto using other tools, such as the colors tool.
Matt Swann
2007-02-10 18:18:09
Great tip. One thing I've noticed about the Auto Levels and RGB tools is that they tend to clip bright values -- that is, a properly-exposed photo (no blown-out highlights) will often end up with areas of overexposure after using the Auto tools.

Is this by design, or are there settings that'll let me control the amount of clipping that goes on there? I've been doing the auto-RGB adjustment by hand in the Levels palette instead so as not to blow out any highlights, which is pretty frustrating when the auto button is staring me in the face.

James Duncan Davidson
2007-02-12 19:43:15
Matt: Leveling tools typically "toe-in" just a bit into the histogram. The theory goes that the first bit of black is typically a bit noisy. I'm not sure what the idea is on the white side, but I know that there's gotta be a reason as so many of the auto tools tend to do it. Some applications let you set the amount of "toe-in", but I haven't seen this in Aperture.
Matt Swann
2007-02-12 20:34:33
Thanks for the response, James!

Does the overexposure concern you when you're preparing photos for print, or do you leave things as-is? I've been careful about not blowing out any highlights were I can avoid it, but not having to worry about it would be a lot easier.

Aperture does have black clip and white clip sliders beneath the main histogram in the HUD, but even at 0.0 they don't seem to eliminate this.

Jim N.
2007-02-13 05:07:42
The auto-exposure is a disaster at least 50% of the time for me. But often if I go Auto Exposure, Auto Levels it works out okay. Just this past weekend I got into the advanced tab of the highlights panel, which I found very productive --if not very time-consuming-- in dealing with blown highlights.

Either way, while I'm really appreciative of Aperture's 'eye' in the auto- field, we're still going to have to subjectively decide for ourselves what the proper exposure, luminance, etc it. So I'm trying to learn to expose as properly as possible in the field, and using Aperture's tools as a back up, and then I usually adjust the exposure manually.

PJ Harvey
2007-03-03 08:22:30
Auto levelling has been completely unusable for me since the beginning (Aperture 1.0) with my D70 and D200 pictures, example:
(however, I had figured out myself that RGB levelling, done manually, is one of the best ways to deal with colour casts in long range landscape photography, particularly these blue-ish shadows)