Aperture: ticking levels controls changes colours?!

[wolfgangarnold]

<p>Hi *,<br>

while editing photos with strongly saturated colours I noticed, simply when activating levels control colours are shifted. This is both visible in the photo and in the histogram - see attached screenshot: left shows levels deactivated, right: only the tickmark in levels control is set - no other changes done.<br>

Any idea whether this is a bug or a feature (or my colour-mis-management...)?<br>

cheers, Wolfgang</p>

[blake_schwalbe]

<p>I think it's a feature. I use levels and curves for color correction. If you change the blending mode to "luminosity" colors will not be affected. For example, if you make a typical S-curve in the curves dialog to increase contrast, you'll also increase saturation unless you change the blending mode to luminosity.</p>

[wolfgangarnold]

<p>Blake,<br>

many thanks for your response - the funny thing is, that colours are shifted already when ticking the "levels" tickmark without touching any other controls. Changing to "luminosity" does not change anything.</p>

[blake_schwalbe]

<p>Yes, I understand. Then I'm not really sure what it means. Perhaps one histogram is cached and the other is re-calculated or refreshed, or is considering other color settings (color space) ... but I of course don't know.</p>