If you have ever used the curves tool to try and mimic a film effect you have probably noticed this problem. A common technique when creating a film like effect (or “cinematic” effect to use the more common vernacular these days) is to use a tone curve with the blacks raised and the whites lowered. You would do this to mimic the more gentle roll off of film (Film’s not actually not really like this – but it’s become accepted as the way to do it). If you look at some of the Cinematic presets that are included in the new version of camera raw and Lightroom, you can see many of them use this type of curve.
So what is the problem?
Well, when you use a curve like this, you may end up with blacks or whites becoming too grey. You might want to fix this, but you don’t want to adjust the tone curve, as you like the results apart from the black and white levels. The problem is, you can’t adjust the black or white sliders in the exposure panel, because this will still lead to clipped blacks and whites. If you look at the histogram when doing this you can clearly see where the hard “brick wall” is. The reason for this is that Lightroom generally evaluates its adjustments from the top down (unless you rearrange them). So because the various exposure controls happen before the curves, there was, until recently no way to adjust this.