Levels Brightness Sliders: how do you use them?
Hi. I use the Levels Brick on almost every Image I process in Aperture. I have long known that I can change the what Aperture calls the "Brightness Levels" by using the Brightness Levels Sliders (the inverted triangular handles along the top edge of the Levels Brick Histogram). (What they do is easier to see than to describe -- a couple slider-slides will show you if you're not familiar. Here is the User Manual's page on the Levels Brick.)
My first question is simple: do you use these, and if so, under what conditions for what goal?
Yesterday I discovered that holding down a modifier key changes the function when sliding the Brightness Levels Sliders. I am trying to understand how this works, and how it could be useful.
Holding down "{Command}" while sliding the Brightness Levels Slider(s) resets the Levels settings and allows one to reposition the half-tone and quarter-tone controls. (Again, immediately apparent in use -- not worth the words it would take to describe.) My second question then is, What is this for; when would it be used?
Holding down "{Option}" while sliding the Brightness Levels Slider(s) changes not just the Brightness Levels Sliders' positions, but also the Black, the White, the Gray, and the Quarter-tone Levels Sliders. (Which begs the question: what are these to be called as a family? I would have called them "Luminance Levels Sliders", but having separate Sliders on the same control for "Luminance" and "Brightness" confounds me.) What is this for?
If I have images that are mostly dark, or mostly light, I have found it useful to shift the half- and quarter-tone controls towards the hump in the histogram. (Which makes me think that this reposition of the default start positions for the controls should be automated: instead of fixed by equal separation of the luminance gamut, they should be repositioned in congruence with the Image's luminance histogram. (If I understand this correctly, they are currently positioned accorded to the median points of a whole gamut. I am suggesting auto-positioning them according to the mean(s) of the selected Image.)
My last question is hard. As with _every_ control in Aperture, I want to know (and strongly feel the right to know) what operation is actually being done (at the pixel level of luminance/channel). And in particular, what is the difference between the Black, White, Gray, and Quarter-tone Levels Sliders and the Brightness Levels Sliders. An explication would be both elucidating and massively appreciated.
Thanks. 🙂
MacBook Pro 13, Mac OS X (10.6.8), 8 G / 500 G internal / 5 TB external / NEC 2490 / ColorMunki Pho