Hmmm, that was fast. An astute reader noticed that one of my screen shots in the Aperture 3 full screen mode post used the black point slider. It never ceases to amaze me that the questions that sort of thing generates are completely unpredictable to me. Her email wanted to know what that control did. I am going to answer that question here along with what I hope are the answers to a bunch of other questions that a lot of people have regarding Aperture's adjustment controls. For that matter any image adjustments. I also hope to answer the "why" you might want to use one versus the other when choosing what adjustments to apply.
Before I begin keep this thought in mind. There are a million ways to get to the same place when adjusting an image. Choosing one over the other is more a matter of comfort and convenience rather than right and wrong. This is especially true of Aperture.
Okay let's start with the exposure control block as a whole. For the most part the sliders are shortcuts to using something more complex like the curves control in Aperture. Meaning they manipulate the tonal values in much the same way you would do it using curves it's just quicker and easier unless you want to do it some specific way or do other things that the sliders are not set up to do. I say this with one caveat - when I mention curves I am talking specifically about Aperture 3's curves control and the new "extended" and "shadows" features.
Let's take an example - the black point control. This slider does two things if we want to talk about what it does in relation to a curve adjustment. The first thing it does is stick an anchor point somewhere in the mid tones to nail down the mids and highlights. The second is it moves the black point slider at the left towards the the right. In effect this makes the darks darker and increases shadow contrast if you move the black point slider to the right. I won't really talk about moving it to the left because A) you almost never do it and B) although you can get close using the "shadows" range of the curves control it is WAY more difficult to get the same precision.
The other thing the black point slider does is makes extreme precision easier because the amount of movement of the slider vs the change is not a whole lot. If you are using curves in the "normal" range the tiny bit you have to move the black end is hard to do with the same precision. Hence that is sort of what the "shadows" range in the curves control does. Here is an image that get's to the same place using both controls. The version on the right using black point at about "10" and the image on the right using a curves control.
Here is what the curves control looks like in the "shadows" range.
Here is what it looks like in the "normal" range. Note the anchor I stuck in the mid-tones.
The same is pretty much true for the rest of the exposure control block as well as the contrast slider in the enhance block. Think of them as shortcuts to doing things another way. Same goes for the recovery slider. It's almost the reverse of black point. It sticks and anchor point in the lower mid tones and drags the right hand end point in curves further right in "extended" range. Contrast manipulates the end points in one direction or the other. You get it.
So why would you want to use anything but the sliders? Well, if you want something a little different than what they do. You want your anchor points in different places for different tones or you want to manipulate them in slightly different ways or maybe do a whole bunch of stuff with one control. Can you do a combination of both? Sure, let your eyes guide you, not some rigorous dogmatic by the numbers approach. Whatever works best for your purpose at the time, don't sweat it too much.
Hope this helps - part of the Aperture super simple stuff series. Maybe I need a new series called "Aperture 3, fairly simple stuff".