Trying to understand the use of adjustment layers

Question

I think I am missing something here so thought I would bring it here for discussion.
I am learning to do Adjustment Layers and when I do an adjustment layer of levels  and then another of  hue/saturation it has no way of confirming my choices.  If  then  want to do Shadows/Highlights it will not work change anything and I think it is because I have not confirmed the adjustment layer. What do I need to do after doing the adjustment layer to go ahead and do something else no as an adjustment layer on the photo OR is that impossible. How to make this work????
Sandie

Answer 1

An Adjustment layer effects all visible pixles in the layers below itself. So if you have a Levels Adjustment Layer and the Background layer below, the entire Background layer is affected. If you have another layer above the Background that is not an Adjustment Layer but only covers have the entire image space, the Levels Adjustment will effect the additional layer and the visible parts of the Background layer. If you add a second Adjustment layer, the effects stack. The lower of the Adjustments is applied first, the second is applied afterward, moving upward.
The Adjustment Layers themselves are transparent, they are a bit like a lens filter on your camera. Just like looking through a pair of sunglasses causes the world to appear darker, an a Levels Adjustment Layer can make the layers beneath it appear darker (or lighter, as dictated by the settings). But as Ken mentions, the strenght of using an Adjustment Layer, as opposed to using an Adjustment, is that you can always come back and change the settings later.