My workflow is as follows. I shoot in raw; title, keyword and adjust/develop in Lightroom; then transfer to Photoshop as a 16 bit TIFF for further work. The final image is stored as an 8 bit JPG. I don't normally keep TIFFs, which are very large files, but I do retain the original raw files.
What is an adjustment layer? My way of thinking about it, which may or may not be the whole story or even correct, is that an adjustment layer is analogous to placing a filter over the camera lens. The filter can change the contrast or colouration etc of the image, but can't change the basic content.
You can't therefore clone bits out of your image on an adjustment layer, you must return to the background layer for that purpose.
Adjustment layers are cleverer than filters however, as you can optionally mask off some of the filter effect so that it only affects a section of the image. Using a so called layer mask, you can choose the sections that you want the adjustment layer to influence. Further, you can return at any time to the mask to change it, using the brush and eraser tools to extend or reduce its active area (use the Channels window and select your active layer mask).
You don't have to have hard edges to your mask selection either, and most of my work is carried out using graduated masks, so that transitions don't stand out in the final version.
It can take a significant amount of time to construct a layer mask to suit your purposes, particularly if you are working at pixel level. Fortunately it is possible to re-select and re-use the mask, or, usefully, an inverse of the mask, later in the process. I recently learned that you can also copy a mask between layers.
The other great thing about adjustment layers is that, unlike a camera filter, they can be removed or made dormant at any time after the shot has been taken. So you can play with the image to your heart's content but then undo any wrong moves at a single key press. This means that you can experiment in a non destructive manner, always able to backtrack without changing the basic image.
You can use as many adjustment layers as you wish, but I rarely get beyond 5.
Why not do this in Lightroom? Well you can't, at least not with my knowledge/edition! True there are various graduated filter effects available, but with Photoshop you can accurately choose the exact area of an image that you wish to work on, working to the nearest pixel if need be.
Here's an example of how I have used two simple levels adjustment layers, sky and foreground, to enhance the appearance of an image. I hope that the differences are not too profound, in my view the best editing leaves a viewer believing that the image accurately represents the scene.
This is the image after developing in Lightroom
This one has had a few additional tweaks in Photoshop.
Ok, so how do you do it?
Easy, to obtain an adjustment layer select Layer, New Adjustment Layer, followed by the layer type that you want, e.g. Levels. At this stage you can give it a name.
It is useful to have the Layers window open in PS, when your new layer should appear above the background layer. There is a little eye symbol to the left of each layer and that determines its visibility, click to remove the eye and the layer is switched off. Another click and it is restored. Right click on the layer and you have the option to delete.
Layer Masks - again easy. Click on the background layer in your layer palette and then select a section of the image using any of the selection tools. You can optionally refine your selection, e.g. Select, Refine Edge. Now open a new adjustment layer. Rather than covering the entire image its scope of influence will be restricted to your selection.
How do you see the area affected? Open the Channels window and ensure that the little eye symbol is selected on the layer mask. The area of the image that is not affected will appear masked in red and your area of interest will not be masked.
How do you change the mask? Use the brush and eraser tools to extend or reduce it.
If you do a lot or work on a layer mask, you might want to use it for another layer. No problem, you can copy a layer mask between layers.
Last edit 8-11-16