There are a myriad of uses for the Adobe computer program known as Photoshop when it comes to post processing your images. And, over the years, Photoshop has introduced many fantastic new features and functions. But there is one tool that I employ regularly when post processing my RAW files which has proven to be the most valuable tool of them all.
It is a tool contained within Photoshop’s RAW file converter plug-in called Camera Raw and the tool is called the “Adjustment Brush”. Many of the adjustments tools designed to adjust exposure are what are known as global adjustment tools. The Adjustment Brush however allows you to make finely tuned local adjustments to small areas of a photo. To elaborate further, a global adjustment affects the entire photo file whereas a local adjustment allows you to work on just an isolated, problematic area of the photo in order to balance it out with the rest of the image.
In the past there were only global exposure adjustment options in Photoshop which would fail in many cases because they would also effect the exposure in parts of the photo that were already perfectly exposed. A local adjustment tool is more precise because you can leave the good areas alone and only work on the areas of the photo that need fixing. Otherwise, a global adjustment might damage an already good, larger area of the picture while trying to repair the smaller, problem area in the photo.
In general I have found this tool particularly useful for wide angle landscapes and architectural shots where you have a big and uncontrollable difference in the amount of light from one part of the frame to another.
To start using the Adjustment Brush tool, first open a RAW photo file in Photoshop. Then, once the Camera Raw dialogue opens up on your screen, select the Adjustment Brush tool on the top of the Camera Raw tool bar by clicking on it. See here:

The next thing is to go over to the Exposure slider and set the level you want to adjust the exposure by for the problem area of the photo. Normally I start with .50 (half a stop) either over or under, depending on whether I plan to darken or lighten the specific area. In the sample settings below I have set it to reduce the exposure by -.50 in order to darken the image by half a stop with the Adjustment Brush:

Once you have set the amount of exposure you want to adjust by, you will need to choose the settings for the brush itself. Below are the settings I use and generally they need to only be set one time when you use the Adjustment Brush for the first time and then Camera Raw will save these settings for your future use. The brush size will vary though based upon the size of the area you are working on, but best to start with a small brush size and adjust as needed. You may also end up changing the brush size many times during your workflow:

Also, uncheck the “Show Pins” function at the bottom right of the Camera Raw window. This seems to restrict the flow of the adjustment brush a bit. So it is best to turn it off when you are working.
Now you are ready to start working on your photo. Just take the mouse, click the mouse button down, and run it across the area of the photo where you want to adjust the exposure. You will immediately see the subject area lighten or darken, depending on how you set the Exposure slider.
Once you have made your changes, you may find the options at the top of the menu of “New” and “Erase” to be very useful.

If you have overshot the area you are working on then take the Erase tool to brush away and remove areas of the adjustment you want to restore. If you want to go back and add more of an adjustment to a certain area then click on New and go back to the area and start brushing it some more to increase the effect of what you did previously.
Note that if you make an adjustment, then use New to add more brush adjustments that you wont be able to erase the adjustments you made with the brush the first time. So if you think you want to erase something, it is best to do it before you use the New option or your original edits become permanent. Otherwise, you can always click on the “Clear All” button at the bottom of the Camera Raw window at anytime to remove all of your Adjustment Brush changes made and start over fresh.
Now, instead of using the New option, you can also move the Exposure slider after you have made your changes with the Adjustment Brush in order to simply increase or decrease the amount of exposure on the adjustments you just made to the picture. Again, you have to move the Exposure slider before you click on either New or Erase if you want to adjust the brush strokes that you laid down already or your changes will become permanent.
I have found that sometimes it is better to work with a larger brush size if you are trying to blend the exposure at the edge of an area in the photo since a larger brush will give you a softer, more evenly blended edge.
You can also darken or lighten an area that you have over adjusted with the brush by inverting the exposure setting of the brush and going over the same area again. This sometimes helps to blend an area better than using the Erase function because the Erase function removes everything and will usually give you a hard edge on the area you are erasing from and this isn’t always desirable.
As you start working with this tool I think you will find it very useful and you will develop your skills in working with it quickly. If you have more questions on how to use it then please feel free to post them below and I will gladly try to help.
Your welcome Snow. Best wishes.
Thank you very much. This is very useful!