There are a bunch of adjustment tools in Photoshop, some of them are interchangeable and you can achieve the same results in a different way. One of the most powerful of these tools is "Curves", it is also very popular, because you can easily change luminosity values across different areas, or work on color aspects of the image. Combine that with "Masks" and you got a heavy weapon in your hand.We are going to cover the "Curves" adjustment in RGB Color space. In modes like CMYK the idea is the same but the directions of the curves is reversed. Before we begin we want to advise you not to be scared from the complicated look of the curve, it is very logical and easy to understand.Curves are primarily used for adjusting the Tonal Range, Contrast and Color. To help you keep track of what you are doing, the curves adjustment tool is equipped with a Histogram. The histogram is essential when correcting Tonal Range.The curves adjustment panel looks like a grid with a diagonal line right through the middle, it also has a histogram as a background. As you can see from the two gradients on the left and bottom sides, the bottom left corner represents the shadows and upper right corner represent highlights. If you pull the line up and left will brighten the image, whereas down and right will darken it. You can also pull on the little squares in the corners to adjust the overall tonal range of the photo.By clicking on the diagonal line, you can add an adjustment point. If you want to target a specific tonal range of the photo, you will need to add two or more adjustment points so that you can isolate a more specific range, rather than affecting the whole photo.Let’s say you shot an athlete skiing down the slope and you forgot to apply exposure compensation. Then your images will be underexposed, the good news is that a quick and easy curves adjustment can fix your photos in no time. Just grab the little square in the upper left corner of the graph and slide it left to where you have some values on the histogram. This will makes your whites…white and fix the problem.Another popular adjustment is the so-called “S” curve. Shooting in “RAW” has a lot of advantages, but the images come out of the camera looking somewhat flat, and usually require some contrast to be added. Creating an "S" curve is as easy as clicking in the upper part of the diagonal line (this is going to add an adjustment point) and dragging it slightly towards the upper left corner. Then adding another adjustment point in the bottom section of the line and dragging it slightly towards the lower right corner. The line will make an “S” shape, which will result in brighter highlights and darker shadows…aka "more contrast."Right above the graph you will find a drop down menu that lets you select between the R, G or B channels or the default RGB. This lets you apply adjustments to the different channels for color correction or to create a distinctive look for your photos.On top of the Curves dialogue is another drop down menu that lets you save your curves adjustments for quick re-use with other images later. There are also presets for brightening or darkening the image, and “Increase Contrast”, which is very similar to the above mentioned “S” curve. Check these out to see what the program does to the curve for each effect if you are having a tough time understanding how this all works.As usual, we advise you to play and experiment with curves, many people say that curves is the only thing you need to know in Photoshop to improve your images dramatically. Keep in mind that often times subtle adjustments are better than drastic ones.