A Selection of Selection Tips
Whether it be brightening shadows, removing color casts or something else entirely, in order to overcome the limitations of photographic capture, it is often necessary to apply adjustments to just a portion of an image in Photoshop. There's no way I can cover everything about selecting in Photoshop, nor is there probably a need for me to try. The basics are simple. For many users though, going beyond the basics can be a source of frustration. Here are a few tricks that may help.
Marque, Lasso and Magic Wand
The basic selection tools most users start with include the Marque, Lasso and Magic Wand. If you need a simple selection, these just may do the trick, but such is often not the case. Still, they are a great place to start. When working on nature images, it is rare to encounter a perfectly rectangular or elliptical area needing to be selected so the standard Marque shapes just never seemed all that useful to me, but I use the free-form Lasso all the time. The Magnetic Lasso revs things up even more by automatically cajoling the selection boundary to follow edges in the image. The Magic Wand lets you simply click and have Photoshop find edges all the way around. It may take you a bit of practice to optimize the settings for the Magnetic Lasso and Magic Wand to convince them to select what you want and leave out what you don't, but once you do both of these can be huge time savers.
Adding, Subtracting and Modifying Selections
Once you create an initial selection with the Lasso or Marque tools or some other method, the question is what to do with it next. Unless you are extremely talented (or the selection you needed to make as extremely simple), your selection may be close, but it is unlikely to be exactly as you want it to be. But rather than starting over, trying again and again to get what you are after, you can easily modify your existing selection. If you hold down the Shift key while using the selection tool again, your new selection will get added to your current one. If instead you hold down the Alt key (Option key on OS X) your new selection will be subtracted from what you currently have selected. You can switch between selection tools too with this trick. If you initially select an area with the Magic Wand for instance, you can switch to the Lasso and add to that selection with the Shift key held down. Just hold down whichever key you need (Shift or Alt/Option) and keep on refining your selection until you like it.
If you need to make an adjustment to an area that is roughly all the same color, try using Select >> Color Range. Although you can select particular colors from the dropdown list in the resulting dialog, you rarely will want colors that neatly fall into categories such as "red," "yellow" or "green." Instead, use the "Sampled Colors" option and simply click on the colors you want in the image. The mask preview in the dialog shows you what is included based on where you click. By default, each time you click with the dialog still open, the old selection is discarded and a new one is made. To expand your selection instead to include additional colors, hold down the Shift key while you click. If you overdo things, hold down Alt (Option on OS X) while you click to remove certain colors from your selection. The Fuzziness slider determines how close colors need to be to what you click on in order to get selected. The "Localized Color Clusters" checkbox option generally helps create more useful selections by factoring in distance from currently selected pixels rather than just color.
If you already have the start of a selection you can expand it to include adjacent pixels with similar colors by using Select >> Grow. If you want to include similarly colored pixels throughout an image rather than just ones near the selection boundary use Select >> Similar. Both of these rely on the Tolerance setting that is hidden in the options for the Magic Wand. If you click on the Magic Wand in the toolbox, you'll see it on the options bar across the top of the main image area. You're likely already familiar with the Magic Wand but may not have known it affected other tools. Adobe hid it. What can I say.
Selections and Masks
To preserve image quality and retain maximum flexibility I recommend performing as many of your edits as possible on adjustment layers. In the world of adjustment layers, selections become layer masks. To the right of the icon on the layers panel for each adjustment layer there will be a second rectangle for the associated layer mask. If you create an adjustment layer without a selection being active, your new layer mask will simply be a white rectangle meaning that this adjustment will affect the entire image. But if you have a selection active, that selection will be turned into the corresponding mask. Each pixel completely inside your selection will be white in the mask, and those completely outside the selection will show black. Pixels in between will become gray in the mask as dictated by their degree of being selected.
Photoshop has menu options for modifying selections by expanding, contracting, or smoothing edges and so on, but if you use adjustment layers you really don't need to mess with them. Instead, click on the rectangular mask icon for the adjustment layer and you can use the paint brush, Gaussian blur or just about any other regular Photoshop tool to modify the mask. Modify the mask and you modify the selection used for that adjustment. If instead you Alt/Option-click on the mask rectangle you will be able to see the mask in black and white in the main window in place of the image itself. Sometimes this can make it easier to see what you are doing when modifying a mask, but in either mask selection mode (clicking or alt-clicking) you can use pretty much anything to optimize your selection mask without having to deal with the limited options for refining masks on the Select menu.
Copying a Selection
Sometimes after making one adjustment on part of an image, you find yourself wanting to make another one to that same area or one that is close to the same. Rather than going to the work of selecting that area all over again, Photoshop lets you create a new selection by copying an existing layer mask by simply right-clicking on the mask icon in the Layers panel and choosing "Add Mask To Selection." And yes, if you already have a selection active, this does what it says it does. Your selection will grow to include what was already selected, plus what was selected in the layer mask. There's also a "Subtract Mask From Selection" in case you need it that works the same way.
Being such a basic action, Adobe built countless ways to create a selection into Photoshop. I'll cover more of them next week.