Another essential tool for the plein air painter is the ViewCatcher. Made by the Color Wheel Company, this handy device has two uses. First, it lets you crop down your awesome, Bierstadt-esque scene to something more manageable. It's better than a camera viewfinder because the little slider lets you match proportions exactly to your panel or paper. Cameras only have common presets such as 16:9, 3:2, 4:3 and 1:1; if you are painting an 11x14, you're out of luck.*
Second, it has a little "isolator" hole in the the slider that lets you judge value, hue and chroma more accurately. The ViewCatcher is made of a mid-value, neutral grey plastic. Patches of the landscape seen through this hole are either lighter, darker or the same value as the plastic; right there, this gives you three values to work with. Because the plastic is netural, a color seen through the hole appears clearly without the confusion of adjoining color masses. You can tell easily if a shadow is blue or violet, and if it is rich or dull in chroma.
By the way, there's a little hole at one corner where you can attach a lanyard. If you like to check color or value frequently, or often need to remind yourself where the major masses sit in your frame, you can have the ViewCatcher easily at hand instead of digging through your backpack. (If the maker would make a round version, it could be worn as a monocle, making it even more convenient.)
You can find more helpful tips and tools in my book, Backpacker Painting: Outdoors with Oil & Pastel, available at Amazon from this link.
*If you have a truly odd size that's not indicated on the ViewCatcher, such as 12x20, then there's a trick you can use to match it. Holding the tool to your eye, adjust the distance from tool to panel so that the apparent width of the paper fills the entire width of the ViewCatcher's window. Now, making sure that the top of the window seems to sit on the top of the panel, slide the slider up until the top of it seems to touch the bottom of your paper. This will give you an opening that matches the proportions of your panel.