This tip is WAY FASTER and WAY more accurate than what I wrote before. I'll add right here, as I have in the past: I really do Not consider myself a Photoshop expert by any means; the tips I post are just things I've picked up over the years on my own, or when I'm lucky enough to have someone look over my shoulder. So if you see something in these posts and you know a better way: Please send it and help us all! Full credit given to you.
Carrie writes: "When you are in levels, you can use the little eyedropper things in the lower right corner of the box to set your black and your white. Just click on the "white" eyedropper, and then click on a place in your photograph that you know is white... then do the same thing with black... and it virtually color-corrects for you."
Here's the step by step: I've opened the original dark file (as you can see, the same funky sketch from last time), and made a copy of it for on screen comparision.
To begin, Click on Ctrl + L (Cmnd+L for Mac) or from the top toolbar, Image Adjustments/Levels to bring up the Levels dialog box.
Note the eyedroppers in the lower right hand corner as shown in Diagram A above. Click on the farthest right one, the 'Set white Point' dropper. Choose an area on your image that you know to be (in life) true white (in this case I clicked on the lower lefthand background), and click on that area with the dropper. Voila!
If the image didn't turn out quite how you wanted, click Ctrl + Z for undo, and try the step again with another 'white' area of the image. (Notice the middle graph, called a Histogram, and how it changes in these steps. The histogram measures the relativve amount of light and dark across your image).
As Carrie pointed out, do the step again using the left hand dropper to set your black point; I've done it here, using the shadow under the babushka's lower lip.