Cropped photograph, © Karen Lynn Ingalls 2010
I hear your question already – why can't you just paint from the photo as it is? Why go through a whole process of cropping what you've got?
First, the proportions of a photograph are different from those of most standard canvas and paper sizes. Consider: what size and shape do you want your painting surface to be? The proportions of an 18"x24" canvas are different from those of a 20"x24" canvas, in significant ways. What are the proportions of your painting surface? Do you want to work on a square shape? Maybe you want to work with a vertical format, and your photo is in a horizontal format. Cropping will allow you to discover new compositional possibilities hidden within the photograph in front of you.
Second, an uncropped photograph will give you too much information. It's too much to ask of your brain to continually refer to the photo and ignore portions of its visual information; it's much simpler to just make sure those portions aren't there to distract you.
Finally, using a photograph as is ties you to what the camera predetermines. An artist needs to allow room for creativity and imagination, for experimentation and play. You don't want to be a slave to the photograph – do you?
Now you can play with the possibilities.
If you are cropping your photograph digitally, your experimentation will all be on the computer. You can try various croppings, save them, and come back to and compare them all later. Then you can print the ones you like best to use as photographic references. I print mine on full-size photographic paper, and put them in page protectors, so I don't get paint on them.
If you have photographs already printed, first protect them. 4"x6" photographs fit nicely inside little plastic sandwich bags, which are clear and don't obstruct your vision.
Cropped photograph, © Karen Lynn Ingalls 2010
Then take pieces of paper – I cut wide, straight strips out of copy paper or whatever else I have to hand – and use them to frame different compositional possibilities. How many different, interesting compositions can you find? This is the part that is like creating thumbnail sketches. It may be possible to get several interesting paintings out of one photograph.
In these two examples, the composition I chose was a very small part of the larger photograph. Sometimes you may want to leave much more of the original photo.
Once I figure out which composition appeals to me most, I tape the strips of paper down around it, so that I will not be distracted by the rest of the photograph.
Then I get my painting surface ready. I can either choose my composition first, and then use a surface that matches its proportions, or I can determine what painting surface I want to use first, and find compositions that match its proportions. Photo ready? Painting surface ready? Paints? Brushes? Water and medium? Hallelujah – now you can paint!