If you get overwhelmed by extraneous details when making a picture, want to paint more loosely vs. following a tight preparatory drawing, or have trouble knowing how to leave parts of your work less refined, try this exercise.
Take a black and white photo and blur it (e.g. using Photoshop’s Gaussian blur) until it’s reduced to a few big unrecognizable blobs. Then paint or draw a copy of the blurred image, using a soft physical medium like charcoal, or painting digitally with a big, soft brush. Take your time to get those big abstract shapes and values just right.
Next, create a slightly less blurred version of the photo (it may help to create a few levels of blur in advance), and using this as your new reference, refine your drawing. You’ll probably find yourself simply subdividing the initial big blobs into smaller component blobs, but you still will have no idea what you are drawing.
When you’re done with that stage, refine the drawing again using a less blurred version of the photo, and so on, until you have something you can recognize (or keep going until there’s no blur left at all, if you want). Maybe you’ll want to refine certain areas but not others.
If you’ve done the exercise correctly, the less refined areas will not need any more detail or development to look “finished.” Maybe your background space is a big area of value, but it works just fine. If you want to add detail to some other area, you can see exactly where those refinements fit in.
You may be surprised to see that you’ve created a very accurate reproduction of the photo, without first creating any kind of tight drawing as a framework, or using any measurement devices. Here's the source photo I used for this exercise:
This exercise is not about learning how to copy from photos – it’s about seeing abstract shapes. Most of us have brains that are surprisingly good at seeing abstract shapes, but our analytical brain gets in the way. It may help you to try to recall the mindset you experienced while doing this exercise, when creating your art.
The exercise also demonstrates that it’s the big shapes and their relationships, not the small details, that create the illusion of form and reality in a 2-d image. Lastly, this exercise may serve as a guide for how to conceive, start and finish a picture, whether painting from life, a photo, or your head – get the big relationships down, then refine as you see fit, where you see fit.
Here's the same approach used with a picture created from the imagination:
Artists have employed various devices to help them see physical subjects and media like this, including squinting, and viewing through frosted glass.