Squinting is bad for your eyes but great for your art.
Back in art school I had a drawing instructor that taught us how to measure with sight and the pencil. You probably have seen artists being mimicked by other people holding up their thumb and squinting at it as if they’re measuring something. Well, that’s what he was teaching us and it works well as long as you add one more thing to the drill – comparing different portions of what you’re drawing or painting to some other part of the same subject.
We use to stand there in class and while drawing a nude model we would have to measure how wide their shoulders were in relation to how wide their head was and how far from the left shoulder in towards the head we would start drawing the neck and so on and so on.
It’s an excellent technique and if you use it throughout the drawing most of the time the pieces all seem to fit together perfectly and voila! It will look just like the real thing!!! Try it some time.
He also taught us how to do “Quick studies”. The model would hold a pose for 20 seconds and in that time we had to “capture” the aesthetic line of the body with minimal drawing and then the model changed to another pose and on and on until we were able to capture the pose quickly and correctly. Terrific drill. Try in a park sometime with a large pad of newsprint paper and a charcoal pencil.
The squinting just seems to come naturally because your eyes can’t focus on your thumb or pencil being held up to use as a measuring device and the subject in the distance being measured at the same time.
Oh…and don’t forget to stop squinting once you’re done drawing.