Look at the picture to the left. Which one looks right to you? The face with the square glasses or the round one?
If you said the square one, good for you! Now, why did you choose it?
The right one, when it comes down to drawing it, is what you would probably draw considering perspective... but that's exactly why it is wrong!
You'll notice a line drawn behind the heads- this is the horizon. Everything points back to a point in the horizon - usually something at the back of the head when drawing someone facing you. But when it comes to drawing glasses, perspective does not apply unless the glasses is overly huge for the person's head or exaggerated.
In real life, the lens of the glasses themselves are small causing the wire frame extending to the ear to bend outwards as in the first example.
Though this may be the case, you can break this rule.. look
below for reason...
The male on the left is an example of a improper way to draw glasses generally. The only way this would pass muster is if the lens are supposed to be bigger than is normal - for example, if the glasses was passed down and is too big for him, or he is unfashionable and prefers big glass lens than the more hip smaller glasses.
The female on the right the right way to draw glasses. In this case, though, it goes a bit further and the bend of the wire frame is exaggerated. Why, you ask? Look at her. What do you think about her and her character because of her glasses?
When I drew her, I had a character in mind who was a bit messy, and unkempt. Notice her ruffled hair and the glasses emphasize this fact even more. You can further speculate that she is a goody-goody student, a bit on the nerdy side, and a good-natured girl. Is this what you thought also?
Drawing quarter glasses is basically drawing parallel (lines like || that never meet) and perpendicular (lines like _|_ or a T that meet) lines.
Just draw a line going across the front of the face that will be the lens of the glasses.
Then, draw lines that are parallel to each other on either ends of the "lens" line.
Finish the picture with the shape of the glasses and erase the lines going across the face.
male here has thick glasses- notice you can't hardly see
the side of the glasses on his right side? It's being blocked
mainly by the frame.
The female there has circular type glasses- the typical type of glasses that manga artist's draw. It shows the eyes openly unlike the glasses of the guys which hides them.
In another note, you can use glasses to emphasize character- for example, wouldn't someone wearing thick framed glasses seem like he is hiding something? Or maybe isn't that out-going? The female, with her wide and open glasses looks like the upbeat, out-going type, right? In effect, glasses are not just for decoration or for seeing purposes- it also adds character.
Pretty easy huh?
The male facing the left has lens that is only a line. It is best to draw a thick line or else the lens will fade into the masses of hair.
Also, if you happen to draw glasses with a sort of this lens like the girl over there, make sure that when you color it, it's not the lens you're coloring but the frame of the glasses! On a side note, remember to color the frames according to the character's personality! You'd think it'll be funny or weird if that guy there had a pink frame- wouldn't you?