A while back I decided not to post any photo studies i do, I do a fair amount of them every now and again. I think it was because some guy kept thinking I was cheating some how by doing it, which was hypocritical as he would trace or copy from photos, getting them very wrong aswell. My point being studying from photos can be good, although in some cases its often bad practice (if your tracing or something like that). It realy seems to be what you're using the photo for, if youre using it directly to draw over, thats kind of crappy however you can use them to study things like values and tones, aswell as shapes and other things.
These are 30-40 minute studies looking at shapes and tone.
How did I go about them? Well I gave my self a few restrictions to stop bad practice.First, I select my image, making sure theres something interesting about the picture, I quite liked the harsh black and white. I than put the image into photoshop, select a boarder around the image and on a new layer, paint in the slection area, move the now painted box to the side and use that as my canvas. Thats the only help I give myself so that the scale is correct.
Im not alowed to use rulers or eracers. The pippet tool (eye dropper) is not alowed to be used on the photo, only your canvas, so no grabbing swatches or piching tones. And I can only use one layer. Only the brush tool is aloud to be used.
This way, I have to measure and get the tones by eye.
So its basicaly just training the eye.