To keep things interesting though I've selected some special objects: a rusty torn-up tin can and a morrocan oven. Mundane, not so much.

In a previous sketch of a breakfast plate, I completely messed up the drawing of a circle skewed by perspective, i.e. an oval lying on its side (so that drawing didn't make the cut and wasn't posted here, hah). That time I tried to use a grid and be all mathematical about it. When sketching the morrocan oven, of which I really like how it turned out, I threw away the technical approach. Using a soft pencil, pressing very lightly on the paper so I could adjust along the way, I got the oval shape more or less right in one attempt, completed in seconds. This went so smoothly that I wonder if I'll ever use a tool to draw circles and ovals again. Maybe if it's something technological.

The rusty torn-up tin can (say this fast a few times without tripping over your tongue!) was a challenge to get the lighting and shadows right. Still not very happy with the result, but as it's a study I decided to abandon the effort after about two hours. The original drawing had very low contrast, so here GIMP came once again to the rescue. In this case I'd say the digital version beats the analog one. And I just played around with the Brightness / Contrast tool. With both drawings I digitally erased all smudges, so we get a nice clear white background.