Why comparing a drawing with an iPhone is bad

I know that this drawing isn’t perfect, but I believe it to be much close than the following digital comparison illustrates.

I thought I was closer than this

52513-drawing

I have noticed a trend with my drawing comparisons, that the drawing becomes more inaccurate the closer to the bottom of the image I go, or the further from the center of the photo.

So I left this setup overnight and today I drew markings for a grid made of 1 inch measurements around the subject, then took another picture of the drawing and subject. I then pulled the drawing into Photoshop and resized it so that the beginning top left inch measurement on the paper was exactly one inch in Photoshop. I then proceeded to rebuild that grid in Photoshop by putting a line every vertical and horizontal inch. What I ended up with is a drawing with an inch grid on it, and a digital representation of those same inches over it. In theory, once I get to the bottom right of the grid, both the digital grid and the drawn grid should match up. As you can see below that is far from the case.

 

Distortion test 1

52513-camera
Not only is the width smaller but so is the height, and not by a little but a very large amount. That seemed to be to much, of a distortion, so as I good scientist would do, I repeated my experiment to test with similar results. And this time I too extra care in insuring I re-size the photo correctly.

 

 

Distortion test 2

52513-camera2

I also did a third test where I took a picture a bit farther away thinking that the lens of the camera would distort only the parts of the image closer to its edge. This was incorrect, the distortion was the same even when taken from a larger distance.

This is much closer to the truth. The distortion is not as much but it’s still there. It looks like an average of 1/4 inch inaccuracy per 12 inches of space vertically and maybe worse horizontally.

What does this prove? That my drawings are not as bad as I thought, but this is no excuse, I still have errors in my drawing. This whole exercise was for me to help realize how the camera distorts an image and to be aware of it.

Moving forward, I’m not sure how I can compare my drawings with a three dimensional subject accurately now.