I was noticing a slight problem with my pictures taken with my Canon SL1 and the 40 mm lens. It didn’t look like the area I focused on was quite as sharp as other areas in the pictures so I started feeling something wasn’t quite right. I then took a quick picture of a ruler at an approximate 45 degree angle. You can see the results below.
The lens was back focusing. I had focused on the 6 inch mark in the middle. The lighting wasn’t great but you can see that the ruler on the far side of the six-inch mark is in better focus (around the 7 inch mark on the left side) than in front of it (the bottom of the picture). With a 40 mm focal length at this distance the focus drop-off is pretty quick. You can see the variation across the word Dietzgen. The D is sharper than the N.
As this point I hoped that it was the lens and not a function of the camera so I put the 24 mm lens on the camera and repeated the test. You can see the results below. The out-of-focus area is pretty evenly distributed on either side of the six-inch mark.
I have decided to return the 40 mm lens and continue my evaluation using the 24 mm lens, which is approximately the same focal length as the Fujifilm X100 series cameras and lenses.