You Can Check Your Lens Focusing

 See if your lens if focusing where you think it is or should be with a simple test using a ruler and/or a yard stick. You can also use this simple and inexpensive technique to see how your DOF extends on either side of the focal point.

Here I've laid a 6" ruler on a page of checks that I've pasted to a foam core backing with a marker line drawn where the 3" mark on the ruler is. I just focused on that spot and then shot some frames with the aperture at 4 different settings. Here it's at the smallest aperture of my Canon EF24-105mm f/4L IS USM lens, f/22 with the focal length at 105mm. Looks like everything is in focus as it should be at that small aperture but this doesn't tell me if the focal point is actually where I set it.

Here I take the same shot with the aperture wide open at f/4 at the same focal length. You can see how short the Depth Of Field is at the wide aperture. It looks like it's about 3" total with 1.5" on either side of the focal point. I can see that the focal point is right and the DOF is evenly split by it.

Seeing that I wanted to do the same test with the lens at its shortest focal length and see what happened.

At f/22 we see that the sharpness covers the entire yard stick. The decrease at the near and far ends is negligible. Even the dining room furniture against the wall looks pretty sharp.

It's the wide open aperture that will give us more information.


Look at that short DOF. Just look at the single numbers on the yard stick in both photos.

I checked the numbers farther away by looking at a close up of the image in Adobe Lightroom3 and found that the focal point was right where I expected it to be and that the DOF length was just about evenly split between toward the camera and away from it.

This simple test may also reveal some slight differences in the images at different focal lengths and apertures that are called aberrations. Many lenses perform their best and most consistently in the focal lengths from about 1 stop wider than the smallest aperture and a stop smaller than the widest aperture. Knowing about little quirks like that may help you get the most out of your shooting.