Tennessee Photography by Bradley Proctor

A common way to get sharper images is to stop the lens down. It is generally excepted that the range of f/8 to f/11 is the sharpest. This has been a loose rule that I have followed while taking my photographs. But how can we be sure? Where in the f/8 to f/11 is the sharpest? How much difference does it make? These are some questions that I’ve attempted to answer by performing a simple resolution test.

The images in this test were taken using a Canon EF 50mm f/1.8 II lens. The camera was mounted on a sturdy tripod and leveled to be aligned as perpendicular to the test chart as possible. Mirror lockup and a remote shutter release were used to prevent camera shake. The images below are 100% crops.

f/1.8

f/8.0

f/22

As you can see from the images above, shooting at f/8.0 is sharper than shooting wide open or stopped all the way down. The reason that wide open is not as sharp is because more of the glass on the lens is used and not just it’s most optimal area. The reason that stopping down to f/22 is not as sharp is because of defraction which causes light to deflect off of the aperture blades making scattered light hit the sensor. But what about that middle range?

f/8.0

f/9.0

f/10

f/11

From the images above, it appears that f/8.0 and f/9.0 are the sharpest. At f/10 and f/11 the image starts to get softer. This came as a bit of a surprise to me as I was expecting f/11 to be sharper than f/8.0.

Does it really matter if we are shooting at f/8.0 or f/11? Probably not. The difference is so slight that except under extremely rare circumstances would the difference be visible in practice. But it’s handy knowledge. Generally in the past, I would stop down to f/11 and leave it there unless there wasn’t enough light or I want a different amount of depth of field. With this knowledge I will now be stopping down to f/8 instead. Hopefully this little test will be useful for others as well.