a7 September 2012

Tutorial: Finding your lens sharpest aperture

Probably you've already heard that sharpness of your image depends on many factors and that one of them is the aperture being used. Each lens have its sharpest aperture which is usually somewhere between f/8 to f/11 (but not always). That means that photos taken with this aperture will be the sharpest ones.

Please note, that sharpest aperture differs from lens to lens and it can even differ for the same lens at different focal lengths.

Ok, we now know that being aware of sharpest aperture is useful and important but how can you find it out? It's very simple in fact.
  1. Put a newspaper (or other text or drawing with strong lines and contrast) on the wall using a tape (make sure it cannot move as it can potentially ruin the test).
  2. Mount your camera on a tripod just in front of a newspaper.
  3. Set your camera to Aperture priority mode (Av for Canons) and set your ISO to 100.
  4. Focus on the newspaper and switch to manual focus to avoid refocusing (what could result in different sharpness due to Auto-Focusing missing the target).
  5. Use shutter release to avoid any camera shake and take photos of the newspaper at different apertures. Start with the widest aperture and close it slowly (I used 1/3 EV increment in my test below). To be honest to avoid touching the camera during the test I connected my Canon 5D MK II with my PC via the USB cable and released shutter and changed exposure parameters using Canon's EOS Utility.
  6. Copy files to your PC.
  7. View your files at 100% magnification and compare them. You will notice that some photos are more blurry than others. There will also be a few which are much sharper than any other. Note aperture of these photos - it is your lens sharpest aperture.
Here are some test shots for my new Canon 24 f/1.4 L lens, taken at apertures of f/1.4, f/7.1 and f/22.
From these images it should be easily visible that for this lens the sharpest aperture is f/7.1. If it isn't view the images in full size - the difference should be much easier to notice then.

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