Why are my f/11 and f/22 shots softer than f/5.6 on Micro Four Thirds, and how can I keep more depth without blur?

Question

I’m shooting a Lumix GH5S with a Leica 12-60mm lens on Micro Four Thirds. I tried stopping down to get more depth in a boulevard scene, but my images at smaller apertures look softer.

Example settings:
- f/22 at 1/20s
- f/5.6 at 1/120s
- f/11 at 1/125s

The f/5.6 image looks much sharper, while the f/11 and especially f/22 shots look blurry. I’m wondering:
- Is stopping down the right way to increase depth of field here?
- Is the softness at f/11 and f/22 caused by diffraction?
- How can I tell diffraction apart from camera shake or focus issues?
- On Micro Four Thirds, what aperture range is usually best if I want both good sharpness and enough depth of field?

Tom Axford · Answer

Diffraction blur

I use Micro Four Thirds cameras and I would very rarely use them at f/22 because the diffraction blur is far too much at that f-number (aperture).

As a general rule the smallest aperture (highest f-number) that I use is f/8. Even at f/8, for someone who examines their images very closely, it is possible to see a very slight diffraction blurring at f/8 in comparison with a similar image shot at f/5.6 or larger aperture (when using a sufficiently high-quality lens).

The amount of diffraction blur is directly proportional to the f-number. So there is twice as much diffraction blur at f/22 as at f/11 (and four times as much as at f/5.6). At f-numbers below about f/4 or f/5.6 lens aberrations often cause more blurring than does diffraction, but that is highly dependent on the quality of the lens and whether you are looking at the centre of the image or the corners of the image (lens aberrations are usually much greater in the corners than in the centre).

Whatever lens you use and whatever focal length, the diffraction blur is always the same at the same f-number. However, it does depend on the sensor size. The diffraction blur at f/22 on MFT is the same as the diffraction blur at f/44 on FF (when measuring the blur in relation to the diameter of the image). The amount of diffraction blur is proportional to the crop factor of the sensor (at the same f-number).

Other causes of blurring

Some blurring may be caused by movement blur: the camera moved slightly during this exposure. This depends on the exposure time and the focal length of the lens (longer focal lengths magnify the image more hence magnifying the effect of any motion blur). However, I think your camera has IBIS (in-body image stabilisation) that would eliminate any blurring at the shutter speeds you used unless your hands were exceptionally unsteady.

The other main cause of blur is the image being out of focus, but I don't think that is likely to be a major cause for the examples you gave.