While I was out on my last shoot, I did a quick test to show the effects of diffraction on image sharpness. If you don’t know what diffraction is, here is the dictionary definition:
Diffraction is the process by which a beam of light spreads out after passing through a narrow hole or across an edge.
Practically what this means is that when you use physically smaller apertures, the light passing through the lens spreads out more. It no longer focuses to a pinpoint on the sensor and makes the resulting image less sharp.
How bad is this effect? Well that’s what I wanted to test for my camera. I shoot with a Sony A7RIV camera which has a 61MP sensor. The individual pixels are very small and hence it is more sensitive to diffraction as the same spread of light will cover more pixels compared with a lower resolution sensor.
Here are 100% crops showing the detail, or loss of detail, with the aperture varying from f/8 to f/22:
All the images were taken with a Sigma 14-24mm lens at 17mm, ISO 100, and with the focus distance set to about 3m. This image has the default processing in Lightroom with a small amount of capture sharpening (amount=25, radius=1, detail =25).
Here is the full image showing the location of the crop.
My takeaway from this test is that the effect of diffraction is very noticeable and I want to shoot at f/8 or lower whenever possible. F/11 is also acceptable although it is slightly less sharp. After f/11 the sharpness drops off rapidly, f/16 looks very soft and I would describe f/22 as mush.
Sometimes it’s not possible to get enough DOF when shooting at f/8 or f/11. In these cases I will focus stack if the scene alllows it. I check the DOF calculator in PhotoPills to see if my photo is possible in a single shot (tip: PhotoPills uses outdated defaults for computing DOF that aren’t relevant anymore for high resolution sensors. I added a custom camera setting with the circle of confusion set to around 0.01-0.013mm for my A7R4.)
Here is the final image from my shoot: