M43 Practice, Diffraction, Unsharpness caused by reducing lens aperture

 

Why post about this ?  I am moved to write this by a post which is running currently on the DP Review M43 Forum, titled "Diffraction Limit". It has generated 142 responses thus far. Some contain such deep questions as ..."are you inebriated with the exuberance of your own verbosity ?"

I have to confess that I could not bring myself to read all the erudite contributions and further have to confess that I did not understand several of them. There is a lot of material about Airy disks and pixel dimensions and all kinds of technical and theoretical stuff.

Numerous contestable assertions are offered.  Much of the "discussion" would, I think be described by Australia's former Prime Minister Ms Julia Gillard as "Argy Bargy".

The Test  My own approach to the evaluation of contestable assertions is to test them. In this case a useful test can easily be designed and carried out in a few minutes.

I set up a simple test chart and photographed it at f2.8 - 22 in 1/3 stop increments. I made all the usual precautions to prevent camera shake: tripod, E-Shutter to 1 second then Shutter delay, 2 sec timer delay, manual focus, ISO 125. I used a Lumix GH3 with Lumix 35-100mm f2.8  lens at 42mm.

The Test Chart. This photo has been reduced and compressed for the net so it is difficult to appreciate the detail. Many Lumix zoom lenses, even some of the budget ones can resolve all the words all over the frame at their optimum aperture.

 The Results  I enlarged a small part of the frame, near the center, 330x343 pixels [0.11 Mpx] from each exposure and viewed these on screen at 200%.

On this particular test with this particular lens I found sharpness/resolution was the same from f2.8 through to about f8, with the first sign of slight softening becoming evident from f9. The image gets progressively softer from there to f22.

F2.8

F4

F5.6

F8

F11

F16

F22

Comment on the test photos  These have suffered somewhat from their compression for publication. The originals appeared sharper. Despite this I think you can see things are definitely softer at f11.

Conclusion  On this test closing down the lens aperture smaller than f8 did have a deleterious effect on image sharpness. I estimate the effect would be apparent in ordinary photographs by f10. By the way, with this particular lens, image sharpness in a large central area (not the corners, which sharpened up by f4) was just as good at f2.8 as f4 or F5.6 or f8.

It has certainly been my experience when photographing out and about that image sharpness/resolution is adversely affected in M43 camera/lens systems when the aperture reaches the f9-f10 range.  

Note on close ups, 30 August  I made some flower photos this morning, trying various apertures. The ones at f11 were the best. The gain in depth of field achieved by stopping down outweighed the slight loss of sharpness from diffraction.

Lumix GH3, 12-35mm f2.8 lens at 35mm and f11, tripod.