I was reading about DOF the last few days and I came across another term that I had not seen before: DLA or Diffraction Limited Aperture.
That triggered a few discussions around me and since it seemed to be a unfamiliar subject I decided to see it for myself. Here are a few pictures of my arm band on top of grass – may not be the best subject, but the grass itself did not have enough details to illustrate the idea. The arm band has those nice little threads coming out of it, which I wanted to observe but are not quite visible on the picute, but its corner provided another details to illustrate the topic.
The experiment looks like this. The camera is on a tripod, 55-250 lens zoomed to 131 mm and auto-focused in the middle of the arm band, after that both the Auto Focus and the Image Stabilization turned off. The set of pictures are taken in Aperture Priority mode with the aperture gradually increase by one stop starting from F/5.6 and going to F/22. The shutter is on 2 seconds delay to deal with the vibrations, no mirror lock engaged.
There are 2 things clearly visible:
- Depth Of Field increases (as seen on the long crops)
- Visual sharpness loss in the middle of the DOF after F/11 (as seen on the small crops)
And here are the actual crops. They are all 100% crops, all images shot in RAW, unsharp mask 3-7-4 and additional 100 points of sharpening on the JPEG, both in DPP.
F5/6
F/8.0
F/11
F/16
F/22
And here are the bigger crops:
What is the moral of the story? If you need bigger DOF you can close the aperture beyond F/11, but you will loose sharpness in by doing it. For more details read the article I linked at the beginning.
