according to tests on dxo analyze 5D III:
In 16 mnm:
the sharpness is excellent from 2.8 in the center and third zone and it becomes very good at F / 4 and very good from F / 5.6 in the corners.
dive down in the angles at F / 16, it is very good for the angles of F / 5.6 to F / 11.
It is advisable to target a little wider because of barrel distortion which is quite present and will be easily corrected in post production with DXo8 or LR4 (see LR5 soon), otherwise the corners will be cut at the correction.
vignetting (darkened edges) is very present at F / 2.8 and still visible at F / 11 no worries in Post production DXo8 or LR4 are miracles.
20 minutes:
Vignette amounted to F / 2.8 and traces F / 8.
Moderate barrel distortion.
the picket F / 2.8 is very good in the center but the edges and third means, it becomes good at F / 4 and excel from F / 5.6 to F / 11 is excellent everywhere so perfect in landscape and loses a F / 16 because of diffraction.
A 24mn:
just the same finding a F / 2.8, it becomes a good F / 4 and excel anywhere from F / 5.6 being the top F / 8 slight degradation to F / 11 and more has F / 16 due to diffraction .
So use at the top of F / 8 F / 11 for the scenery or group photo.
35 minutes:
the sharpness is always excellent at full aperture for the center, third edges and areas become a good F / 5.6 the corners are recessed.
the best yield is F / 8 F / 11 the highlight being F / 11.
So zoom on top of overall F / 8 F / 11 ideal for the landscape report can be used F / 5.6 center is always good and the edges and third zone.
However this objective requires the use of post production software for correction of vignetting and barrel distortion.
From this perspective DXo8 performs best for optical corrections, LR4 is effective but less than DXO.
must not therefore hope to have sharp angles for a top opening at F / 16,
I advise you to use the hyperfocales tables (or software android / iphone) to calculate your focus range following the aperture and focal length used to not open a F / 22 as do many photographer by saying all will net ....