Originally posted by rfkiii It is quite possible for a lens to be sharp across the frame wide open, even at f1.4. The Zeiss Otus series proves this and is famous for this. I was first introduced to wide open "landscape shooting" relatively recently (where "landscape shooting" presumes not only good sharpness from front to back but good sharpness across the frame into the corners and edges, the latter being a function of how well lens aberrations are handled by the designers) with the Canon 24-70 II which can be used at 24mm/ f2.8 (wide open) and produces acceptably sharp corners/edges. This lens was a huge eye-opener. If one's composition is at infinity distance across the frame (nature's flat wall), no DoF essentially, then such lenses can render such compositions sharp (in focus) across the frame. When objects are placed closer than infinity to the shooting position, then DoF comes into play and one must stop down to render this object "in focus". Obviously, how much one has to stop down is determined by how close that object(s) are placed to the shooting position (assuming one wants good front to back sharpness). What one does not want to do, if possible, is to stop down just to fix lens aberrations as in the case of compositions featuring infinity across the frame. Nothing about lens design demands that a lens' "sharpness" (in focus) must drop off in the corners at wider or wide open pertures other than meeting a price point and/or customer satisfaction.
finally, someone gets it.
however, given that i already posted an f/2 example photo that had practically no field curvature characteristic, other than defects, it really shouldn't have been such a difficult concept for other people to understand
Originally posted by rfkiii Amazingly, I have stopped down to f32 on the 645z without the visible softening associated with f16 on a FF system (impressionistic not scientifically derived)..
that's because, in part, the lower pixel density of the 645z makes diffraction less visible, but afaik, it doesn't change the amount of diffraction there is... now ask yourself what it'll look like with a 100mp mf camera.
---------- Post added 04-08-16 at 11:30 AM ----------
Originally posted by rfkiii I wasn't suggesting a direct causal effect but there is an indirect effect. Stopping down has pros and cons. The pro for landscape being that stopping down brings objects into focus (including edges and corners rendered out of focus by lens aberrations) but stopping down can also bring on diffraction. If one's composition has no close objects and the only reason to stop down is to fix out of focus corners and edges, then a better designed (and more expensive ) lens would eliminate this need and as an extra benefit, avoid possible diffraction issues.
that's it exactly.
---------- Post added 04-08-16 at 11:41 AM ----------
Originally posted by 2351HD There is a big difference as to what a DOF calculator says will be acceptably sharp and what actually "IS" acceptably sharp in the real world. These calculators are well off in my experience.
i agree, yes they are off, the entire concept of dof is based on what is an acceptable amount of visual blur near the area of critical focus.
but the general differences will be accurate, be it two feet or three feet in the 28mm example... if someone is blindly stopping down to f/11, for no reason, it's a waste of expensive camera gear.