You've probably figured out I'm a big fan of one of Fuji's first lenses in the X system line-up. The now mostly ignored 18mm f/2 R. I've written more than a few times that in my opinion it doesn't deserve it's rumored reputation as the one bad lens Fuji makes. In my view it's fantastic. It still confounds me as to how the collective internet comes to the conclusion it's somehow not up to snuff.
While looking through a few pictures for the last post on the XPro-2 I made with Alice I noticed most of them were made with the 18mm f/2 R at large-ish apertures like f/4 and f/2.8. That in turn reminded me that the XPro-2 has a new setting that you may want to check out for yourself if just for education. The setting I'm referring to is the choice you now have for display of depth of field on the distance scale when displayed. They are film format and pixel basis. I kinda remember that the previous X cameras might have been pixel basis, if so that will do for the purposes of this discussion.
You'll notice when set to pixel basis you may be shocked as to how much depth of field you don't have. Practically none at even remotely short camera to subject distances like four to six feet. That's kind of where I live. Heck even wide-ish lenses like 18mm show depth of field that's not even worth considering as depth of field. This is something I've known since I was pretty much a kid way before digital, or pixels. How? Well I printed my own pictures so I was constantly using a grain microscope to focus negatives. When viewed with that kind of magnification you can clearly see there's really no such thing as "depth of field" when it comes to the old-school method of pixel peeping or the new-school 100% view method. There's really only a shallow in-focus and varying degrees of out-of-focus.
The only question is will anyone notice it at reproduction sizes/magnifications that you'll be using it for? That's where the "film format" comes into play. It's more in line with Leica's tighter DOF scales on their lenses. Tighter in the sense that they printed DOF scales that assumed larger magnification than most other manufacturers. Still nothing as tight as "pixel basis".
For better or worse since that early "pixel peeping" I did way back I've treated my use of cameras and lenses as having no practical depth of field. Well at least I did in terms of making decisions about what particular point I was choosing to allocate absolute perfect focus. Sort of like one does when using a long lens wide-open. Sure I realized there was practical depth considerations but always treated that selection of exact point with more than healthy respect.
Take a look at the picture above. Shot at f/4 about five or six feet away at best. Here I chose the close side of Alicia's face/glasses as the focus point. I also was shooting up to a large degree meaning that the focus plane was nowhere near parallel with Alicia. Unlike a large format view camera (or a digital with T/S lens) you can't manipulate your focus plane other than tilting your camera. Take a look at what that means below.
What are we looking at? Well just assume I was telling the truth about the face/glasses being in perfect focus. Take a look over on the right side of the frame behind Alicia, the rail and better yet the edge of the brick wall. See how in the lower parts of the frame the bricks are in pretty decent focus, close to perfect? Take a look at what's going on has you look higher in the frame. Wow, that's pretty soft huh? To state the obvious, tilting the camera up pushes that plane of focus at the bottom back behind "the subject". Imagine how out-of-focus the bottom of "the subject" is?
Here's another but choosing the mid-part of Alicia as the focus point.
Now take a look at what's going on with the background vs. a large portion of the dress with about the same tilt.
Ask yourself a few questions. What's more important in this particular composition to have the most detail? To be the sharpest? The answer depends on many things including what the picture is for and where you'll display it. Alicia's face is not horribly out of focus now and since this is more of a fashion kind of picture I would say the old rule about always choosing the face as point of focus is probably not the best way to go.
Internalizing how cameras actually see it pretty important and takes a bit of time. Probably more important that ultimate lens "quality" to be honest. Many photographers don't really think this through much and let the mind's eye take over sorta like… "Hmmm, I'm shooting wide, f/4 or f/5.6 or f/8 should be plenty of "depth of field" to get a full-length person in. Heck that's less than one foot, I'll just focus on the eye, or maybe anywhere on that person." Wrong. Now think about this in a landscape context with a close foreground element vs the background.
The Price Of Tea In China
What's this have to do with the 18mm and image quality, etc? Actually everything. For the most part even a tiny tiny tiny bit of out-of-focus-ness at most apertures will completely kill the resolving power of the best optic ever made. Within reason any old lens will slaughter that "best lens" if it happens to be in perfect focus at the same point where the "best one" is even a tiny bit off.
Combine the above with the real world and you'll see why moronically small resolving power differences between lenses in the absolute far reaches of the corner (where there are no focus points) are about the last thing I care about when evaluating lens quality and look. As mentioned before, unless you shoot flat things exactly parallel or relatively flat things very far away it's moot. It's far better to take into account pictures that you make and what characteristics will actually are important than stuff like the very very very outer corner resolution wide-open. Unless there are flaws that are horribly visible and distracting (there' aren't on the 18mm f/2 R) they won't enter into the equation at all. Other things like how the lens renders overall and what do out of focus area's actually look like are far more important. Oh, by the way the center and how big that center is are crazy important IQ wise.
All pictures made with the Fuji XPro-2 and 18mm f/2 R. Processing via Lightroom CC with some homemade preset plastered on at import with a bit of fake grain.