The issue of lens mount / sensor parallelism can be separated into swing and tilt:
swing = deviation left/right of image plane versus lens mount
tilt = deviation up/down of image plane versus lens mount
1 micron = 1/1000 of a millimeter
Camera aside, a lens can also have swing and tilt relative to its lens flange (and also have optical decentering, further muddying the waters).
Distinguishing swing and tilt of the camera from that of the lens is difficult without a lens adapter that enables rotating the camera 180° relative to the lens. Ideally one would have a reference camera known to be ±5 microns or some such, but I know of no way to be so-blessed, and it’s nowhere close to realistic given manufacturing challenges.
A tilt that causes the top of the frame to focus farther away and the bottom to focus closer is advantagous for many shooting situations (camera in horizontal orientation). But a tilt that causes the top of the frame to focus closer than the bottom is a sharpness robbing disaster in with the lens working against against the photographer.
Standard view camera operation for landscape work involves the Scheimpflug principle: nearly always involved intentionally throwing the lensboard (front standard) and the film plane (rear standard) out of parallel by a small amount (e.g. 1 to 15mm sort of thing with a 4 x 5 camera) in order to cause the plane of focus to not be perpendicular to the optical axis. This capability helps to overcome the extremely low depth of field of large format cameras. This is especially effective when the subject is flat, e.g. flat ground, and it’s at widely differing distances from the camera in different parts of the scene.
The three planes, those of the focus, the front standard and the rear standard, always converge in a line (except when there is precisely zero swing or tilt i.e. the lens mount and the film or sensor are perfectly parallel. Typically this means using back tilt or sometimes front tilt to get land in front of the view camera into focus. Small adjustments of the camera cause large effects on the orientation of the plane of focus. Instead of objects nearer or further away than the plane of focus being progressively more out of focus, objects above or below a strongly-tilted plane of focus are progressively more out of focus. So you can stand on a floor with the camera on a tripod, pointing down at say a 30 degree angle from the horizontal and focus on the floor perfectly at a big aperture with a lens with very little depth of field, such as a normal lens on a 4 x 5 camera (150mm focal length). The entire floor area shown in the picture could be in focus. But anything above or below the floor will be increasingly out of focus. To focus on a ceiling you’d adjust the camera’s tilt the opposite way. To focus on a wall that’s not perpendicular to the optical axis, you would use a swing adjustment of either the front or rear standard, where one or the other moves rather like a door being opened.
With today’s miniaturized, high-resolution cameras, incredibly tiny misalignments of the lens and the sensor can have visible effects on the image. A typical sheet of office paper is 100 microns thick (1/10th of a mm), but we can sometimes see swung or tilted focus planes in an image when the camera and lens are misaligned by as little as 10 microns. If we’re working at medium or small apertures, rather than very large ones, tolerable misalignment is apt to be up to roughly 30 microns or higher, for example. None of this matters much if getting an entire scene into focus isn’t your bag. In such cases it would be unusual to find manufacturer tolerances for swing and tilt in either a camera or a lens insufficiently fine. — Joseph Holmes
The ideal scenario is a camera with zero swing and zero tilt, a near impossibility with current manufacturing techniques (meaning in terms of quality control cost). It would require not only tight machining tolerances but extremely fine tolerances for assembling everything, lest errors add up. And then some means of ensuring the sensor was mounted for no deviation from the plane of the lens mount. All of that is surely prohibitively expensive.
The camera and lens add their errors together. If a camera has a swing of +20 microns and a lens a swing of -20 microns, they net out at zero—perfect. But +20 and +20 nets out at +40—not nice at all. Thus a particular lens and camera can work well together, or 'fight' each other.
* Indeed, of the two Sony FE 12-24mm f/2.8 GM lenses I tested, Sample1 has an advantageous tilt that yield superior foreground *and* background sharpness relative to Sample2. It is not necessarily sharper, but its tilt makes it effectively sharper for many scenes that lie in a relatively constrained near-to-far planar shape.
Expected errors for swing and tilt
For production cameras, tolerances in the ±20 micron range for cameras is a reasonable guesstimate, but it could be better or worse—little data exists. Joseph Holmes and Samuel Chia state that ±10 microns is excellent (the goal of their shimming efforts) and 5 microns would be fantastic, but that is likely to require meticulous shimming with most cameras (tedious and time-consuming).
What can we expect for left/right symmetry?
The three A7R II cameras that I measured were tilted and/or swung by over 45 microns in one case, and about 30 microns in the other two cases. I would say that it’s unlikely that many more than half of the Sony Alphas are accurate to better than 20 microns, but I really don’t know. One would simply have to measure a whole lot of cameras, and each one is hard to measure.
This is the most important single finding of all my work with this and I’ve yet to publish it... so as to potentially egg Sony on to figure out how to cut their typical alignment error by ⅔ or so... assuming the small sample I was able to see, three cameras, all from very different parts of the production run, plus Samuel’s A7r II, which was also off by maybe 25 to 30 micron...
For giggles, let’s stipulate an unrealistic figure of exceptionally tight build tolerances ±10 microns of swing and and ±10 microns of tilt (1/100 millimeter). The reality is probably twice that, or more.
A camera with ±10 micron lens mount / sensor parallelism should be considered outstanding, yet at 12mm even a ±10 micron deviation shows an easily-seen asymmetry. The table below makes this plain.
For example, with a 12mm lens focused at 9.8 feet, a ±10 micron swing will push focus to either ~8 feet or ~12 feet on the opposite side. At 15 feet, a ±10 micron swing will push focus to about ~11 feet or ~23 feet on the opposite side. That’s the kind of error is just what I am seeing on the Sony A7R IV loaner camera. Still, it speaks to very tight tolerances from Sony, based on the numbers—and I cannot attribute it all to the camera versus the lens.
Stopping down masks errors to some extent, but my experience says it is not a cure, especially on distance scenes with fine detail across the frame.
It is not just left/right asymmetry but also impaired depth of field on one side versus the other. Both are at work in Big Tree at Creek Bend. On top of all that are all the various optical aberrations. But by all numbers, it is as good as we might hope for.
Table based on formula supplied by Joseph Holmes. Gray areas show focus extensions varying less than ~15 microns with each 25% increase in focus distance.