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Analyzing Swing and Tilt of Sony A7R IV with Two Samples of Sony FE 12-24mm f/2.8 GM

re: Sony A7R IV in for Repair of Lens Mount / Sensor Parallelism Problem
re: Lens Mount / Sensor Parallelism Afflicts All Brands

My gratitude to B&H Photo for getting me a brand-new Sony A7R IV while mine is in for repair . The A7R IV is currently the best mirrorless deal on the market at $2998, IMO. Please give B&H Photo your business using any link or add on this site, or things like wishlists.

Sony A7R IV

I’ve had a chance to shoot two samples of the Sony FE 12-24mm f/2.8 GM on the brand-new loaner Sony A7R IV from B&H. The loaner shows similar results to my camera sent in for repair for two samples of the 12-24/2.8 GM and also the Sigma 14-24mm f/2.8 DG DN Art.

If there is interest, I might publish the full f/2.8 - f/8 aperture series for both samples.

While the error in alignment is very small (at or below what is likely buildable), it is troublesome in the 12-18mm range in particular.

The root cause is surely some sensor swing, and some lens variance.

Terminology

Sample1 = Sony FE 12-24/2.8 GM used for initial work
Sample2 = Sony FE 12-24/2.8 GM received later
My A7R IV = the Sony A7R IV I purchased early late last year, whose sensor and shutter modules were replaced in late Feb 2020
Loaner A7R IV= Sony A7R IV, brand-new

Contradictory sharpness between lenses

When there is swing and/or tilt in a camera system, confounding results can be found.

My series show that Sample1 is sharper than Sample 2 both near and far*, because the system (camera + lens) is tilted slightly to favor both foreground and distance, similar to a view camera or tilt/shift lens with a mild tilt adjustment to improve foreground to background sharpness.

Similarly, the amount of swing (left/right) favors Sample1 overall.

* If the system were perfect, this would be impossible; a small unavoidable focus difference when comparing lenses should cause one lens to win-out over the other either near or far. But this is NOT seen. What it means is that the system is swung and tilted, which alters the zone of focus.

Focus Distance vs microns at sensor (imaging plane)

At 12mm, the requirements for lens mount / sensor parallelism are absurdly tight, on the order of 10 microns (1/100 mm). Cameras just cannot be built that “tightly”.

Continues below...

Blue lines are lens Sample1, Yellow Lines are Sample2
Thanks to Samuel Chia for anazlying annotating this image.

f2.8 @ 1/1600 sec, ISO 100; 2020-10-17 15:13:03
Sony A7R IV + Sony FE 12-24mm f/2.8 GM @ 13mm RAW: Enhance Details, LACA corrected

[low-res image for bot]

Samuel Chia’s analysis

Thanks to Joseph Holmes (www.josephholmes.com) and Samuel Chia (cacaoeditions.com) for background information on their knowledge about swing and tilt in cameras and lenses through years of research.

Samuel Chia has been cross-checking my analysis, and in a more rigorously numerical way too. Joseph Holmes has also been very helpful. I made my conclusions, then emailed him the images

Analysis near the edges is made difficult by optical decentering issues (both samples show signs of it), and generally inferior performance overlaid with aberrations including astigmatism. It is also generally impossible to match focus exactly; certainly AF is incapable of doing so (recently I've seen pairs of lens samples repeatedly focus differently in 100% magnified Live View). And manual focus I can’t do better than AF on a 60MP camera. However, slight focus differences are accounted for by assessing the zone of focus shape, not its absolute placement.

Sample1: swung left to distance, tilted at top to distance
Sample2: swung left to distance, tilted at top to foreground
This matches my morning shoot where I saw the same lens (first sample) sharper at both top and bottom in spite of being focused further back—the tilt accounts for that.

What Samuel is saying is that a mere 6 microns of swing accounts for a focus of 15 feet on the left side versus 12.5 feet on the right side—easily visible. Although that looks bad to me at full-res, it is about as good as it gets.

Excellent setup!

Yes, I'm seeing the same too. I looked very carefully for the center plane of focus, to help quantify the amount of swing and tilt of each, relative to the camera's position, taking the x point as 10.5 feet.

1st sample, swing: 15' on left, 11.5' on right. This is 10 microns closer on the right image edge. Looking again, I may have drawn the far limit on the right edge too near. It should be about 15' on left, 12.5' on the right. This would be 6 microns worth instead.
1st sample tilt: 21.5' on the left, 15' on the right. This is 9 microns closer on the bottom image edge (when re-orientating the camera to landscape position).

2nd sample, swing: 27' on left, 13' on right. This is 18 microns closer on the right image edge.
2nd sample, tilt: 14' feet on left, 17' on right. This is 6 microns closer on the top image edge (when re-orientating the camera to landscape position).

I've layered your files and marked out the plane of focus, which shows field curvature too, as well as the near-far limits of the DoF at f/2.8 for the vertical as well as horizontal frames. You might find them to be a useful reference (~2.5GB file size for both)

Notice that the DoF is about twice as deep on the right image edge for Sample1 vs Sample2, because of the tilt of Sample2. Strangely I don't see a corresponding phenomenon for the DoF in the vertical frames. I need to look at them again. Also, my markings are not 100% precise, don't worry too much about that since the direction of the tilt and swing are in agreement with yours, and if I go over them again I would re-draw the lines slightly differently each time. More time looking will allow a more precise determination. This is part of the reason why it takes so long to get the correct judgment for shimming work. This morning I discovered that my Sigma 35mm shimming adjustment is a little off still. And a few days ago I fixed my V65 yet again. Very irritating.

If we do not consider bayonet machining differences, the LoanerA7R IV is probably a bit worse, a little more swung (~2-6 microns worth) nearer to the right image side than the A7R IV you sent in for repair. But only just. It would be more precise to say the two cameras are virtually alike in their swing positioning, assuming you used lens sample #1 on the repaired camera. [diglloyd: correct on Sample1]

The errors are actually pretty small, all things considered! It's only this noticeable in part because the DoF at the edges are so thin at f/2.8, there's rearward bending of field curvature at the edges and the GM 12-24/2.8 is so sharp, the transition from in-focus to out-of-focus is so obvious, coupled with a high resolution sensor, tiny differences are highly magnified. [diglloyd: but 24MP or 42MP would still make it pretty obvious in net effect]

I also spent this morning looking at many 40+MP full resolution samples of Leica SL 50mm and 35mm F2 APO lenses. I found serious swing and tilt issues exceeding what you are seeing here. In $5000 lenses! I also noted many people have issues with the manual focus rings of these lenses being too tight, needing factory re-adjustment, and way too often the repair doesn't fix the problem. Occasionally it does. And that the fly-by-wire feel of these SL lenses are awful (no personal experience). How did you find the by-wire focus feeling of these lenses compared to say the V50? The SL 35, when it's in focus, can be fairly sharp right to the corners, although it has big problems with keeping PSFs round wide open, affecting a wide region off-axis. I sure wish Cosina Voigtlander would make their version of this lens, an APO-Lanthar with manual focus only. It would be better than Leica's and smaller and lighter and cheaper. I hear Leica is making 28, 24 and 21mm versions too, so it seems that wide focal length lenses can also enjoy the advantages of this superior optical design in a relatively small package. Presumably, Cosina could make them too if they wanted. I dearly wish!

DIGLLOYD: the exact details don’t matter much since all estimates are subject to eyeballing how things fall out. Bottom line is that Sample1 is a solid match for the Loaner A7R IV and Sample2 is very good also.

Samuel Chia’s estimate is ~6 microns swing and 8 microns tilt. Both of these guys feel that this is impressively good for an out-of-the-box camera. Their past efforts to get within 10 microns with shimming should be context enough.

As it turns out, Joseph Holmes characterizes this situation for Sample1 as “... 7.3 microns swing. LOL—in any case this system is CLOSE to swing free.  Squeezing this much total information from a sensor so tiny is just insane”.

Reader Comments

Paul B writes:

I want to let you know that I have really enjoyed reading the posts about your Sony A7R IV and swing and tilt. I particularly enjoyed reading that the are two other people out there trying to push the limits of what is possible. Twenty years ago I was doing similar things with my view cameras trying to eke out the ultimate image performance. So I remember the pain of trying to figure this sort of thing out.

I have a few comments related to these posts that I hope are helpful.

First, for context, over 30 years ago I worked as a Mechanical Engineer in test, evaluation, and failure analysis. During this time frame our internal machining facilities were Aerospace Certified and could hold tolerances to +/- 0.0005 inch (12.7 microns), but for normal “high” precision work tolerances of +/- 0.005 inch (127 microns) were used; required tolerances depend on the application. Considering that machining technology has advanced considerably and we are discussing a mass produced consumer item, tolerances for sensor position of +/- 0.001 inch (25.4 microns) is probably reasonable. These and tighter tolerances are certainly possible, but the time and cost to do so goes up considerably with each increment.

When I read the conclusions about the amount of swing and tilt, and looked at the provided image with the blue and yellow lines, things did not add up in my mind. The estimated amounts of swing and tilt, even though they are additive, don’t seem to be enough for the result shown. Because of this I duplicated your table, but things still did not add up for me.

Looking at the results of unintended tilt or swing in an image, the effect should be centered around the focus point. A swing located at the image center that is forward on one side should have an equal amount back on the opposite side; with a proportional effect as the swing point moves off center. From your image the focus points appear to be point #15 on the tape measure for blue and point #12 for yellow. This centering would occur at the focus point even if the swing or tilt were applied at the sensor edge. While your blue line is close, the yellow line is not, so something else must be happening.

My first thought was, that in addition to tilt & swing, there may be some shift involved. Or the axis of the lens may be offset from the center of the sensor. After further thought, I realized that I have seen similar images from my Linhof Super Technika IV camera. In this case, due to an extreme drop, the revolving back was bent, and the springs in the ground glass holder would bend a plastic film holder and the film inside.

Looking at the solid blue and yellow curves gives us more information. If we look at the right side of the image it appears that both curves will overlap. This is what we expect to see. In fact, the more the focus point is moved to the right edge the more sense the swing/tilt explanation makes. It is when we move to the left side of the image that things don’t add up. As both curves seem to become steeper as you move to the left. The blue curve beginning very close to the tape measure and the yellow very close to the right hand “)” in the 10.5 ft comment. The fact that the yellow curve significantly deviates from the blue curve shows that the yellow lens has its own problems.

Without being able to see A&B images, inverted from each other, such as the Mirex adapter allows, we can’t say anything about any deviations due to the alignment of the sensor and the lens mount.

Though, the above leads me to the conclusion that this camera’s sensor, or a sensor stack component, may be slightly bent (or curved) towards the left side of the image, in addition to the suspected swing and tilt.

Finally, the fact that Sony A7IV and Sony 12-24mm lens combination allows us to read a tape measure to within 1/4 inch at 10.5 ft is very impressive. The fact that this also lets us see how well the manufacturing process works is awe inspiring.

Though this brings up serious questions for photography and the the photo industry:

Does the current IBIS system position the sensor to account for variances in manufacturing tolerances? If not, can a future IBIS system do so?

If an IBIS system can’t account for tolerance variations, can the process be improved to correct this situation and keep camera prices affordable? If not, it might be possible that the precision needed for 100+ MP will make Leica prices seem reasonable.

PS. You may want to look at Novoflex bellows options. One of these may provide the flexibility to rotate a camera or lens independently of the other.

DIGLLOYD: there is definitely more to it than sensor swing and tilt. Lenses themselves are far from perfect. It’s also true that that without the proper gear that it can be difficult to do more than deduce a likely camera swing/tilt.

The imaging system of lens + camera totals its errors—errors can cancel out to varying degrees, or can be amplified (additive).

When a group of lenses all establish the same general pattern of swing and tilt, it is fair to conclude that the camera is involved, especially if no other exceptions have been seen with other lenses (indeed, this is the case with my A7R IV which I sent for repair). With this brand-new loaner camera and the one I sent in for repair, 5 different lenses all are swung to the distance on the left side. That might argue it is all the lenses, except *no* lens has ever shown swing to the other right on my camera (the one in for repair).

There is no perfect lens. All lenses have their own swing and tilt. Furthermore, for one lens to be sharper in both the foreground and background (as in this scene) is proof enough that one of them is tilted (probably both, but in different directions). Also, both lenses look to be optically decentered slightly, even Sample1, which is the better of the two (it has trouble on the left side, as seen in House Under Construction ).

On this scene, the Sigma 14-24mm f/2.8 DG DN Art shows a similar swing to the distance at left, but it also looks to be tilted more than the two Sony lenses.

Furthermore, zoom lenses are notorious for failing to place the center of the image circle at the center of the frame. This is easily seen by switching lenses; the center of the image can be displaced considerably between two lenses, showing that one (or both) of them is nowhere near centered about the frame. See for example the awful quality control of the Nikon NIKKOR Z lenses in Shootout vs Nikon Z 14-30mm f/4S: Bonsai Above Beaver Pond in which there is a displacement of 28 pixels horizontally and 54 pixels vertically! This drove me nuts when comparing lenses. The two Sony zooms here have no significant displacement, but I have seen Sony zooms with displacement issues.

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