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Reader Question: Testing for Focus Shift

In the Deck Planks example, I included a short explanation of why using a planar target is not the right testing protocol, reproduced towards bottom.

See Testing for Focus Shift and Secondary Color and Detecting and Mitigating Focus Shift in the How to Test A Lens section of Making Sharp Images.

Christopher C writes:

I fastened some words of the smallest print I could find to the wall, carefully focused my Sony A7R IV on a tripod at f/2, then carefully keeping the focus the same, took the same picture at f/5.6, and f/8, and magnified the three images on the back of the camera as much as I could. The lettering seemed equally sharp.

Would this show that my lens is OK?

I have recently bought the CV 35/2 APO, but I have no knowledge how test it. Is testing straightforward? I am thinking of returning it in exchange for a Sony 35 GM, even though that will cost me quite a bit. (I have two weeks to make up my mind.)

DIGLLOYD: as described above, four mistakes are being made: (1) using a planar (flat) target shot orthogonally instead of an target angled obliquely to the camera, and (2) the key apertures are usually one stop and two stops down from maximum, e.g, f/2.8 and f/4 for an f/2 lens, (3) conflating an “OK” lens with not having focus shift (most lenses have it), (4) examining on the camera (hard to evaluate near/far, depends on the low quality JPEG).

Shoot the whole series from wide open through f/8, focusing only once, wide open. Ideally, use a finely detailed ruler (eg a yardstick as I show in Rulers and Soil).

re: Zeiss LensSpire Articles especially the “Focusing” articles. See especially the Focusing a lens with focus shift section in Focusing Zeiss DSLR Lenses For Peak Performance PART TWO: Tips and Best Practices for Sharply Focused Images.

Very few lenses today avoid focus shift entirely. Some are problematic, like the Fujifilm GF 50mm f/3.5 (at close range). Furthermore, focus shift with some lenses can be in one direction in the outer zones and another in the center! Add on field curvature with some lenses and with some lenses it van be difficult to know where focus will land. So it’s not a trivial topic to master.

Sony FE 35mm f/1.4 GM might also have focus shift; I don’t know yet. I hope to have it soon.

This Voigtlander FE 35/2 APO behavior has me flummoxed—it defies my past experience with hundreds of lenses. Yet I can show the behavior at-will on different subjects. The good news is that it has nil field curvature and if you fine-tune focus, you can obtain better results than with any 35mm lens I have yet tested. When the 2nd sample arrives tomorrow, I will see if the behavior is the same.

Brief notes on evaluating focus shift

A shift in the zone of focus can have substantial influence on the visual impact of the image, not to mention whether the foreground and background are blurred or sharp as intended. For example, slightly blurred eyes and sharp ears or just sharp eyes and sharpness behind them but not the face (rearward focus shift). Or blurry distance and sharpness much closer (forward focus shift).

Focus shift is often evaluated incorrectly. It is not a check as to whether a thin slice of the subject remains in focus, since depth of field gains often are enough to compensate for the shift in part. Put another way, when the depth of field gains exceed the focus shift, there is always a slice that remains as sharp (or sharper) than wide open. So you get fooled thinking there is no focus shift. Most lenses have less shift than DoF gains, so you’d be fooled every time.

Focus shift must be evaluated by assessing the centering of the zone of sharpness. If the center of that zone of sharpness displaces itself with stopping down (allowing for a slight near/far asymmetry), then the lens has focus shift and it is only a question of degree relative to how much depth of field masks/compensates for the shift. The casual observer might complain of “frontfocus” or “backfocus”, not realizing the camera focused wide open spot-on, but the capture being stopped down will not have the zone of sharpness where expected, even if the focus point is fully sharp.

In my experience, most people won’t notice unless it is pointed out. But picky people who want the very best sharpness do notice. There is no substitute for mastering shot discipline, one factor of which is understanding lens behavior.

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