Auto-Focus Direction Sensitivity Error Analysis

Camera lenses don’t all focus the same way. Manufacturers make many trade-offs when designing a focus system, including speed, accuracy, and cost.

I’m going to show a couple of different lens investigations here. One lens focuses fast, but has some focus accuracy and directional problems. Another lens is slow, but much more accurate and repeatable focus.

The ‘fast-focus’ lens uses a linear stepping motor (the Meike STM motor). The ‘slow-focus’ lens uses a Nikon SWM (silent wave motor) that rotates.

A ‘focus position’ chart analysis result, Meike 85mm f/1.8 lens

The screen shot above shows a close-up of a ‘focus position’ chart after analysis using the MTFMapper program. The lens was focused on the chart center (the chart was rotated 45 degrees relative to the camera sensor), indicated by the set of orange arrows. The actual sharpest focus found by the software is indicated by a vertical blue line (-7.5mm here). The MTFMapper program was configured to only use the green sensor pixels here for its analysis, since each sensor pixel color can have the focus at a different distance if the lens has any longitudinal chromatic aberration.

The lens focused at a distance of “-7.5mm” beyond the chart center, although the camera focus point was targeting the chart center. Most cameras allow the user to “fine-tune” the focus and force the focus to be closer or further from the camera. Sometimes that can work for a lens that consistently misses focus, but as you’ll see, that doesn’t always fix the problem.

A spreadsheet showing focus position measurements

The spreadsheet shown above contains the results of analyzing two different 85mm lenses. The left-hand portion of the spreadsheet shows the results from the Meike 85mm f/1.8 Z-mount lens. The top group of numbers was obtained when the lens was manually set to infinity focus before initiating auto-focus on the chart center. The bottom group of numbers resulted from manually setting the lens at minimum focus prior to auto-focusing the lens.

The top group averaged an error of about -6mm (focused too far from the chart center). The bottom group averaged an error of about +6.5mm (focused too near from the chart center).

This kind of lens defect causes the lens to stop focus before it gets to the target. The lens considers the focus to be “close enough”, and stops.

No focus fine-tune setting can fully correct for this type of problem. The best you can do is to configure the lens to center the two groups of focus problems.

The right-hand portion of the spreadsheet shows the measurement results from a Nikkor 85mm f/1.4 AF-S G lens, set to the very same f/1.8 aperture. Nikon chose to design this lens to auto-focus at a fairly slow pace, preferring focus accuracy over raw speed.

A ‘focus position’ chart result, Nikkor 85mm f/1.4 lens at f/1.8

Just like the Meike 85mm, the lens had the focus manually set to infinity-focus in the top group of measurements, and then minimum-focus in the bottom group. Notice that this lens also fairly consistently stops focusing prior to reaching the target. It just gets a lot closer to the target before it considers that it’s “close enough”.

The Nikkor 85mm results indicate that a slight auto-focus fine-tune adjustment might be useful, to push the focus slightly further from the camera sensor and balance the near and far error groups.

Focus error for Meike 85mm

Focus error for Nikkor 85mm

The error plots shown above give a better visualization of how the focus distance error is grouped according to which direction the lens had to move to focus on the target.

So, what can be done to ‘fix’ the focus problem here? First of all, note in the shot at the top of this article that the chart center still looks pretty sharp, even though it isn’t in perfect focus.

The first step to fix focus is to use focus fine-tune on the lens to at least balance the pair of missed-focus groups to be equally wrong on either side of the correct focus distance.

The second step would unfortunately be to override auto-focus and manually adjust focus to get it perfect. Unless you use a tripod, this is easier said than done. My Nikon Z8 and Z9 cameras let me zoom in using the viewfinder to more accurately see how to manually adjust the focus. I also use focus-peaking indicators, although this feature is generally too coarse to perfectly focus the lens.

A third option is to stop-down the aperture, to hide the focus error inside deeper focus. Not a great option.

A fourth option would be if Meike were to make a lens firmware update that moves focus a little further before stopping, but this is probably wishful thinking.