Mirrorless AF Calibration – Part 6: The Evidence

This is the culmination of a series of posts about how the whole autofocus system works and what issues might still lead to a requirement to calibrate lenses even though the autofocus measurements are now obtained from the same sensor as the image is captured from.

I’ll link back to the previous posts at various places – there’s a lot of information in them.

First, we’ll take a look at some empirical evidence gathered from FoCal user results uploads which show the sort of focus adjustment required for cameras. I’ve used information from the Camera Database at FoCal IQ (April 2023 data) for this information. 

About this Series

This post is part of a series looking at on-sensor phase-detect autofocus, the whole autofocus system, and why mirrorless autofocus might still need some calibration.

The full list of posts are:

• Part 1: You SHOULD Calibrate your Mirrorless Autofocus!
• Part 2: On-Sensor Phase Detect Autofocus
• Part 3: On-Sensor Phase-Detect Issues
• Part 4: The Lens
• Part 5: The Autofocus Processor
• Part 6: The Evidence

DSLRs

Although this post is about mirrorless calibration, I wanted to briefly touch on DSLRs, with their old-school technology of an SLR camera, using a separate phase-detect autofocus sensor. 

The chart below shows the percentage of all Nikon DSLR cameras and lenses tested with FoCal that require certain AF Fine-tune values (ignoring direction), across all tested lenses:

So, we can say that around 4% of all DSLRs don’t need any focus adjustment. Or to put it another way, over 96% of DSLRs could benefit from autofocus calibration!

Here’s that latter point made a little clearer with a chart. It shows the percentage of cameras that require at least the value on the horizontal axis:

We can again see that around 97% of DSLRs require an adjustment of at least 1 unit. Moving along the chart, around 50% of all DSLRs would benefit from an adjustment of up to 7 units! At that sort of adjustment, that’s a significant visual change improvement and just shows that it really is pretty much essential to calibrate your DSLR to get the best from your lenses.

A word about the results

We process the data to remove erroneous results due to testing errors, but this data includes all lenses – from the cheapest to the most expensive – that have been tested with FoCal on all Nikon DSLRs, from 2007 through to the latest.

Lower-end lenses and DSLRs will generally – but not always – require more adjustment, but this data does give a fairly representative view of the calibration requirement for DSLRs.

Mirrorless

The story is a bit different for mirrorless cameras, and here I’m specifically talking about the Nikon Z-series mirrorless.

Here’s the chart showing the percentage of mirrorless cameras requiring each adjustment value:

So, for DSLRs, almost 97% of cameras required adjustment. For mirrorless, it’s much lower – it certainly looks like getting rid of the separate AF sensor was a good idea, at least from a focus accuracy point of view!

Although lower, there is still some obvious need for calibration, and there’s variation between native Z-mount lenses and adapted (via the FTZ adapter) F-mount lenses. 

For native lenses, a little under 70% of all lenses could benefit from calibration, and for adapted lenses, this rises to greater than 80%.

Let’s look again at the percentage of cameras that need adjustment:

For native (Z-mount) lenses, around two-thirds of all lenses could benefit from a tiny adjustment, but realistically an adjustment of 1 unit is not going to make a significant visual difference, although it will slightly increase your likelihood of getting a perfectly focused shot.

Around one-quarter of all F-mount lenses on Z-cameras fall into the “visually significant” category of greater than 2 units of adjustment.

It’s worth thinking about the lenses with a larger requirement too. Let’s take the AF Fine tune of 4 units, which would definitely produce a visible difference to image sharpness. 10% of tested lenses require at least that adjustment. That might not sound much, but if Nikon sold 1,000 of a specific type of F-mount lens, 100 of them could benefit from a significant adjustment. What about the one on your shelf?

Specific Examples

I adjusted the data to just show the calibration required for Z-mount Nikon cameras with both Z-mount and F-mount lenses.

Here are the AF Fine-tune values required for the popular F-mount 105mm f/2.8 macro lens across all tested Z-mount cameras:

Over 75% of all lenses require an adjustment of 2 or more AF Fine-tune units, and around 20% (1 in 5) require an adjustment of 4 units. In this instance, an uncalibrated lens would be obviously, noticeably softer than a calibrated equivalent.

And for the 50mm f/1.8:

Again, we’re looking at over two-thirds of all lenses needing a visually significant adjustment.

One of our commercial partners, CameraCal, with whom we work closely has reported specific examples of F-mount lenses on Nikon Z cameras requiring greater than 10 units of adjustment, and even native Z-mount lenses requiring 6 units.

At the very least, it’s worth checking the calibration on your mirrorless camera!

But, why?!

If you’ve come to the site through this post, it’s worth heading back to the first post in the series – there, you can dig into:

• how the AF Sensor works,
• the sort of problems that might arise due to manufacturing issues on the sensor.
• how the lens optics, mechanics and calibration data can contribute to errors, and
• how the Autofocus Processor implementation can affect the focus results.

In Conclusion

I think there’s a pretty compelling set of reasons to carry on checking the calibration of your Z-series mirrorless cameras if you want the very best performance from them, doubly so if you’re using a mount adapter with F-mount lenses.

FoCal offers both Desktop and Mobile software to make this easy, and if you use FoCal Pro you can find out a whole lot more information about the performance of your gear from repeatability to focus-shift to vibration-reduction system performance.

And I’ll leave you with a comment from a freelance sports photographer on a Z9 Facebook group:

“Just a heads up…. Just because the Z9 is a mirror-less body doesn’t mean you don’t have to AF Fine Tune your lenses, especially your adapted f mount lenses.  I shot a dock diving event last weekend and over half my shots were back focused with the 400mm f2.8. and the 70-200mm fl was hit and miss … I just fine tuned three of my most used lenses, and two of the three had to be adjusted.  The 400mm was -3, the 70-200mm fl was -2 for tele and +1 on the wide end, and the 24-70mm 2.8 vr was spot on at both ends of it’s range”