Mirrorless AF Calibration – Part 1: You SHOULD Calibrate your Mirrorless Autofocus!

“Mirrorless cameras don’t need calibrating.”

You hear that a lot, but is it true?

At Reikan, we’ve been gathering autofocus calibration results for almost 10 years now from users of our FoCal software, which includes data from users of Nikon Z-series mirrorless cameras… and it suggests that you should calibrate your mirrorless camera. 

I wanted to try and understand why there is still a need to calibrate mirrorless systems.

Overview

I’ve been working with camera autofocus systems at a pretty detailed level for over 10 years now, but when I sat down to write what was supposed to be a quick post explaining a few potential causes of needing to calibrate, I kept going into more details and the whole thing has grown a bit out of hand!

I’ve been digging around in patents, reverse-engineered lens protocols, snippets of information from camera manufacturers and obscure posts and comments in the dark and dusty corners of the web. With the help of some software I haven’t used for almost 30 years, I’ve modelled the operation of sensor components and lenses, adding intentional defects to see what effect this has on the components of the autofocus system. And I’ve written, deleted and rewritten an awful lot of words!

And so, the point of this series of posts is to show why you should calibrate your mirrorless camera. 

Obviously, I’m biased – Reikan develops software that helps with the process of calibrating cameras, so of course we’re going to show you that you need to calibrate!

With that in mind, I wanted to explain in some detail various potential causes of autofocus issues – not just some arm-waving statements, but actually delve into the why, and show that there are a lot of little things that can all combine to cause noticeably soft images which can be correct by fine tuning your system.

As Richard Feynman said: “If you want to understand something, teach it.”

So, here goes nothing!

Posts in this series

I’ve broken the information down into a bunch of posts, the order of which hopefully helps explain things to build upon. I’d suggest reading this post through to the end and then using the other posts to understand more details and see the justification for my arguments as to why you should calibrate mirrorless autofocus.

Part 1 – Introduction (this post)

Part 2 – How Phase Detect Autofocus Works

Part 3 – Issues with Autofocus Sensing

Part 4 – The Lens

Part 5 – The Autofocus Processor

Part 6 – The Evidence – results from FoCal IQ

Phase-Detect vs Contrast-Detect

First off, it’s worth understanding the difference between the two types of autofocus used in DSLR and mirrorless cameras.

Contrast-detect autofocus is what a human would do when focusing a lens: you take a look and see an out-of-focus image, turn the focus ring in a random direction and see if the image gets sharper. If so, keep going until it’s just past its best, and then return back to the best position. Contrast-detect autofocus is performed with the same sensor that you use for capturing the image.

Phase-detect autofocus works by entirely by magic… ok, not quite, but it can seem like it. By looking at light rays from two different angles, the sensors can determine both the direction and amount of defocus in one go. I’m going to explain it in simple terms with lots of pictures to help, as I wanted to see the effect for myself as well. In DSLRs, phase-detect autofocus is performed by a separate sensor from the one that captures your final image, but in mirrorless cameras that support phase-detect autofocus (pretty much all nowadays), the measurement is taken using special pixels on the image sensor.

I’m going to focus on phase-detect autofocus during this series as, while it is fast, it’s not infallible and is generally the cause of more focus issues than contrast-detect.

Mirrorless vs DSLR

The main argument for not needing to calibrate comes from the fact that mirrorless cameras don’t use a separate autofocus sensor, instead measuring autofocus information using the same sensor that captures the image.

This is fundamentally different to how autofocus works with a DSLR when you use the viewfinder.

The biggest cause of needing to calibrate autofocus comes from any physical discrepancy between the autofocus sensor and the image sensor (the bit that captures your photo), i.e. you can end up with a situation where the autofocus sensor thinks the image is in focus, but it’s actually not quite right at the image sensor.

There’s a general opinion that with mirrorless cameras, as there’s no longer a separate autofocus sensor, you don’t need to calibrate. But as we’ll see, that’s not the case.

Most camera manufacturers now offer mirrorless cameras in their lineup, but I’m going to focus on Nikon Z-series mirrorless cameras here as they offer the ability to adjust your autofocus, and the whole Nikon Z-series line is fully supported by FoCal, so we have a wealth of data from our users which we can dig into.

Spoiler Alert!

To whet your appetite, here’s a chart showing the percentage of Nikon Z-series users requiring at least the fine tune shown on the bottom axis:

Taking F-mount lenses attached using the FTZ mount adapter (the blue bars):

• around 50% of users will get immediately sharper images with calibration
• over 80% of users will get some improvement with calibration

We define 2 units as the threshold for a “visually significant” change, meaning that you’re likely to see a small but noticeable improvement in image sharpness.

But even a change of 1 unit can improve autofocus speed, which in turn improves accuracy under shooting conditions that push the limits of the camera autofocus – high-speed movement tracking or low-light shooting, for instance.

So there’s a benefit to mirrorless calibration… but why?

I’m not trying to write a clickbait post, so I don’t want you to have to read all the way to the very end to find out why you might need to calibrate (I’m kind of hoping at least some of you will though, as it’s been a lot of work putting this lot together!)

So, why might you need to calibrate the autofocus on a mirrorless camera with on-sensor phase-detect autofocus?

This isn’t an exhaustive list, but below are the things that I dig into a lot more detail about and explain potential issues that can cause a reduction in the sharpness of your images. (Don’t worry if they don’t make a lot of sense to you now – I’ll explain exactly what the issues are in simple terms over the next few posts)

The AF Sensor:

• Sensor phase-detect mask size and placement errors
• Microlens orientation and placement/form errors

For more details, see the Issues with Autofocus Sensing.

The lens:

• Optical path angle offset due to camera/lens mount offsets, mount adapters, lens element decentering or tilting, stabilisation unit issues and optical aberration.
• Limited lens control precision and accuracy
• General mechanical lens wear-and-tear
• Out-of-date or missing internal lens calibration data

For more details, see The Lens.

The AF Processor:

• Algorithm implementation decisions
• User settings

For more details, see The Autofocus Processor.

Mirrorless Cameras supporting Focus Adjustment

There are a (growing) number of manufacturers that support some sort of user focus adjustment on their cameras.

• Nikon: Reikan FoCal fully supports the Nikon Z mirrorless lineup, so you can automatically calibrate your autofocus quickly and easily.
• Olympus: the EM1 allows adjustment of every single focus point!
• Panasonic: the brand new S5 Mark II had just brought phase-detect autofocus to the S5 line, and offers focus micro adjustment.

(Note: Reikan FoCal will allow file-mode calibration with the Olympus and Panasonic cameras, so you can use the analysis tools to check and calibrate your autofocus)

There are some notable exceptions, however: Canon, Sony* and Fuji do not allow any form of user focus tuning on their mirrorless cameras.

(* Sony offers AF Fine-tune of the phase-detect sensor in their LA-EA2 and LA-EA4 mount adapters, but not tuning of autofocus at the image sensor)

Interestingly, with development versions of FoCal, we’ve seen hints at a need for calibration on Canon EOS-R line mirrorless cameras under certain conditions, but it’s difficult to get clear results fromdue to their automatic aperture control. We’re working on adding support for checking calibration on these cameras, however.

The Autofocus System

Before we start digging into the details, I’m going to define a few things to make sure it’s clear which bits I’m talking about going forward.

There’s a lot of talk about autofocus sensors, but that’s just one part of the whole Autofocus System, which is a bunch of components split across the camera and the lens:

Camera

• AF Sensor – determines the amount of focus offset and in which direction
• AF Processor – takes inputs from the AF sensor and the camera lens, and instructs the lens to move. The actual autofocus algorithm resides here.

Lens

• Optics – the glass bits you (and the camera) look through.
• Mechanics – motors, gears, rails, cogs – anything that moves!
• Calibration Data – factory programmed adjustments in the lens

Whats Next?

Without further ado, let’s get on with seeing how on-sensor phase detect autofocus works…