You NEED to tune your DSLR autofocus!

I try my very best to avoid making statements that I can’t back up, so I’m going to do just that for the following one:

“You are almost certainly not getting the sharpest shots from your DSLR – and it’s not your fault!”

What if I then told you there’s an easy fix? Read on to find out more…

Image of FoCalCam-3000 with some light rays

About this article

I recently wrote a series of 6 articles going into a whole bunch of reasons as to why you should calibrate your lenses on mirrorless cameras, during which I made some hand-waving statements about it being obvious that you need to do the same with DSLRs as well. What I wanted to do here was clarify that latter point.

Autofocus on a DSLR – a high-level view of how the AF system works on your DSLR

Problems with Autofocus – it ain’t perfect… and here’s why

Fixing the issue – Autofocus Calibration! (Just a heads up – this is the bit where I try and sell you FoCal… sorry!)

Evidence – charts to prove that you really, really should take a look at your DSLR AF system

Conclusion – A review of everything

A Sneak Peek at the Evidence

Just so you can see how important it is to tune the autofocus on your DSLR with all your lenses, take a look at this chart showing the results from over 170,000 calibrations with FoCal on the flagship prosumer DSLRs from Nikon and Canon:

A chart showing that the Nikon D850 and Canon EOS 5D Mark IV typically benefit significantly from autofocus calibration
Focus Adjustment required for popular Nikon and Canon DSLRs

The three groups – Significant, Noticeable and Minimal – represent the amount of adjustment required for each camera.

  • Almost 9 out of 10 Nikon D850s, and 8 out of 10 Canon 5D Mark IVs would give you sharper images after calibration
  • 6 out of 10 Nikon D850s, and 4 out of 10 Canon 5D Mark IVs would show significant sharpness improvements.

Hopefully, those numbers convince you that there is a need to at least take a look at your autofocus performance – I’ll go into more detail later in the article – but for now, I want to show you why you need to calibrate your DSLR.

Autofocus on a DSLR

To help me explain autofocus on a DSLR, I’ve invented a brand new camera. Allow me to present… the FoCalCam 3000!

A rendering of an imaginary camera, the FoCalCam 3000
The FoCalCam 3000 – not the most practical of cameras!

It’s not really all that practical, to be honest – all those floaty lens elements tend to fall on the floor in a terrible mess. In fact, it’s absolutely useless at taking photos, but hopefully it will be handy to show the components of a DSLR and where the issues with autofocus arise.

Setting up the shot

Let’s start with a typical scenario: you’re looking through the viewfinder at the back of the camera, setting up your shot.

I’ve added a light ray to show the passage of light through the lens optics and into the camera (the blue ray), through the insides of the camera, and out the viewfinder (the green ray you can just see coming out the back, where you’d put your eye):

A camera rendeirng with a beam of light showing the path through the eyepiece
LASERS! Well, beams of light, anyway.

Let’s get rid of the lens and the camera body and take a closer look at what’s happening to the light inside the camera:

A view of the light path through the main mirror to the AF module and up through a pentaprism to the eyepiece
The light paths in the camera – red to the AF Sensor, green to your eye

The light goes through the lens (not shown above) into the camera and hits the main mirror (the blue ray). This mirror is semi-transparent, so a good chunk of light is sent upwards (the green ray) into the pentaprism, where it bounces around and comes out the back of the camera, through the eyepiece and into your eye.

But, some of the light goes through that semi-transparent main mirror, hits a secondary mirror behind and is sent down into the AF Module sitting in the bottom of the camera – you can just see this as the red beam of light behind the main mirror.

Here’s a clearer view of the light that goes through the mirror and down to the AF Module at the bottom, shown as the red ray:

A different view of the light path through the main mirror to the AF module and up through a pentaprism to the eyepiece
Clearer view of the path to the AF Sensor off the secondary mirror

So, you’re holding the camera to your eye, and setting up the scene, and you half-press the shutter button to get that comforting confirmation autofocus beep.

Remember – you’re not the only thing looking at the scene – the AF Module at the bottom of the camera is too, and it uses this view to let the Autofocus Processor in the camera do its thing.

Taking the shot

When you fully press the shutter button to take the shot, quite a lot happens in a very short period of time:

  • The main mirror lifts up towards the pentaprism
  • The secondary mirror folds up behind the main mirror
  • The shutter in front of the image sensor opens

So now, our ray of light goes to one place and one place only – the image sensor:

Internal components of a DSLR, mirror up and sensor exposed.
Mirror up, shutter up, sensor sensing – the shot is captured!

As a side note, this explains why you get a blackout through the viewfinder when you take a shot – because the main mirror has lifted, you can no longer see through the lens.

Focus is NOT measured at the Image Sensor

The absolutely critical takehome from the above section is this:

The measurements for autofocus are NOT done by the image sensor, but instead in a totally different part of the camera.

This is really, really important, so let’s run through a quick example to be absolutely clear. Here’s a flow-chart of what happens when you take a shot:

A flowchart showing what happens when you take a shot with a DSLR
What happens when you press the shutter button and take a shot
  • You aim your camera at a scene and press the shutter button
  • Light comes in through the lens, bounces off the secondary mirror, goes into the AF Module and reaches the AF Sensor. Some magic happens and the AF Sensor* reports a measured focus offset to the AF Processor. For this example, let’s say the AF Module reports a focus offset of +27.
  • The AF Processor in the camera gets the measurement of +27 from the AF Module and then has to use various bits of internal information to determine that it needs to move the lens 85 steps towards infinity to make the image in focus at the image sensor.
  • The lens is moved – and the whole process may be repeated a few times depending on the situation – then the shot is taken.

Note that at no point is the focus checked at the image sensor. The accuracy of the whole system is reliant on there being a fixed, known relationship between the focus offset measured by the AF Module and the focus position required for a sharp photo at the Image Sensor.

(* I’m deliberately not going into any detail about phase-detect autofocus as it’s not really necessary to understand how it works, but if you’re interested in a visual overview, take a look at this article about On Sensor Phase Detect Autofocus)

Factory Calibration

When your camera is manufactured, in the final stages of commissioning the Autofocus System is put through its paces and calibration data is written to the camera. This calibration compensates for any tiny positional differences in:

  • AF Module and internal components
  • Image Sensor
  • Main-mirror, including the mounts in which it sits
  • Secondary mirror
  • Lens mount

The factory calibration stage mentioned above compensates for the fixed relationship between the autofocus measurement – what the sensor measures – and the autofocus result – how sharp your image is. This calibration adjusts for slight discrepancies between the perfect design and the real-world position of the mechanical and optical components that take part in the autofocus operation… on the day the camera was calibrated. Which may be months, years or even decades ago!

Next up

On the next page, I’ll dig into the sort of issues that affect the autofocus system on DSLRs…

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