Issues with the DSLR Autofocus System
Now we understand that the focus of your captured image depends on an unchanging relationship between what the AF Module and the Image Sensor see, let’s look at some ways that can change this relationship.
The Autofocus Module
Let’s take a closer look at the AF Module. Here’s one I rendered earlier…
The AF Module sits quietly at the bottom of your camera, guiding light to the AF Sensor so it can take measurements of the focus offset at the focus points. Here’s the AF Module shown in red:
The AF Module is held in place with various mechanical components – brackets, nuts and bolts – and the position of the module even has some adjustment capability via an adjustment screw.
Without going into too much detail, the AF Module itself is quite a complicated optical assembly.
It contains a set of carefully designed field lenses, masks and baffles to ensure only the intended light reaches the sensor, a mirror and a set of separator lenses, as well as the actual autofocus sensor chip itself. All of these components are mounted in an assembly and glued together.
What would happen if the AF Module moved position? Or any of the components in the module – the lenses, the mirror, the masks or the sensor itself – slipped fractionally from their initial mount points?
Or what about the microscopic change in the position of components through thermal expansion between freezing outdoor conditions and baking hot sunny days?
In operation, the sensor chip is measuring image offsets in the order of 1 millionth of a metre, so it’s not inconceivable that the internal components could move enough to upset this tiny measurement.
When this (inevitably) happens, the AF Module takes a measurement, but the relationship between its position and the Image Sensor position isn’t the same as it was when it left the factory. Subsequently, the internal calibration data – which allows the AF Processor to determine where to move the lens focus for a sharp image at the Image Sensor – is no longer correct.
The end result – an image that isn’t as sharp as it should be!
The exact position of the AF Module is important, but what it’s really measuring is the focus offset in the light coming into the camera. This light doesn’t travel directly to the AF Module but instead goes through the camera lens, bounces off a couple of mirrors and heads into the sensor module.
The main mirror raises and falls with every shot you take, and when at its lowest position (as shown above), it settles into position against some mechanical seats after a few bounces.
Mechanical wear and tear, temperature changes and microscopic amounts of dirt or grit inside the camera could change the position that the main mirror comes to rest, which will have a direct impact on the secondary mirror behind.
The same applies to the secondary mirror – it’s on a set of hinges so it can fold into the back of the main mirror when a shot is taken, and the repeated folding and unfolding can cause tiny changes in the position in which the secondary mirror comes to rest.
Remember, some of the light entering the camera passes through the main mirror and bounces off the secondary mirror into the AF Module, so any tiny changes in the position of the secondary mirror can affect the result from the AF Module.
The Image Sensor Assembly
The Image Sensor captures your image, and its position relative to the AF Module is critical for sharp, in-focus images. We’ve seen that changes in the position of the AF Module or the mirrors guiding the light to the module can cause focus errors, so it’s not unexpected that the position of the Image Sensor itself is also critical.
Like the AF Module, the Image Sensor is mounted against a back-plate with a series of brackets and fixings, and again, its position can be changed during commissioning or maintenance via adjustment screws.
Although firmly fixed in place, the position of the Image Sensor would only need to change by millionths of a metre to result in focus issues in your images. Months or years of general use of the camera – hanging from a strap and knocking against hips, moving around inside luggage, knocks and bangs through general use can all potentially cause tiny internal movements.
And don’t forget that for every shot you take, the shutter just in front of the sensor is raised and lowered – a tiny mechanical movement on the shutter assembly that happens thousands and thousands of times.
The Lens Mount
As we’ve seen above, the position of the mirrors can affect the path of light reaching the AF Module, therefore resulting in incorrect focus information.
The same thing can happen with the lens mount – any slight change in the position of the mount itself can affect where the lens sits, slightly adjusting the light reaching the AF Module.
When you’re walking along with that heavy 70-200mm f/2.8 lens on the front of your camera, supported only by the shoulder strap on the camera, there’s quite a lot of potential for tiny movements in the lens position with each step you take!
The lens you attach to the front of your camera is a complex machine in its own right and can influence the autofocus result in many ways, through issues with
- The mount on the lens
- Lens optics – tilting or decentering of lens elements, incorrect placement or issues with the image stabilisation module
- Lens mechanics – the motors, gears and rails can all wear or be imprecise
- Temperature changes causing expansion or contraction of lens components
- Internal correction data within the lens
Details on the sorts of problems above can be found in this article if you want to dig a bit deeper.
Now we’ve got a bit more of a view of the problems that affect the DSLR autofocus system, next I’ll show you what you can do about it…