Here, we’re going to talk about DSLR cameras with a separate phase-detect autofocus sensor.
The critical thing to note is that the sensor for phase-detect AF is NOT the same sensor as the one that captures the image. The cutaway image below of a Canon EOS 500D shows the image sensor and AF sensor highlighted:
Cutaway view of a DSLR
The AF processing system (which interprets the waveforms from the AF sensor and determines how to drive the focus of the lens) knows how to adjust the lens to achieve the correct focus for the picture to be sharp at the image sensor - which is where you want the image to be sharp as that’s where the final, complete image is captured.
But what if something is not quite right… maybe the AF sensor was positioned just a tiny bit incorrectly when the camera was manufactured, or the lens mount has become slightly looser over time (fractions of a millimetre difference), or you’re shooting in an environment where the temperature is very different to “normal”, or maybe a knock to the camera has moved part of the complex AF sensor assembly by a tiny amount…
The AF system cannot know about this problem, and so continues to work in the same way as before, determining focus for where the image sensor should be. The AF sensor does the job and as far as it’s concerned the image will be in focus… but the shot you take is not quite in focus where you expected.
And this is where AF microadjustment comes in.
You don’t need to physically adjust components in the camera – you just tell the camera that things are slightly different from “factory perfect” and by how much it needs to shift the AF result so the image is perfect at the image sensor.
Well, sort of. There are two problems remaining:
- AF microadjustment doesn’t fix every situation, and
- How do know what adjustment to make?
As explained above, AF microadjustment simply shifts the result of the phase-detect algorithm a bit to compensate for differences between the ideal factory setup and the current setup. So, it can fix static situations, such as:
- the camera has suffered a knock and the sensor or AF sensor has moved fractionally
- the lens mount is slightly out of alignment
- the optics in the lens are slightly misaligned
- things have moved due to temperature variations (e.g. the difference between shooting outside in Finland in winter, and in a Moroccan desert in summer)
- the secondary mirror is in a slightly different position due to mirror-cushion wear
- and other situations which create a constant difference between the AF sensor and image sensor result.
It cannot fix differences that change every time the camera is used, for example:
- an AF sensor or image sensor that can continuously move fractionally due to damage,
- or problems that cause the result to vary during normal focus operation, for example seriously misaligned lens optics.
The next step is to determine the best microadjustment value for your setup, and all of the following aspects affect the microadjustment value required:
- The camera body in use
- The lens in use
- The distance to the thing you want to focus on
- The focal length of the lens if it’s a zoom lens
- Even the colour of the lighting you’re shooting under!
In theory, to calibrate the AF microadjustment setting on a camera, you simply take some shots at various different microadjustment settings and pick the best, but as ever things are not quite that simple as you will see below.
Continue on to Autofocus behaviour