Auto Focus Micro Adjustment (AFMA)

Auto Focus Micro Adjustment is a mechanism Canon have built into the firmware on a number of their models, it is to allow a user to correct small errors on front or back focusing.
Canon have also made available a free  Micro Adjustment guidebook available here: http://www.canonwatch.com/canon-publishes-free-canon-eos-af-microadjustment-guidebook/
Front and Back focusing creep in during the manufacturing process, everything is built to a certain tolerance + or - xxx for example.
What this can equate too is if I have a lens that is +10 and a body that is +10 when you put the two together your focal point is going to be out by +20 which will mean that no matter where you focus on a subject the actual focal point will be out by a factor of +20.
This can and does make a lot of difference as I found out from experience, time for a little story.

I had arranged to meet up with a friend who was bringing with him a Canon 1dmk4 for me to use for the day, skipping to the end of the day when I got home and checked what I had nothing was sharp had the odd shot that was acceptable but not one was tac sharp, we couldn't understand what went wrong for starters, we met up again and this time my friend bought his AFMA alignment guide with him and we MA'd my lens to his 1dmk4 and realised that the issue was my lens was back focusing by a factor of 10, this was enough to throw the focus out resulting in a virtually wasted day.

From that day on I built my own very crude but effective AFMA guide and have religiously MA'd all my lens combinations to my body, its a simple process that cannot harm you camera or lens in any way and can simply be disabled in the AF menu.

I think it is crucial to carry out this procedure if you want to obtain the best performance from your camera and lens combinations, after all we pay a lot of money for these items it seems false economy not to perform a free calibration procedure to fine tune the focal accuracy, as when using a long lens with subjects that are relatively close the DOF can be pretty shallow and if your camera and lens combination is either front or back focusing the resulting image will not be truly sharp as you intended it to be.

A simple test using a scale printed on a sheet of A4 can be used, here is an example.

This was a test shot taken using my Canon 100mm f2.8 L IS, the above shot is uncalibrated I am using the centre line as my physical focal point, notice that the centre line is reasonably sharp as is the red text, but it is plainly obvious from the scale that this lens is front focusing its actual focal point is around the 8-10 lines, looking behind the centre line we can see that the scale is not sharp and beyond 14 is not really legible, yet 14 in front of the centre line is clearly legible.

This shot was taken after using the AFMA menu and applying a setting of +10 as we can see the centre line and text are lovely and sharp and the scale is sharp pretty much equally in front of and behind the centre line, we have successfully moved the actual focal point to the physical focal point if anything it might just need knocking back to +9.

It does show just how much difference AFMA can make to your focal point

My MK2 AFMA adjustment guide is copied from a friend of mine Dave, it cannot get much simpler, three bean cans screwed to a piece of wood, I have added a scale in the form of a bit of an old tape measure to give an added guide.

 So all I did was enjoy the beans on toast, then mark a centre line on my bit of wood and from that centre line I measured 100mm in front and behind and marked out two more lines, drilled the centre of all 3 empty cans and screwed them to the bit of wood turning them so that they are all presenting the same text etc for comparing the focus, finally I added the tape measure.

To use it is simple Canon recommend 50 x the focal length of the lens as a minimum distance to the target for AFMA, which equates to 25m for a 500mm lens, however with lenses of 300mm upwards this can be reduced to 20x the focal length which is a much more manageable 10m.

1. With your camera on a good solid tripod measure out your distance (in this case I will stick with 10m) to target as per your focal length.
2. Place your target at the same height as your camera and place it at approx 45 degree angle across the focal plane.
3. Ensure you have good lighting when doing the test.
4. Set AV Mode
5. Using your widest aperture (in my case f4)
6. Set ISO to 100
7. Set AF Mode to One Shot
8. Set Drive Mode to Single Shot
9. Set AF area to Spot and use centre spot only for focus point accuracy
10. To minimise camera shake you can also use shutter delay or mirror lockup or remote release

OK with that done you are ready to go , now using AF, focus on the centre can and fire off the first shot, this can be reviewed on the rear screen, zoom in and see what is sharp.

  Shot 1 no AFMA adjustment when zooming in on this picture if we look at the 410g on each can, front can soft, centre can sharper, back can sharpest of the three, baring in mind our focal point is the centre can this tells us that this camera and lens combination is focusing towards the rear can or back focusing.

To adjust this is simple we hit the "MENU" button scroll to the purple AF tab on my 5dmk4 it is page 5 (it may be on a different page depending on camera model) last option "AF Micro adjustment", firstly you need to Enable AFMA, then choose "Adjust By Lens", then press "INFO" in the case of my prime lens I am presented with a scale of + or - 20 (if you are using a zoom lens there will be two scales one for "Wide" and one for "Telephoto") as my lens is back focusing I need to dial in a - setting to bring the focal point forward towards the centre can.
I am going to dial in -5 for starters and see how that goes press "SET" to access the adjustment use the wheel to dial in your setting and press "SET" to save it and press "MENU" 2 times to come back out.

The resulting shot with  -5 AFMA dialled in, to me this appears to be a big improvement on the initial shot after a few more shots I settled on -6.
One thing to do between each shot is de-focus the camera, de-focus towards infinity and then macro, this is so that the AF mechanism has to work from both sides of the focal point which will show up any slop in the AF mechanism.
Finally when you are happy with the setting from the adjustment menu press "Q" to enter the lens serial number (if it has not been automatically detected) to register your lens setting in memory.
Now whenever you put this lens combination on your camera it will apply the relevant AFMA setting.