Thursday, 11 June 2015

Fine tuning the Canon 7D and 7D Mk2 autofocus

This is the "technical" blog I have been threatening for some time, it is an extension of the autofocus tips and advice I gave in a previous blog

In that blog I talked about the choice of 7D Mk2 autofocus settings for photographing birds in flight. Here I want to address the issue of fine-tuning, or micro-adjusting, the autofocus system to make sure it gives you sharp images.

I do not ever read very much about  this, but in my view it is absolutely essential. If you have not done it, there is every chance that your lens is seriously under-performing.

When you activate the AF system by half-pressing the shutter button prior to the shot, the camera obviously tries to focus on the subject. It normally does a good job, but it is prone to two types of error which will affect how sharp an image it comes up with. 

The first type of error is a random error. So if, say, you asked the camera to autofocus on the same object a hundred times in succession, on average it will home in on the correct distance, but there is a plus-or-minus factor affecting each attempt. So, there will be a variability in the sharpness between images depending on how successful the AF system was on a given shot. The AF system is therefore not perfect, but this is how things are in the real world with everything of this nature.There is nothing we can do about this except take a lot of shots and hope that a few are as sharp as we would like. This type of error is like a darts player aiming for the bull; over several hundred shots there will be a scatter of points at different distances from the aiming point, most will be in or close to the bull, whilst some will be a bit more wayward and others in between. It is called a "distribution" of results.

The second type of error is a systematic error. This means that the correct distance mentioned above may be no such thing. If a systematic error is present then the AF system will home in on an average focus distance which is always displaced from the actual subject, either a little too near to the camera or a little too far away. We can do something about this and we should otherwise the image will be blurred to one degree or another. Oddly, of course, the random variation always present could mean that some of your shots could occasionally be sharp, but we do not need to confuse ourselves with this.

Here is how to do it on your Canon 7D Mk2. It is a simple procedure, but sometimes difficult too. All will become clear. This is where it all starts.

Go to the AF menu, fifth item as shown, and scroll the highlight down to the bottom. Press the SET button. This is how my camera looks when I do that because I have already microadjusted my lens..

Scroll down to the third option, as shown above, and this time press the INFO button which is towards the top left on the rear of your camera. Then you will see this (my screen photo is a little out of focus itself, but I hope that you can read it OK.)

Now, you will find that you can move the pointer left and right using the large wheel on the back of the camera. This pointer is showing you the relative adjustment you are making to the average focus position. Negative values mean you are moving it further away from the camera, and positive ones mean you are bringing it closer. The aim is to make it coincide with the actual position of the target you are focusing on. This is done by trial and error as described below.

So you will need to set up a target to photograph. In my examples I have used a printed page from a TV guide. You need something with fine and coarse detail to help you decide whether or not it is in focus.

The target should be set  up at a distance of between 25 and 50 times the focal lengh of the lens. In my case I was adjusting a 400mm lens with a 1.4x teleconverter attached, I had it about 25 metres away at the bottom of my garden.

Then all you have to do is adjust the AF microadjust pointer, press SET to activate it and take a picture. It is best to set the camera to ONE SHOT AF and manually adjust the lens focus wheel to blur the image a bit before activating the AF to make it work a bit harder.

I suggest that you work methodically through the AF microadjust settings from -20 to +20 in increments of 5 initially. You may need a note pad to record what you are doing and as an aid to interpreting the images afterwards.

You will end up with series of images which are in focus to different degrees. This one, for instance, is decidedly  out of focus:

Whereas this one is better:

 But is it better than this one? I think not.

And therein lies the rub!! It is quite easy to eliminate AF microadjust settings that give horrible results, but very difficult to choose between the three or four that give the better results. You can, of course, repeat the tests with microadjust settings varying by just 1 or 2 points over a more restricted range to see if that helps you decide. You can see that, in the end, I chose a setting of -9 for my lens combination.

One tip is to go through the initial test images eliminating the obvious bad ones then if, for example, you are left with reasonable results ranging from settings of -5 to +15, then it is odds on that the best result will be obtained somewhere in the middle of that range, i.e. at about +5. You can investigate. Always remember, that your efforts will be somewhat confounded due to the random variation which also affects the results. You may need to do it a few times or, as I did, get the best possible setting like this then experiment further when shooting in the field.

I reiterate here that, in my view, this is an essential adjustment to make. I have found huge differences in the required microadjust settings for each of my lenses and again if teleconverters are used. Without making these corrections you could conclude that you had a bad lens. This is why I am somewhat mistrustful of lens reviews where they compare sharpness - how did they focus them?

There are other ways of doing this and you can buy special target apparatus, but the above worked for me. Good Luck!

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You may also like to read my Blog article on the effect of Lens Extenders, or Teleconverters, on the quality of wildlife images.

It has some perhaps surprising conclusions!

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