AF Sensor Behaviour - More complex than it might appear

After a shot in which the focus was not where I expected, and the AF point was spot on where I wanted it to be, I started looking for the cause. I couldn't find anything.

Then I started searching for AF sensor accuracy/precision/rel​iability, and found a number of very informative posts. Those who tested their AF sensors either used depth of field charts, or stickers on walls. I tried a few things, and found that I could repeat the focus problem.

But, it seemed to me that the behaviour of the AF sensors must be quite complex. The posts I'd read mainly concerned the location of the AF sensors compared to where they appeared to be in the viewfinder. But, each sensor has a complex design, and that makes the way it detects patterns complex, but it also means that near objects could attract more 'attention' than the object under the centre of the sensor.

I've just been taking some shots of a table full of Lego. Because there's lots of depth and pattern, it turned out to be a very good target. I chose a day when it's not too bright (cloudy), indoors, but with lots of window light.

I used a Canon 60D with 70-200mm f/2.8 II, at f/2.8 and ISO 400, with AI servo, and using the AF-on button. On a tripod, using the shutter button. I selected on AF point manually.

I started pointing at spots and pressing the AF-on button, releasing and then pressing the shutter, but that doesn't record the AF point used for DPP. I found it better to press the AF-on button and very slowly move the camera around and look at the behaviour of the focus. After some experimenting, I added a 'bridge', so I had a hole with things on all sides.

What I found is that each sensor square doesn't match where the actual sensor is, but more importantly, high contrast objects near to the sensor caused that object to be focussed on and not the object I was trying to focus on.

Having read about this recently, I suggest this applies to all cameras. I suggest that it's quite instructive for people to try this themselves, to get a feel for how the AF sensors behave. That way, when trying to focus, you will have a better chance of anticipating problems, and also you may slightly offset your AF point knowing it will 'grab' the object you want.

Anyway, here are some pictures. This is just a small subset of what I took.
The 'failures' are in the main more interesting than the 'successes'.

These are using the top sensor.
This is what you'd expect:

The top-right sensor was surprisingly good:

Great test, keep 'em coming!

Thanks

A little more background:

http://www.the-digital-picture.com …D-DSLR-Camera-Review.aspx

http://photo.net …gital-camera-forum/00XhwJ

The sensor needs to be looking at some detail to be able to focus and unless using spot the area covered is larger than the indicated area.

I don't see anything much surprising in your test shots, all that is well documented in Canon technical documentation.
To summarize:

1) AF sensors are about 3 times bigger than focusing points.

2) Different focusing points have different AF sensor arrangement and different sensitivity.
Some of them are horizontal, some are vertical, some have cross pattern, some are more, some are less sensitive.

Example:

HTTP response: 404 | MIME changed to 'text/html' | Byte size: ZERO

If you look through the viewfinder of a 40D and compare what you see to a 7D, you'll find that the representation of the AF points in the 7D is quite a bit larger than in the 40D. Still, they are in reality even larger than that in the 7D too.

Point is, I'm not so sure you can say that a ratio like three times larger holds for every camera. But it doesn't matter too much either, as long as you understand that they are bigger.

I agree with you about the focus area being larger than the focus point displayed in the viewfinder, but I found troublesome that the area in NOT centered on that focus point.

citro wrote in post #11959890
I agree with you about the focus area being larger than the focus point displayed in the viewfinder, but I found troublesome that the area in NOT centered on that focus point.

I'm sorry, I didn't understand the part in bold.
Can you explain a little bit better, please!

apersson850 wrote in post #11959876
If you look through the viewfinder of a 40D and compare what you see to a 7D, you'll find that the representation of the AF points in the 7D is quite a bit larger than in the 40D. Still, they are in reality even larger than that in the 7D too.

Point is, I'm not so sure you can say that a ratio like three times larger holds for every camera. But it doesn't matter too much either, as long as you understand that they are bigger.

You are probably right, exact ratio is most likely different for different models, but generally speaking, it is significantly bigger!

Hermeto wrote in post #11959897
I'm sorry, I didn't understand the part in bold.
Can you explain a little bit better, please!

I'm guessing he's referring to the top sensor box appearing to be high vs the real center of the sensor, as seems to be indicated by pics 2 and 6 of the top post, where the selected af point was on what appears to be a sharp contrast location but the sensor selected another point significantly further down in the image.

Why were you on AI Servo?

I would definitely like to see spot af on more cameras. Can you select spot af on any af point on the 7D? I've found it problematic from time to time when shooting with shallow depth of field the af points are too big on my 500D. Like someone said it grabs the eyelashes for instance instead of the eye. I've also found when shooting birds, spot af would be really useful since the af point sometimes grabs branches instead of the bird for me. It makes me want to get a 7D for focusing alone

Xcelx wrote in post #11960979
I would definitely like to see spot af on more cameras. Can you select spot af on any af point on the 7D? I've found it problematic from time to time when shooting with shallow depth of field the af points are too big on my 500D. Like someone said it grabs the eyelashes for instance instead of the eye. I've also found when shooting birds, spot af would be really useful since the af point sometimes grabs branches instead of the bird for me. It makes me want to get a 7D for focusing alone

Spot AF works on all 19 focus points.You would not be the first person to buy a 7D for focus system either.

toxic wrote in post #11960955
Why were you on AI Servo?

I always use AI-Servo and the AF-on button. That way I can press and release the AF-on button, or I can press and hold the AF-on button. In this case I needed to press and hold in order to have the AF sensor used recorded for DPP. It did also enable me to get a feel for the sensor behaviour by very slowly moving the camera and watching how the focus changed.

A couple of things I didn't mention.

1. In some situations the lens hunted backwards and forwards slightly and didn't stop. It "couldn't decide" which object to lock on to. Not those posted.

2. I did release the AF-on button and then press and hold again to get the camera to start the focus operation on a new fixed part of the subject. I did this because I think the AI-servo operation is different to the initial capture of focus. I think all the pictures I've posted were done this way, so I think in effect, it would be the same without AI-servo. After the initial focus, I didn't see any focus change if I kept the camera still (apart from the hunting).

3. IS was off. This is relevant not just for image quality (which is worse on a tripod with IS on), but also because the IS can cause the subject to move, which will affect the location of the AF sensor. Yet another complication.

I'm mainly concerned about situations in which the camera focuses only slightly differently to what I expect, so that I can't tell the focus is wrong in the viewfinder. I'd prefer not to review and magnify every picture just to check whether the camera focused on the point I put the AF point on!

Notice in one of my links the highly regarded 7D is not immune from this problem:

7D af point size and alignment
http://forums.dpreview​.com …age=35760121&ch​angemode=1