Multi point AF - question

ajf · 17 Mar 2012 at 12:33

This may seem a basic question but something I have always been unsure of.
When DSLRs are advertised as have multi point AF, how does the camera use these to focus?
Is it purely for when you want to select a focusing point other than centre or does it use them to help track moving objects?

My 350d I think has 9 but I see the latest 5D has 60!

Andrew

Ksanti · 17 Mar 2012 at 17:01

As a basic answer, yes to both. You can choose more points with better AF systems and having more AF points allows for both faster AF and better motion tracking.

Lunawolfy · 17 Mar 2012 at 19:53

More AF points allow for more intelligent auto focus. if you take a pic of the missus at the zoo, and theres 3 points all in the edge of her face, the AF will be a tad off.

Now with 60 points there will be a fair few more covering the face and the camera will have a better choice of focal point.

Theres also standard and Cross-Type AF. I don't know the difference other than Cross type is better

Columbo · 18 Mar 2012 at 09:25

Lunawolfy said:
Theres also standard and Cross-Type AF. I don't know the difference other than Cross type is better

The reason its better is because they are able to detect phase (or contrast) differences on both vertical & horizontal axis, hence the 'cross type' naming. Though the functions of these are often affected by the aperture being used.

Where as non cross type only detect on a horizontal or vertical axis.

ajf · 18 Mar 2012 at 12:32

Lunawolfy said:
More AF points allow for more intelligent auto focus. if you take a pic of the missus at the zoo, and theres 3 points all in the edge of her face, the AF will be a tad off.

Now with 60 points there will be a fair few more covering the face and the camera will have a better choice of focal point.

Theres also standard and Cross-Type AF. I don't know the difference other than Cross type is better

Interesting. So, even if I select the centre point for AF, does it still use the others to gain a better focus of the subject? I'm thinking of when I am using one shot AF rather than AI Servo.

With either though, I had always gone on the basis it used the selected point only for AF?
From my understanding of what you say that is not quite correct?

I have to be honest it was one of those things I accepted it did the job rather than fully understanding how it does it!

Andrew