Area covered by an AF point

[rj79in]

I've heard that the actual AF point area is larger than what appears as the rectangle in the viewfinder ... does anyone have any info what the actual difference is?

 

[dr croubie]

I'd actually heard the opposite, that it was a lot smaller than the rectangle.
But no solid info.
Probably depends on camera and which point, too...

 

[neuroanatomist]

The actual AF point area is larger than the little box in the viewfinder that represents the AF point. With Spot AF, the size of the actual AF point is reduced, and in that case, it's only slightly larger than the box in the VF.

This issue causes confusion for many people claiming poor AF performance, because there's a high contrast feature at a different distance just outside the VF box that they don't think they're focusing on, but the camera does.

The graphic below (from TDP, which Bryan modified from a Canon graphic) gives an idea of the relative sizes.

 

[P_R]

Thank you neuroanatomist for the answer,

I've been wondering that too rj79n - never thought to ask.

So if the area is larger than the box, which part of the box does the camera use? Is it an average, or which ever area within the box which is closer to the camera?

I ask because I love using my 35L wide open, but because the depth of field is then so small, it might help knowing how the camera calculates focus.

Cheers,

P_R

 

[neuroanatomist]

So if the area is larger than the box, which part of the box does the camera use? Is it an average, or which ever area within the box which is closer to the camera?

It uses whichever feature within that area that generates the greatest phase difference, i.e. has the highest contrast in the proper orientation relative to the sensor line (and the orientation depends on the AF point geometry, line vs. cross vs. dual cross).

 

[rj79in]

Thanks neuro ... that's pretty much problem solved. This should be a help while focusing

 

[Mt Spokane Photography]

Yes, it is something to take into consideration. It can affect those autofocus test charts that are found on the internet. Properly designed targets take the autofocus point into consideration and make for fewer test errors where a camera actually focused on the wrong point.

Here is one that helps you focus on the correct spot on the chart.

http://regex.info/blog/photo-tech/focus-chart