Help me understand AF points.

jyc860923

Veteran
Aug 29, 2018
104
Shenyang, China
贾一川
Having used film SLR and RF in the early years and now mirrorless for about 8 years, some of you know I've got myself a used DSLR dirt cheap and I intend to have some fun with it. Although I've used some DSLRs briefly in the past I really want to understand how to get the most with its AF system.

Take PDAF for example, with the on-sensor phase detection AF you get an illustration of AF points, but there aren't actually "points", they are arrays of thousands of AF pixels, translated by the processor to give the user the ability to choose points to focus on.

So what about the AF points found in DSLRs? They use the dedicated chip under the mirror box which provides, for example 9 points, 11 points, 45 points, does that mean they're actually the only points you can focus on or do they have something that works to fill in the gap between points, because I often find it funny that people even bother to test the tracking capabilities of DSLRs that have a few or dozens of AF points, how are they supposed to track if there's nothing between those tiny little dots?
 
Feb 6, 2015
124
Central Ohio, USA
Andrew
A lot of it depends on the company making the camera and their proprietary algorithms. There is a lot of math going on under the hood to help in predictions of where a subject might go next and all that kind of stuff.

The mode helps too, as with Nikon, you can have AF-C with single point, group, dynamic area, 3D, Auto modes to name a few.

Single point uses that one single point to determine everything, where as dynamic area uses the center point as the main focus area, but supplements that with the selected number of ancillary points around it (9, 25, 125, etc). Group modes use a cluster, but also take other factors into account like subject colors during initial focus acquisition. 3D and Auto use every focusing sensor, but limits your ability for accuracy of the target because you are essentially letting the camera determine the subject of focus.

Not sure how Canon, Pentax or others might do it....but research and experimentation are the best ways to get to know the strengths and weaknesses of each system/mode.
 

jyc860923

Veteran
Aug 29, 2018
104
Shenyang, China
贾一川
Thank you for the help, with the 40D there's not much to choose from, and it works differently from on sensor phase detect and in low light the 40D is faster when it isn't too dark to focus, it isn't as accurate as the m50 when the subject is reflective like dog fur, I've seen the same with some fuji lenses and camera combo.

To answer my original question, I still don't know but that doesn't seem to matter as much as I thought.
 

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