Selecting the AF Point: Automatic or Center Point

[Keith B]

The wider the lens the bigger the issue you will have recomposing.

[Colin]

Hey, that makes sense too!


WOW. I'm more edjamacated!

[Benjamin]

I do not use auto selection either. As I found at least 50% of the time the camera will not choose the subject which I have in mind to focus on, am I just unlucky?


Anyway, I use the center point most of the time as it has a good accuracy and it's fast. For sport and stage photography I'll certainly use auto servo withONLY the centre point. For stationary like landscape and portrait I will take a little bit time to choose the right point to focus right on the subject. If and only if the subject is completely off the area which is covered by points, I'll choose the nearest pointto the subject and recompose after focusing since there isn't too muchI can do beside this.


Open for corrections on my focusing habit[]

[Citronsyra]

Yeah, like Benjamin said, I have also had the same unlycky experience when the camera choose the subject, so I always use the centre point as a start and then change when there is time and a need for it.

[Stephen Probert]

The wider the lens the bigger the issue you will have recomposing.

Looks like Keith is right (just thought I would prove it to myself). I got all nerdy and opened up Solidworks (a CAD program) to do the trig for me to see if this is correct. Assuming we are talking about getting the same framing on a 24mm vs. a 50mm lens (for example), we recompose the same distance, assume a flat plane of focus and the same aperture....


I got (from Wikipedia), that a 24mm lens has a horizontal angle of view of 73.7 deg. and a 50mm lense, 39.6 deg. I assumed that you were to photograph a frame that is 5 ft. wide and you are recomposing 2 ft. DOFmaster gives the depth of field about equal between the two focal lengths (at f/2.8), ~1 ft. Both have almost equal in-front-of-subject focus distance of 0.44 ft. The difference is that the subject in the 24mm case moves out of the focal plane .588 ft. while in the 50mm case it moves out 0.288 ft. So in the wider case the subject you want to be in focus no longer will be.


I just picked these numbers, so this in no way proves that this is true for all cases, but it seems that Keith is correct (not that there was a controversy).


Here are some screen shots to show the geometry I am working with:


[img]/cfs-file.ashx/__key/CommunityServer.Components.UserFiles/00.00.00.22.65/24mm-lens.jpg[/img]


[img]/cfs-file.ashx/__key/CommunityServer.Components.UserFiles/00.00.00.22.65/50mm-lens.jpg[/img]


The greyed-out numbers are the outputs (distance to subject and distance subject moves out of focal plane).

[Jarhead5811]

I use manual select up close portraits.


I use auto on more distant subjects when there are no obstructions in the foreground..


I use the center point when I'm taking pics of deer while I'm hunting. It helps when focusing though thick brush. Also the center point seems to focus in lower light than the other points.

[UK_Scotty]

<p class="MsoNormal" style="margin: 0cm 0cm 0pt;"]<span style="font-family: Times New Roman; font-size: small;"]I still use my T90&rsquo;s so I find manual focus the best, if the shot comes out focused on the wrong spot; I just slap myself round the face.<span style="mso-spacerun: yes;"] Not get pissed at the camera.

[pkChileHead]

I have my focus point selector mapped to the eight-way on the the back of my 40D; center click picks the center point and the AF button resets.

[alexniedra]

I usually use the center and recompose.


Sports are done with Al Servo and center. I find that auto will jump back and forth, and my keeper rate will take a dive.