Selecting the AF Point: Automatic or Center Point

[Mark Elberson]

Is there an advantage in letting the camera automatically select the AF point vs. manually selecting the center point? I find that manually selecting the center point often works better for me because it allows me to have more control and many times what the camerathinks is the best focusing point is outside of where I would like it to be.


Am I losing any speed or accuracy in doing this though?


Also, what if I was in AI Servo as apposed toOne Shot? Would it matter then?

[Dann Thombs]

I use center point, half-press the shutter, then recompose. I supposea holdover from my cheapo Pentax camera years ago.

[MVers]

None of the above. Using my 5D and 40D's I manually select the AF point via the small wheel behind the shutter button. With my MKIII I use both the small wheel and large rear wheel to select points (half press the shutter button for AF lock). For shooting fast action Center AF point is most accurate on most, if not all, non-1-series bodies but it depends on what exactly your focusing on and where its located in the frame. AI servo, for the most part, should be used when focusing on a quicker moving subject and One shot on slow moving or still subjects. Focus and recompose works, but only when in 'one shot' and shooting at more narrow apertures while shooting very slow moving or still subject matter. Read up in your manual for a better understanding.


-Matt

[peety3]

I normally shoot AI Servo, center point only, with AF-ON mapped to be an AF-STOP button instead. Allowing the camera to choose focus points can really slow down the AF system in my experience, and you REALLY have to know how the camera wants to behave. The joystick allows me to choose an alternate point on the fly for off-center compositions, but I gravitate back to center point quickly.

[Keith B]

I never use auto. I rarely even use the center point. I find recomposing often compromises tight focus especially on close subjects.


I am constantly switching between points manually.

[Mark Elberson]

Thanks everyone for your input!

[Oren]

Center point for me.

[mark]

focus and recomposing is in general a bad idea ...it lets the camera focus however when you recompose you are actually changing the focus distance from where you held the shutter at half way ..and what you now are actually wanting to focus on ...there for you do not get optimium focus... its ok if you want the focus to be on the obejctive to the right or left dependimng which way you went when you recomposed ...but if you want the focus for example to be on a persons right eye in a shot then use the coresponding focus point for that ye for best results ...or use MF

[Dann Thombs]

Of course most of the time, I'm in MF anyway, so I guess my bad AF practices are somewhat moot, heh.

[Colin]

The problem with the recomposing after focusing is that the focal plane is not a hemisphere. I.e., even if you can manage to keep the camera equidistant from the point of focus, the focal plane is not equidistant from the camera, even if it's curved. Recomposing may, though, be the best you cann do in a quick situation, and if your depth of field is deep enough, more than just fine.


Still, I figured out that hard way that while it can be convenient (and was suggested in my XT manual, as I remember), it didn't seem to work consistently. With wide apertures, it became really frustrating. Before reading up on Bryan's website and realizing that'the focus distance'was, if not perfectly, close to a plane and not really a fixed focusing distance, I thought I was just misfocusing.


If I have the time, I prefer to dial the closest autofocus point into something it can grab some lines on, eyes or eyelashes being the obvious human targets,and take the picture like that, or maybe recompose slightly if it's a really minor shift.


Sometimes, though, when I'm lazy, I like to use all points, and try to line up the point I want to use with a detail, and see if I can duck the others into areas that either lack quick transitions in contrast, or are simply farther away, in which case the closer points seem to get priority. I may halfway press the shutter button a few times, and when I see the point I want blink, squeeze.


In AI servo, I usually use the center point if it's something I can keep a good aim on. However, if it's something that is moving enough laterally or vertically, I may use all points just to give me a better chance of not accidentally losing the lock and getting a nice blurry subject with a really clear background.

[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[:D]

[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:


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


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


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.