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Newbie question about focus points and depth of field Anonymous
Hi /p/, I've just started playing around with my first SLR and have a question about focus points and depth of field.

From what I understand, the depth of field is well, the depth in which there will be acceptable focus or sharpness. So for a certain distance back and forth of the subject, that is the area in focus, while everything else is out of focus.

Now, where do focus points come into play? I've only been using the center point focus because the AF is just iffy and selects points I just don't want sometimes, so I just center AF and recompose. I have a 30D by the way, picture is the viewfinder screen.

Let's say I let the camera pick AF points and only the left and right most points lock on, does that mean anything in the middle will be out of focus? And if yes, by how much?

I don't think my question is clear yet, so I'll rephrase it a bit in the next post along with an example image.
>> Anonymous
     File :-(, x)
OP here, okay see the picture attached.

I've shot some group pictures like this one using the camera and with the AF point on automatic selection, it selects where there is the best contrast I guess, right? So again, sometimes it really picks out weird points where I don't want it to, like someone's legs.

So I'm thinking, I'll override it and go manual, but can only select one point. Do I simply pick center AF point and close up the aperture and hope everything is in focus?

See picture, parts of the legs on the left and right are OOF while the middle chicks have sharp legs. How do I make sure everything in the picture is in focus?

Sorry if my thoughts are a bit scrambled right now. Again, as I understand DOF, it only deals in depth = back and front of subject. How do I deal in the um, horizontal plane? so that everything on the same horizontal plane is in focus?
>> Butterfly !xlgRMYva6s
It will pick out a certian distance away, anything in a plane (ie a sheet) normal to you that distance away will be in focus.
>> elf_man !!DdAnyoDMfCe
Look at it this way. Everything the same distance from you as what's in the focus point will be in focus. Anything too far forward or too far back will be out of focus, depending on your dof.
>> ac !!VPzQAxYPAMA
It's apparently "People who don't have any grasp on the absolute most basic concepts behind optics" day on /p/...
>> Butterfly !xlgRMYva6s
>>109391
its just a specific idiot day, which is everyday!
>> Anonymous
>>109390

Okay, so if I my center AF locks on and I have an acceptable DOF, everything on the same horizontal plane as that object/person should be in focus?

What happens if say the camera picks up 2 focus points?
>> Butterfly !xlgRMYva6s
>>109398
it decides which it likes more, then chooses that one.

it wont choose two different ones, if you see it lock on to more than one its because it thinks those two are in the same plane.
>> ac !!VPzQAxYPAMA
>>109398
Generally, the full-auto focus mode picks the closest focal plane with something in it. What's happening when it lights up two focal points is that the plane it's chosen has something at both of those points.

I.e., if two people are standing next to each other, forming a plane parallel to the one you're standing in with your camera, the focus point on Sally and the focus point on Janine will both light up.

(I've decided the two people in this hypothetical are named Sally and Janine).

If Shaneequa stands right in the middle of Sally and Janine, the focus point on her would also light up. However, if she took a step back, it wouldn't.

But knowing what a camera hog Shaneequa is, she'd probably step forward to hog the limelight. Result? The camera will focus on her instead. Her focus point will light up, and Sally and Janine will be out of luck (although they might be in focus still, depending on your aperture).

The reason you can only select one focus point is simply because that's the only way that makes sense. When you choose the focus point yourself, it focuses on whatever that focus point is over.
>> Anonymous
>>109409

Oh, I see. Thanks, ac.

So to resume, if I center AF only and lock on to something, everything on the same parallel line will be in focus, regardless of it's in the corners or in the center of the frame?

If there are people behind the subject I've locked on, I'll have to increase my DOF by stopping down to get them in focus?
>> eku !8cibvLQ11s
I would suggest you try Custom Function 13.1. IMO the best way to handle af selection point.