Question about focus

[blueofspirit]

I have a question about how the AF function works on cameras - say if I half press my focus on my subject, and then without letting go of the half press I move the shot slightly so the subject is not in the center - in one corner for example, does the AF still focus on the subject or does it try to shift the focus to whatever's now in the middle of the frame?

Wondering if this is what's causing the slight OOF in some photos where I have the subject on a third line instead of the center.. (yes, I am a noob photographer, yes I know that might be the real reason..:lmao

 

[BuS_RiDeR]

From what I understand, the half-press of the shutter button freezes the focus on the target that your AF had captured. (the squares/dots that light up in your view-finder when you half-press the shutter.

If you maintain the half pres and change the composition (area you are pointing the camera at)... The AF point remains the same; Unless you release the button and re-focus.

 

[PJL]

^ Agreed. It won't refocus unless you let off the shutter release and do it again.

Most SLRs have the ability to, in "P" or another user-adjustable mode, select which specific auto focus point you want to use when you're focusing if it's not doing it in auto mode.

 

[fokker]

Yeah, what you're doing is a legitimate method of focusing (focus-recompose method as recommended in canon user manuals). At some distances and focal lengths however it is not perfectly accurate, as the plane of focus is not a sphere but a plane, so when you focus and recompose you shift the plane behind the subject. It is best to select the closest AF point to the subject - you can recompose from here if needed as the error will be less than if using the middle point.

 

[Dominantly]

YES!
It is a belief of mine that quite a few people who post on here are doing this very thing, and then perplexed by everyone's comments on a soft shot. This plays larger role when you are shooting at the widest f/stop available.

BUT, your AF can move when you readjust your camera.
With Nikon I can go into AF-C and once I lock onto a subject, the camera will track it where ever it goes in the frame as long as I have the shutter "half pressed" (I use the AF-L button though).

 

[blueofspirit]

Thanks all for the comments - very useful

Dominantly - I am one of these people who get called out for a soft shot. I'm using a Canon 450D/XSi. What would be a way to remedy the soft shots? On a good day I try to shoot f16 or f22 on ISO100 with a tripod.

 

[PJL]

AI Focus AF or AI Servo AF modes would accomplish the same AF Dominantly described. Check out your manual, pages 60-61.

 

[Dominantly]

If you're shooting a person, try and place the focus point right between the eyes, f/8, a good shutter speed, plenty of good light (off camera flash), and a steady camera.
That should be it really.

If you're shooting something outdoors, a tripod works great. Make sure to turn any type of vibration reduction off and try to shoot with a remote or shutter timer.

 

[Josh66]

Read this, it will explain everything:

Why Focus-Recompose Sucks

 

[Josh66]

Wondering if this is what's causing the slight OOF in some photos where I have the subject on a third line instead of the center..

Yes - this is what's causing it.

You are moving the plane of focus without refocusing.

 

[blueofspirit]

Thanks all!

 

[fokker]

Also, shooting at f16-f22 will cause diffraction, which robs you of sharpness. don't shoot this narrow unless you really have to (macro or a very expansive landscape)