Aperture, DOF & Focus Points... Somewhat puzzled...

Started 10 months ago | Discussions thread
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Greg Nold
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Aperture, DOF & Focus Points... Somewhat puzzled...
10 months ago

Hello Folks!

I love how my f2.8 80-200 zoom blurs backgrounds, but lately I've learned some embarrassing  lessons with it... namely, at f2.8, the DOF is extremely shallow at 200mm... so shallow that I've ended up with photos in which a person's ear is nicely focused but their eyes are not.  I've learned to be far more careful with my camera... generally, using the center focus point placed on the nearest eye, holding shutter halfway down, recomposing the photo then taking the shot. (I often photograph adrenalin-pumped kids at quickly-moving sporting events, and if I was to try to select a focus point closer to the side of screen using the arrow keys,  I would miss the shot completely.... so 'Focus and Recompose' seems to be the only way to go for me.)

This brings me to some confusion.  I understand how aperture controls DOF.  What I don't understand is how the Focal points work within the framework of the aperture setting.  If an aperture setting determines a 'plane of sharpness' over a person's face in a portrait, why do we need to place a focal point on the person's closest eye to ensure that the eye will remain in focus?  Shouldn't every feature within the plane of sharpness be in focus?

The only explanation I can come up with is that the 'plane of sharpness' is not absolute, nor is it 'over' a person's face.  It must be an imaginary zone which extends into a person's face.... in other words, it is the 'depth' we speak of in 'Depth Of Field' and it must contain varying degrees of sharpness.  If true, I would guess that in the very center of this zone would be a 'plane of perfect focus'... and any position a little forward or behind this plane would be less than perfectly focused -- but still acceptable to our eyes -- (until we travel so far away from the plane of perfect focus that we simply call it 'unfocused'.)

If I am correct on this I could be happy, but I still don't understand the role of the selectable focus points of my camera.  Is it because when we select a certain focus point, we are telling the camera exactly where within this imaginary zone we want our focus to be applied?

Put a simpler way, do the focus points control the focus of the image within the DOF we've specified through our aperture setting?

I'm a master at making mountains out of mole hills, so If I've got this all wrong, your thoughts would be appreciated!    The good news is that I'm doing better with my lens and its shallow DOF, but I would like to know how it all works for my own curiosity's sake.  I've tried to find info for myself online which explains Focus Points in detail but have found nothing but the basics.  Thanks for any thoughts you may have!  -- Greg

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