Confused with this AF bit.....

[Robbo]

What does this mean

The central AF point offers sensitivity up to f/2.8.

What about the other point on the sensor? What if the lens is over 2.8?

Silly me I know lol

 

[Canon Bob]

A quick precis.....

If the lens is f/2.8 or faster AND the centre AF point is selected then the target focus is within 1/3 DoF (that's depth of focus, not depth of field)

If the lens is slower than f/2.8 then the target is simply within DoF (again, focus, not field)

Bob

 

[Robbo]

So say I had a 2.8 lens, but set the apperture to f11, I am assumit that will still give the centre point the 1/3 DoF?

So what about the other AF point around the centre?

Thanks

 

[Jayst84]

So say I had a 2.8 lens, but set the apperture to f11, I am assumit that will still give the centre point the 1/3 DoF?

So what about the other AF point around the centre?

Thanks

Focussing is done wide open, so yes.

Just your centre spot has the extra sensitivity. Although in theory focussing on any point should be more accurate with a faster lens, as you have a smaller depth of focus.

 

[Canon Bob]

....Although in theory focussing on any point should be more accurate with a faster lens, as you have a smaller depth of focus.

That statement needs to be qualified a little by adding "of the same focal length".
A 600/4 targetted at 10m will be more accurate than an 85/1.2 at 10m.

Depth of Focus (ie, Depth of Field at max aperture) is dependant on aperture and magnification.

Bob

 

[Robbo]

right thanks for the info.....so that mean if I was using a 4-5.6 lens, it would matter much which AF point I am using?

looking into the 7D and it says the middle point 2.8 and the other 5.6

I take it that is better than just having one centre point of 2.8?

 

[MatBin]

Remember also the centre point is generally/always(?) a "cross" type AF point and outer ones not always, certainly not all of them anyway(cant recall if its horizontal or vertical sensitive , but its one and not both). So if you use an outer point and choose the wrong format it wont necessarily work as well as it should or the centre point.

Just a centre point makes it very awkward to compose a shot using an off centre point of interest (or what you want to focus on). It can be done though. Use single af mode (i.e. one that doesnt change when you move the camera around, so no servo/ai servo), lock onto your point of interest, focus (using Af) then re-compose but be careful not to do it in arc motion or you'll be off focus again.

Matt

 

[Jayst84]

That statement needs to be qualified a little by adding "of the same focal length".
A 600/4 targetted at 10m will be more accurate than an 85/1.2 at 10m.

Depth of Focus (ie, Depth of Field at max aperture) is dependant on aperture and magnification.

Bob

Of course, apologies, I do tend to be a lazy typer.