40D high-precision focus point question

I understand that the center focus-point on the 40D is horizontal/vertical/an​d diagonal when "used with" f/2.8 and faster lenses. Now my question is, does the aperture have to actually be set at f/2.8 or faster for the high precision focal point to be active? Or can you be using an f/2.8 lens with the aperture set to, lets say for example f/8, and still have the high precision points in the center focal point horizontal, vertical, and diagonal active?

Since focusing is always done with lens wide open, working aperture has nothing to do with AF precision.
When they say f/2.8 or faster, it means lens’ maximum aperture.

Hermeto explanation is correct, when you take your camera and point to a target, you try to focusing (by half shutter press) and the diaphragm of the lens is driven to its maximum aperture (If you have a EF 50 f/1.4 is open to f/1.4; if you have a 70-200 f/4 then is opened until f/4 ...and so on); then once the AF has been achieved and also the metering system has got the correct exposition values (for speed, diaphragm - at a specific ISO), when you press till the bottom the shutter, the diaphragm is driven back to the exposition measured value, let´s say f/8 and the photo is taken.

I was aware of this, but got to thinking about shooting at 10fps. That's quite a workout for the aperature blades in the lens. I wonder what the limit is for them to keep up with the shutter.

Az2Africa wrote in post #3940278
I was aware of this, but got to thinking about shooting at 10fps. That's quite a workout for the aperature blades in the lens. I wonder what the limit is for them to keep up with the shutter.

Didn't know the 40D did 10 fps.

lol i will post this for the 3rd time

evrey single focus point is cross type, honestly this focus point appears to be larger than the others, it's actually bigger than that little box on the view finder, also imagine that you had this hot babe infront of you, and you wanted to focus her eyes instead of her nose, and you had an aperture of 1.2. how would you handle this with your 40D.

it would focus on her nose instead of eyes since nose is mostly big and bright item, so her eyes would be OOF.

you can find more information here, with farrukh threat :
https://photography-on-the.net/forum/...=3766​05&page=6

ofcourse it works on large targets, there for i do not call it high precision.

i know i'm getting slightly off topic here, but i'd like to see some 40D testing about this and warn users about it.

PacAce wrote in post #3941091
Didn't know the 40D did 10 fps.

It doesn't. I just thinking in terms of a camera that will shoot at that rate. How fast can the aperature blades cycle?

"Warn users about" what, SilverOnemi - that the sensor is of one arbitrary size instead of another arbitrary size?

Keith R wrote in post #3942457
"Warn users about" what, SilverOnemi - that the sensor is of one arbitrary size instead of another arbitrary size?

https://photography-on-the.net/forum/showthre​ad.php?t=376605

lol did you even read my post ?

SilverOnemi wrote in post #3941153
lol i will post this for the 3rd time

evrey single focus point is cross type, honestly this focus point appears to be larger than the others, it's actually bigger than that little box on the view finder, also imagine that you had this hot babe infront of you, and you wanted to focus her eyes instead of her nose, and you had an aperture of 1.2. how would you handle this with your 40D.

it would focus on her nose instead of eyes since nose is mostly big and bright item, so her eyes would be OOF.

you can find more information here, with farrukh threat :
https://photography-on-the.net/forum/...=3766​05&page=6

ofcourse it works on large targets, there for i do not call it high precision.

i know i'm getting slightly off topic here, but i'd like to see some 40D testing about this and warn users about it.

I think maybe you might be a little confused. The AF sensors are designed to "lock" in on high contrast targets. Between the nose and the eyes, I think the eyes would be considered much more high contrast than the nose would be.

Also, the precision is not determined by how large, or rather how long, the focusing sensor is. If if were, the 1D series camera would be doing worse as far as focusing is concerned than the xxD cameras. The size only determines how much of an area will be considered when looking for any high contrasts target. Whether an AF point is high precision or not is determined by how far apart the AF sensor pairs are placed from each other. Didn't know there were two AF sensors used for each AF point, did you? The farther apart the AF sensor pairs are from each other, the more accurate the focusing will be. That is the reason high precision AF sensors requires a lens with an aperture of f/2.8 or wider. Lenses with smaller apertures don't allow the AF sensor pair to be spaced as far apart as f/2.8 lenses do.

Below is a diagram of the 40D sensor which I pulled from the 40D white paper.

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Nice explanation, Leo .. Thank you !!

SilverOnemi wrote in post #3941153
lol i will post this for the 3rd time

evrey single focus point is cross type, honestly this focus point appears to be larger than the others, it's actually bigger than that little box on the view finder, also imagine that you had this hot babe infront of you, and you wanted to focus her eyes instead of her nose, and you had an aperture of 1.2. how would you handle this with your 40D.

it would focus on her nose instead of eyes since nose is mostly big and bright item, so her eyes would be OOF.

you can find more information here, with farrukh threat :
https://photography-on-the.net/forum/...=3766​05&page=6

ofcourse it works on large targets, there for i do not call it high precision.

i know i'm getting slightly off topic here, but i'd like to see some 40D testing about this and warn users about it.

You may not realize this, but the 40D is no different in this respect from any other AF camera out there. Any indicators in the VF are just that, indicators. They do not correspond 100% exactly with the actual area used to determine "correct" focus.

yup but the cross sensors are just an abuse.