Understanding AF Accuracy?

[Playdo]

I've done a lot of reading up on AF accuracy and I have a question. I know that there are a lot of technical people here, so put me straight if I'm going wrong.

Accuracy: This is the distance of achieved focus from perfect focus.
Precision: For multiple shots, this is the distance (or range) between the shots. Precision can not be measured from a single shot.

- Canon has cross-type sensors sensitive for f2.8. They state that these sensors are accurate to 1/3 x DOF. Therefore using an f2.8 lens, at any aperture, the lens will focus at the f2.8 aperture. So regardless of what aperture it is shot at it will always be as accurate as 1/3 x DOF at f2.8?

- Canon has cross-type sensors sensitive for f5.6. They state that these sensors are accurate to 1 x DOF. How does this relate to an f2.8 lens? Would it have the accuracy of 1 x DOF at f2.8 or would it have the accuracy of 1 x DOF at f5.6?

Please add in-depth links if you know of any.

 

[Canon Bob]

- Canon has cross-type sensors sensitive for f2.8. They state that these sensors are accurate to 1/3 x DOF. Therefore using an f2.8 lens, at any aperture, the lens will focus at the f2.8 aperture. So regardless of what aperture it is shot at it will always be as accurate as 1/3 x DOF at f2.8?

That's a correct assumption

- Canon has cross-type sensors sensitive for f5.6. They state that these sensors are accurate to 1 x DOF. How does this relate to an f2.8 lens? Would it have the accuracy of 1 x DOF at f2.8 or would it have the accuracy of 1 x DOF at f5.6?

The accuracy would be that of an aperture of f/5.6.

The "DOF" to which you refer is Depth of Focus, not Depth of Field
Bob

 

[Playdo]

That's a correct assumption

I think I'm also right in assuming that, on an f2.8 sensitive focus point, an f1.2 lens would focus at 1/3 x DOF at f2.8, and would therefore be no more accurate than an f2.8 lens?

The "DOF" to which you refer is Depth of Focus, not Depth of Field

Thanks for clearing that up.

AFAIK all of Nikon's focus points are f5.6. This would mean that an f1.4/2.8 lens would focus with the same accuracy as at f5.6. If this is the case then how can Nikon cameras be accurate enough when shooting at f1.4? What is their specifications for an f5.6 sensitive focus point (ie ? x DOF)?

 

[Canon Bob]

I think I'm also right in assuming that, on an f2.8 sensitive focus point, an f1.2 lens would focus at 1/3 x DOF at f2.8, and would therefore be no more accurate than an f2.8 lens?

Optically speaking, this is true but the wider aperture would cause the convergent light to fall on a different part of the sensor and allow software to play a part in the "guesstimate".....how it's achieved, I'm not sure. Depth of Focus definitely decreases with lenses faster than f/2.8 as evident by the change in effect of Micro-adjust units

AFAIK all of Nikon's focus points are f5.6. This would mean that an f1.4/2.8 lens would focus with the same accuracy as at f5.6. If this is the case then how can Nikon cameras be accurate enough when shooting at f1.4? What is their specifications for an f5.6 sensitive focus point (ie ? x DOF)?

I'm not well versed in Nikon stuff but it could simply be that their AF sensors are capable of better assessment than Canon's at the same aperture.

Bob

 

[HoppyUK]

I'm not really understanding the point of the question.

To get to grips with actual numbers you need a measure on the depth of focus, but since that is generally fixed in most cameras (eg all DSLRs) and is not a variable that anybody uses, I don't know of any handy on-line calculator. You might have to do it by hand with a formula off wiki or something.

Depth of focus behaves differently to depth of field - it gets deeper with longer focal length. But it is also increases with higher f/numbers. Very roughly from memory, I know that with a wide aperture wide angle, say 35mm f/1.4, depth of focus is a small fraction of 1mm; it rises dramatically and with something like a 300mm lens at f/5.6 it will be a couple of cms. That kind of order.

Which says to me that this '1/3rd of depth of focus' is pretty meaningless by itself, and needs to be further defined. Taken at face value, it suggests a very sloppy standard of AF with longer lenses, but we know from experience that modern AF systems are incredibly accurate, very fast and amazingly consistent. Ten times better than I can focus manually, that's for sure.