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Author Topic: 6D Center Cross-Type ?  (Read 3862 times)

libertyranger

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6D Center Cross-Type ?
« on: September 17, 2012, 05:48:34 PM »
Looking at the specs of the new 6D on Canon's website, I find myself a little confused.  It states:


AF Points
11 points

Center: cross-type at f/5.6; vertical line-sensitive at f/2.8.

Upper and lower AF points: vertical line-sensitive AF at f/5.6.

Other AF points: Horizontal line-sensitive AF at f/5.6.


So does this mean that the center point is a cross-type point at 5.6 or 2.8?  Currently, I own a T3i and I know the center point become a cross-type point when using f/2.8 or faster lenses.  Otherwise, with my kit lens it acts a single type focus point.  I use the center point only and usually get recompose after gaining focus.  The 6D looks appealing if the cross-type point is activated at 5.6, but I'm just not sure on the wording here.  Any ideas?

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6D Center Cross-Type ?
« on: September 17, 2012, 05:48:34 PM »

dswatson83

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Re: 6D Center Cross-Type ?
« Reply #1 on: September 18, 2012, 03:03:52 PM »

11 points
Center: cross-type at f/5.6; vertical line-sensitive at f/2.8.
Upper and lower AF points: vertical line-sensitive AF at f/5.6.
Other AF points: Horizontal line-sensitive AF at f/5.6.

So does this mean that the center point is a cross-type point at 5.6 or 2.8?  Currently, I own a T3i and I know the center point become a cross-type point when using f/2.8 or faster lenses.  Otherwise, with my kit lens it acts a single type focus point.  I use the center point only and usually get recompose after gaining focus.  The 6D looks appealing if the cross-type point is activated at 5.6, but I'm just not sure on the wording here.  Any ideas?
It will be cross type with any 5.6 lenses and it will add an additional highly sensitive vertical line for f/2.8 and wider lenses. You don't want to recompose after focus if you are shooting with limited depth of field as your focus could go off and full frame cameras have less depth of field than the T3i (or other cropped sensor cameras) so you need to be more accurate with your focusing with a FF camera. When you recompose, you change the angle and can change the distance to the subject which will throw of your focus if you are shooting really shallow. This focus system is not appealing at all. Plus, if you are tracking anything such as for sports or wildlife, 11 points is not enough, especially when only 1 of those points is cross type.

tnargs

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Re: 6D Center Cross-Type ?
« Reply #2 on: October 04, 2012, 12:43:05 AM »
...When you recompose, you change the angle and can change the distance to the subject which will throw of your focus if you are shooting really shallow. This focus system is not appealing at all. ...

Shooting 'really shallow' is best done with MF using live view and magnified view. Your typical dSLR AF system doesn't focus precisely with lenses wider than f2.8.

A focusing system isn't meant to 'appeal', it is meant to perform. And that is yet to be seen.

neuroanatomist

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Re: 6D Center Cross-Type ?
« Reply #3 on: October 04, 2012, 09:44:56 AM »
It will be cross type with any 5.6 lenses and it will add an additional highly sensitive vertical line for f/2.8 and wider lenses.

Exactly.  This is a new AF point configuration for Canon. 

Previously, they had single lines (f/5.6, one otientation only), crosses (two lines with the same aperture threshold, like the outer points on xxD), two-line hybrid crosses (two lines with different aperture thresholds, like the center point on Rebel/xxxD and 5D/5DII, and all of the cross-type points on 1-series bodies before the 1D X), and dual crosses (four lines total - two pairs of lines with different aperture thresholds, superimposed as an 'x' on a '+').

The 6D's center point is a hybrid cross with three lines - an f/5.6 cross ('+') with an additional f/2.8-threshold horizontal sensor line.

Your typical dSLR AF system doesn't focus precisely with lenses wider than f2.8.

False.  The AF system is specified to a given level of precision - the f/5.6 lines are precise within one depth of focus at the lens' max aperture, the f/2.8 lines within one-third the depth of focus at the lens' max aperture.  It doesn't matter whether the lens has an f/2.8 or an f/1.0 max aperture, the AF system will be precise to that degree relative to the max aperture of the lens.  Note that precision is specified, not accuracy.  Precision ≠ accuracy.

The DoF at apertures wider than f/2.8, especially with reasonably close subjects, is so thin that any misalignment in the AF system will be exposed - that's the accuracy part.  That's why we have AFMA - to correct for the inaccuracy introduced by misalignment of the AF sensor with the image sensor.  A properly microadjusted lens will be accurate, and precise within the limits stated above, i.e. the obtained focus will be distributed around the 'true' focus in a manner bounded within 1/3 or 1 depth of focus (probably not hard boundaries, but likely about 3σ, which is a standard tolerance in most processes).

However, manually focusing a lens wider than f/2.8 through the viewfinder with a stock focus screen is not accurate or precise, because the stock screen does not show the true DoF of a lens faster than ~f/2.8.  The AF system will do a lot better (if properly adjusted).
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joshmurrah

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Re: 6D Center Cross-Type ?
« Reply #4 on: October 04, 2012, 10:15:56 AM »
Yeah I take exception to the "AF with large aperture doesn't work"... I shoot 1.4-1.8 all the time and the AF is great, and I don't have to stop and toy with live view, magnification, etc, just snap and go.

I do agree that you won't see the precise focus via your viewfinder, you're totally counting on the AF to do it correctly for you.

I found is that it's critical to AFMA your lens... I had to do +5 on my 50 f/1.4, it was no good without the adjustment @ f/1.4.
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tnargs

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Re: 6D Center Cross-Type ?
« Reply #5 on: October 04, 2012, 11:32:37 PM »
Your typical dSLR AF system doesn't focus precisely with lenses wider than f2.8.
False.  The AF system is specified to a given level of precision - the f/5.6 lines are precise within one depth of focus at the lens' max aperture, the f/2.8 lines within one-third the depth of focus at the lens' max aperture.  It doesn't matter whether the lens has an f/2.8 or an f/1.0 max aperture, the AF system will be precise to that degree relative to the max aperture of the lens.  Note that precision is specified, not accuracy.  Precision ≠ accuracy.

The DoF at apertures wider than f/2.8, especially with reasonably close subjects, is so thin that any misalignment in the AF system will be exposed - that's the accuracy part.  That's why we have AFMA - to correct for the inaccuracy introduced by misalignment of the AF sensor with the image sensor.  A properly microadjusted lens will be accurate, and precise within the limits stated above, i.e. the obtained focus will be distributed around the 'true' focus in a manner bounded within 1/3 or 1 depth of focus (probably not hard boundaries, but likely about 3σ, which is a standard tolerance in most processes).

However, manually focusing a lens wider than f/2.8 through the viewfinder with a stock focus screen is not accurate or precise, because the stock screen does not show the true DoF of a lens faster than ~f/2.8.  The AF system will do a lot better (if properly adjusted).

Apologies, thanks for correcting me on this point. I was thinking about MF, my error. With AF there is the issue of focus shift, but for phase detect AF that problem is smallest at the widest aperture, heh heh. With live view focus, focus shift might catch you out at very wide apertures because some cameras auto control the aperture during live view.

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Re: 6D Center Cross-Type ?
« Reply #6 on: October 05, 2012, 01:49:17 AM »
It will be cross type with any 5.6 lenses and it will add an additional highly sensitive vertical line for f/2.8 and wider lenses.

Exactly.  This is a new AF point configuration for Canon. 

.....

The 6D's center point is a hybrid cross with three lines - an f/5.6 cross ('+') with an additional f/2.8-threshold horizontal sensor line.



Neuro, don't you mean 'vertical' sensor line (for the 6D's centre AF point?)   ???

And am I correct in believing that AF points with vertical sensor lines pick up actual horizontal contrast in scenes easier, eg the actual horizon? (and visa versa, that horizontal sensor lines pick up vertical lines easier, eg the sides of buildings?

Your typical dSLR AF system doesn't focus precisely with lenses wider than f2.8.

False.  The AF system is specified to a given level of precision - the f/5.6 lines are precise within one depth of focus at the lens' max aperture, the f/2.8 lines within one-third the depth of focus at the lens' max aperture.  It doesn't matter whether the lens has an f/2.8 or an f/1.0 max aperture, the AF system will be precise to that degree relative to the max aperture of the lens.  Note that precision is specified, not accuracy.  Precision ≠ accuracy.


When you wrote "It doesn't matter whether the lens has an f/2.8 or an f/1.0 max aperture, the AF system will be precise to that degree relative to the max aperture of the lens."... does that mean that the AF system will or will not have more precision with a f/1.0 lens than a f/2.8 lens?

I do understand lenses are held 'wide open' until they are actually stopped down when taking the photo [or when the depth of field preview button is used.]

And finally, Neuro... can you define both precision and accuracy... comparing / contrasting the two?

Many thanks....   ;)

Paul
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Re: 6D Center Cross-Type ?
« Reply #6 on: October 05, 2012, 01:49:17 AM »

neuroanatomist

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Re: 6D Center Cross-Type ?
« Reply #7 on: October 05, 2012, 04:10:28 PM »
Neuro, don't you mean 'vertical' sensor line (for the 6D's centre AF point?)   ???

And am I correct in believing that AF points with vertical sensor lines pick up actual horizontal contrast in scenes easier, eg the actual horizon? (and visa versa, that horizontal sensor lines pick up vertical lines easier, eg the sides of buildings?


No, I mean a horizontal f/2.8 sensor line for the 6D center point. 



But yes, a horizontal sensor line is sensitive to vertical lines in the scene, like the side of a door frame or building (and vice versa).

When you wrote "It doesn't matter whether the lens has an f/2.8 or an f/1.0 max aperture, the AF system will be precise to that degree relative to the max aperture of the lens."... does that mean that the AF system will or will not have more precision with a f/1.0 lens than a f/2.8 lens?

I do understand lenses are held 'wide open' until they are actually stopped down when taking the photo [or when the depth of field preview button is used.]


Same relative precision, but in an absolute sense, an f/1.0 lens should be more precise. Honestly, I'm not absolutely certain of this myself - in this case, I'm basing this on an explicit statement from Chuck Westfall.

And finally, Neuro... can you define both precision and accuracy... comparing / contrasting the two?


Precision is repeatability, how tightly a series of repeated measurements cluster together.  Accuracy is how close the average of a repeated series of measurements, or even a single measurement for that matter, is to the true value.

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pj1974

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Re: 6D Center Cross-Type ?
« Reply #8 on: October 05, 2012, 06:24:00 PM »
Thanks Neuro... you have answered all my questions SO clearly and well.  I now understand.

You're a legend!  Again, your contribution on this forum is so much appreciated.

Paul
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Re: 6D Center Cross-Type ?
« Reply #8 on: October 05, 2012, 06:24:00 PM »