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Author Topic: Why does Focal tell users to use 1-Point instead of Spot AF?  (Read 1069 times)

entlassen

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Why does Focal tell users to use 1-Point instead of Spot AF?
« on: December 19, 2013, 04:15:02 PM »
I've been doing AFMA tests using Spot AF.  I recently read through Focal's documentation (I haven't bought a license but am considering it) and it says that specifically for Canon bodies that testers should be using 1-Point AF and not Spot AF.  I've always used Spot AF before because I somehow thought that the more precise focus area would be more appropriate for AFMA tests, but Focal's directions make me wonder if I've been doing it wrong this whole time.  For example, if I use the LensAlign ruler, should I be shooting the target in 1-Point and not Spot?

If it's the case that using one is "correct" and using the other is "incorrect", I would greatly appreciate if someone could clarify me how the incorrect might might skew or even corrupt AFMA/focus tests.

As a related question, in the 5DM3 manual, the section titled "Manual Focusing" (p. 112), it says "with 61-point automatic selection, when the center AF point achieves focus, the focus light [dot] will light up."  I'm a little confused as to what they are saying.  When a lens is set on Manual focusing mode, Spot vs 1-Point vs Area vs etc. is irrelevant right?  Perhaps I'm reading the sentence wrong but I'm wondering why they are mentioning "automatic selection" at all when talking about Manual focusing.  Perhaps someone could clarify.

Thanks!

revup67

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Re: Why does Focal tell users to use 1-Point instead of Spot AF?
« Reply #1 on: December 23, 2013, 11:37:57 AM »
The Spot AF is ideal for macro focusing and ties in with Canon's Macro lenses such as the 100 2.8 II (and other 3rd party macro lenses I am sure).  This AF is not an everyday practical choice of AF and since you are shooting at a distance it would rule out the usage of Spot AF for macro.  Focal chose the Single AF point as its the optimum choice of Canon's AF choices for this application.

Rev
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Don Haines

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Re: Why does Focal tell users to use 1-Point instead of Spot AF?
« Reply #2 on: December 23, 2013, 11:50:22 AM »
The Spot AF is ideal for macro focusing and ties in with Canon's Macro lenses such as the 100 2.8 II (and other 3rd party macro lenses I am sure).  This AF is not an everyday practical choice of AF and since you are shooting at a distance it would rule out the usage of Spot AF for macro.  Focal chose the Single AF point as its the optimum choice of Canon's AF choices for this application.

Rev

Never knew that!

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neuroanatomist

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Re: Why does Focal tell users to use 1-Point instead of Spot AF?
« Reply #3 on: December 23, 2013, 12:53:39 PM »
Spot AF is useful in many circumstances, not optimal in others.  Many people don't realize that the actual AF point on the AF sensor is larger than the little box that represents it in the viewfinder. Spot AF restricts the region of the AF point used to an area only slightly larger than the VF box (the outer box, not the tiny inner box). Spot AF isn't any more accurate that Single Point AF, it just restricts the active area.



Spot AF is slower, so it's not a good idea for fast-moving subjects.

With a large, flat target like the FoCal printout, there's no need for Spot AF - but I wouldn't expect it to cause any issues unless you were so close to the target that the shorter length of the active AF sensor lines didn't cross an edge on the chart.

To your question on AF point selection during manual focusing, the camera has AF confirmation, meaning it uses the AF system to confirm that focus is correct with a manual focus lens.  You can actually AFMA a manual lens so the focus confirmation is accurate.  The point about 61-pt auto with AF confirmation is that it will default to confirming based on the center point only (normally, auto selects the closest subject under an AF point, but with MF it can't do that).
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entlassen

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Re: Why does Focal tell users to use 1-Point instead of Spot AF?
« Reply #4 on: December 23, 2013, 03:19:00 PM »
Thanks for the clarification.

I have a couple of questions related to the topic of AF:

1) When your lens is on manual and you select the 4-point or 9-point "expansion" AF modes, does the focus confirmation dot only care about the center point of the 4/9-points, or will it actually evaluate the surrounding expansion points?  And what about if you're in "Zone" AF mode?  I ask this because the Canon manual made a specific case about the 61-point auto selection (i.e. that the focus confirmation dot will only be evaluating the center point) and didn't talk too much about the two expansion modes or the Zone mode.  Just to clarify, I would never consider actually being in any of these modes when my lens is on manual (I'd just stick to Single Point AF), I'm just curious what the system would actually do.


2) A related question I had was regarding AF during Live View.
Does being in x1 vs x5 vs x10 magnification affect Live View's contrast-detect AF system at all?  In other words, does engaging the AF while in x10 have different behavior than engaging the AF while in x5 or x1?  (Perhaps more precise AF while in x10?)

One of the reasons I ask this question is because on the 5D Mark III, the AF box on the rear LCD screen actually changes size when I change the magnification.  I think the x5 and x10 AF box is the same size relative to the screen's size, but the x1 AF box is different.  The practical effect is that when I'm shooting, say, a focus chart on the wall, when I'm in x1 the AF box is surrounds the chart.  As I zoom in to x5 and x10, the AF box size changes and is now "inside" the chart.  Basically, the region of space that falls under the LV AF box changes between the three zooms, so I'm not sure if the AF is changing along with it or if it has no bearing at all and only affects what the user is seeing on the LCD.
« Last Edit: December 23, 2013, 03:32:05 PM by entlassen »

neuroanatomist

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Re: Why does Focal tell users to use 1-Point instead of Spot AF?
« Reply #5 on: December 23, 2013, 04:41:03 PM »
1) No idea.  Perhaps you'd like to test it and tell us?  ;)  An AF lens set to MF also has focus confirm.  My guess is it will ignore expansion points, and maybe pick the center of a zone, or just the center point in Zone.

2) My impression is that it uses the magnified view for Live View AF, i.e., a smaller area as you magnify. But I haven't tested it.
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