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Author Topic: About Cross Type Focus Points  (Read 3455 times)

rihanishtiaq

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About Cross Type Focus Points
« on: March 01, 2013, 01:26:26 AM »
Hellow

I need some about the cross type focus points.
The cross type focus points (combination of phase detection & contrast detection) are they mostly useful in the Live view mode for Video/Stills?
And If I consider stills only, I mean using the viewfinder will the focusing be lot faster?
For Example, the 600D/T3i' focusing system only has 1 cross type..So does this matters for stills?
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About Cross Type Focus Points
« on: March 01, 2013, 01:26:26 AM »

Drizzt321

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Re: About Cross Type Focus Points
« Reply #1 on: March 01, 2013, 01:47:06 AM »
Phase detect AF points are actually lines which detect horizontal, vertical or diagonal lines. The normal AF points are generally vertical lines which detects horizontal lines on your subject to determine focus, while the cross-type combines vertical and horizontal AF lines which will detect both horizontal and vertical lines on your subject. The dual-cross points have an additional X shaped lines across the normal horizontal and vertical AF lines, which can help get even better precision.

http://photographylife.com/how-phase-detection-autofocus-works seems to be a decent summary of how Phase detect AF works.

Now Phase detect traditionally cannot be used for AF during Live-View or Video as it requires the mirror to be down to bounce a small bit of light down into the bottom of the mirror box which is where the AF sensor sits. The way it does the Quick AF during Live View is it drops the mirror down briefly, does it's auto-focusing, then pops the mirror back up. Otherwise, during Live View it uses Contrast Detect which is a more accurate, but slower way to get the correct focus. However, you currently can't use that during video recording on Canon DSLRs, excepting the T4i. An advantage of Phase detect over Contrast is that Phase detect can tell you the direction you need to move the focus towards (near or far), while Contrast has to hunt in one direction, and if it doesn't get focus hunts all the way in the opposite direction.

Onto the T4i, that camera happens to have some phase-detect AF sensors put onto it's main sensor, which allows it to use Phase detect to help determine the direction it needs to move the focus in, and Contrast detect to fine tune it and achieve focus. This is likely similar to how the EOS-M does it, since it has the same sensor in it as the T4i.

Now, if we are talking only stills, the Phase detect which is always used when you look through the viewfinder (the mirror is down) or when using Quick Focus in Live-View is generally what is used. It tends to be faster, and usually reasonably accurate. However, if you want extremely accurate, you need to switch to Live View and use the Constrast detect AF. However, this is usually best done from a tripod, unless you have a very large Depth of Focus/Field because hand-held, especially holding it out in front of you to view the Live View, you generally are less steady and have more hand movement. This means you're more likely to move your subject out of focus, especially if it is near to the lens and/or you have a very wide aperture (small f-number).
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David Hull

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Re: About Cross Type Focus Points
« Reply #2 on: March 01, 2013, 01:50:02 AM »
Hellow

I need some about the cross type focus points.
The cross type focus points (combination of phase detection & contrast detection) are they mostly useful in the Live view mode for Video/Stills?
And If I consider stills only, I mean using the viewfinder will the focusing be lot faster?
For Example, the 600D/T3i' focusing system only has 1 cross type..So does this matters for stills?

A cross type focus point is a focus point that is responsive to detail in two dimensions.  It consists of two co-located line sensor arrays arranged like a "+" sign or an "x" that are capable of responding to details in the scene which are oriented horizontally or vertically.  A non-cross type point (visualize it looking like a “-“ sign) is only sensitive to detail in one orientation (the orientation perpendicular to the layout of the sensor array).  All of these work by phase detection.

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Mt Spokane Photography

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Re: About Cross Type Focus Points
« Reply #4 on: March 01, 2013, 11:24:13 AM »
Hellow

I need some about the cross type focus points.
The cross type focus points (combination of phase detection & contrast detection) are they mostly useful in the Live view mode for Video/Stills?
And If I consider stills only, I mean using the viewfinder will the focusing be lot faster?
For Example, the 600D/T3i' focusing system only has 1 cross type..So does this matters for stills?

You are starting out with a incorrect assumption, cross type points have nothing to do with contrast detection, they are used for phase detection.
 
Contrast Detection uses the actual image from the sensor and measures the highest contrast in the selected area to set focus.
 
Phase Detectiion AF (also referred to as fast AF is the fastest.
 
Contrast Detection is only available in Live view and often referred to as "Live AF" and its very much slower.
 
Some cameras offer a choice of either in live view, the T4i has a hybrid sensor which works differently from other Canon DSLR's.
 
The availability of additional cross points is just another feature enhancement which makes AF easier for off center subjects.  Some think its worth a lot, some don't.  I used cameras with no AF for 40+ years and did fine.  I love AF now because my vision is getting weaker.  Its a individual choice based on your skill level.
 
IMHO, don't worry about extra cross points for the first few years.  Once you get a few wide aperture lenses, then they are more useful.

rihanishtiaq

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Re: About Cross Type Focus Points
« Reply #5 on: March 01, 2013, 04:28:44 PM »
Thanks guys, for the precise replies!


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Sporgon

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Re: About Cross Type Focus Points
« Reply #6 on: March 01, 2013, 05:07:05 PM »
http://www.the-digital-picture.com/Photography-Tips/Canon-EOS-DSLR-Autofocus-Explained.aspx

Interesting article, but I'm curious to know why some lens designs AF more accurately than others. For example the 200 f2 has a superb reputation for accurate AF, but the 135 f2 is notoriously bad. I'm not referring to individual lenses front or back focusing.

The 50 f1.4 has  reputation for hunting focus, but the 40 f2.8 is pretty precise.

Any explanations ?

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Re: About Cross Type Focus Points
« Reply #6 on: March 01, 2013, 05:07:05 PM »

Mt Spokane Photography

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Re: About Cross Type Focus Points
« Reply #7 on: March 01, 2013, 05:44:35 PM »
http://www.the-digital-picture.com/Photography-Tips/Canon-EOS-DSLR-Autofocus-Explained.aspx

Interesting article, but I'm curious to know why some lens designs AF more accurately than others. For example the 200 f2 has a superb reputation for accurate AF, but the 135 f2 is notoriously bad. I'm not referring to individual lenses front or back focusing.

The 50 f1.4 has  reputation for hunting focus, but the 40 f2.8 is pretty precise.

Any explanations ?
I'm curious about what data you have to show that the 135mmL is nortorius for inaccurate autofocus or that the 50mm f/1.4 hunts.
I've had two 135mmL's which were very repeatable, and three 50mm f/1.4's.  One did hunt when I bought it used, I sent ti to Canon and they found broken parts inside and fixed it.  After that it was perfect.
 
Here is the conclusion of a photozone review of the 135mmL, and it reflects what I've seen.
"The Canon EF 135mm f/2 USM L is a highly desirable lens with excellent performance figures in most aspects. It is very sharp and contrasty straight from f/2 and it's outstanding at f/5.6. The bokeh is exceptionally smooth and buttery at f/2 and f/2.8 although it suffers a bit from bokeh fringing at these large aperture settings. Neither distortions nor lateral CAs are field relevant. The only real weakness is the amount of (visible) vignetting at f/2. Flare is generally no big issue but you should use the supplied lens hood in critical situations. The build quality of the lens is excellent and the AF performance is on a very high level. Regarding all the goodness that we've seen from this lens we have to praise it with our rare HIGHLY RECOMMENDED! "

bseitz234

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Re: About Cross Type Focus Points
« Reply #8 on: March 01, 2013, 07:40:53 PM »
same, I've heard nothing but glowing reviews of the 135, including AF performance. If I hadn't been asleep when the sale started I would have bought a refurb this week...
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Re: About Cross Type Focus Points
« Reply #8 on: March 01, 2013, 07:40:53 PM »