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Old 20th of February 2010 (Sat)   #1
DarthMTS47
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Default AF Points: High Precision vs. Cross-type in Low Light Question

I've searched the forums and Canon's white papers, but I don't have a good answer to my question concerning the current generation of cameras and their ability to AF in low light conditions. Let me see if I have this correct (assuming camera is set to One Shot mode):

Canon 7D - 19 AF points. Cross-type sensitive to f/5.6 (vertical and horizontal f/5.6 sensors). High-precision f/2.8 sensor center AF point.

Canon 5D Mark II - 9 AF points. Horizontal f/5.6 sensors. High-precision f/2.8 sensor center AF point.

Canon 1D Mark IV
- 45 AF points. 39 cross-type sensitive to f/5.6 horizontally and sensitive to f/2.8 vertically. 6 horizontal f/5.6 sensors. High-precision f/4 horizontally and f/8 vertically center AF point.


That being said, what kind of AF point is most sensitve in low light conditions, assuming you're using a f/2.8 or faster prime lens?

Is it the f/5.6 cross-type sensors in the 7D, the high-precision f/2.8 center AF sensor in the 7D & 5D, or any of the f/5.6 & f/2.8 cross-type sensors in the 1D? I know that the f/2.8 sensitive sensors are supposed to be more accurate with the small DoF from fast lenses, but are they any better in acquiring focus in low light? If so, why, compared to the f/5.6 sensors? To me, if a sensor is capable of focusing with a f/5.6 lens, wouldn't it actually be more sensitive to low light since the lens itself lets less light in at its max aperture?

Thanks for helping me understand.

-Mike
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Old 20th of February 2010 (Sat)   #2
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Default Re: AF Points: High Precision vs. Cross-type in Low Light Question

The best low light sensors are the high precision center point ones, which are cross type with an added adjacent sensor which is activated with a f/2.8 or wider aperture (or f/4 for the 1d4). The flat sensors are only sensitive in 1 plane - a parallel line will be impossible for it to focus on. Obviously, in low light the wider the aperture the better the AF performance - with f/2.8 or wider to get the added benefit of the high precision portion.
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Old 20th of February 2010 (Sat)   #3
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Default Re: AF Points: High Precision vs. Cross-type in Low Light Question

If the high-precision center points are best, then why does Canon list the 1D Mark IV as having slightly better low light AF (down to -1 EV)? It doesn't have the high precision "X" f/2.8 center sensor point.

Guess I'm still trying to understand it...

-Mike
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Old 20th of February 2010 (Sat)   #4
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Default Re: AF Points: High Precision vs. Cross-type in Low Light Question

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Originally Posted by DarthMTS47 View Post
If the high-precision center points are best, then why does Canon list the 1D Mark IV as having slightly better low light AF (down to -1 EV)? It doesn't have the high precision "X" f/2.8 center sensor point.

Guess I'm still trying to understand it...

-Mike
To get you to buy the 1 series, it has the best AF available. Perhaps it is the materials, software, or something else - but they put the best in the 1
Oh, and th center point of the 1 does have a high precision assist
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Formerly: 7D, 300D, 5D, 5DM2, 20D, 50D, 1DM2, 17-55IS, 24-70, 28-135IS, 40/2.8, 50/1.8, 50/1.4, 70-200/2.8IS, 70-300IS, 70-200/2.8, 100 macro, 400/5.6, tammy 17-50 and 28-75

Last edited by timbop : 20th of February 2010 (Sat) at 15:26.
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Old 20th of February 2010 (Sat)   #5
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Default Re: AF Points: High Precision vs. Cross-type in Low Light Question

something else that might be confusing you is the terminology of the "5.6 sensors" vs. "2.8". The 2.8 sensors are sensitive all the way up to 5.6; 5.6 is the minimal aperture at which any sensor can function other then the 1d's center sensor - which can work down to f/8.
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Old 20th of February 2010 (Sat)   #6
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Default Re: AF Points: High Precision vs. Cross-type in Low Light Question

The ability to focus in low light is mainly a question about the design of the AF sensor. It's a widespread misunderstanding that a lens with a larger opening is better for focusing in low light. But the AF system has its own "effective" aperture, so it doesn't matter if the lens is f/2,8 or f/2, for example. Instead, the larger apertures are interesting when forming the secondary image on the AF sensor, as a wider opening is necessary to establish a longer baseline in the sensor, and thus achieve higher accuracy.
So the question posed by the OP should probably be answered with "Use any point you like, with the camera you have".

But a general advice is that in low light, the sensors will have more trouble finding a suitable contrast to measure. Hence, a cross-type sensor is usually better, as it has twice the probability to find a contrast to work with.
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Old 20th of February 2010 (Sat)   #7
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Default Re: AF Points: High Precision vs. Cross-type in Low Light Question

The center point is the most accurate (or should be) since it receives the most light. This is why only center points so far are high-precision.
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Old 20th of February 2010 (Sat)   #8
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Default Re: AF Points: High Precision vs. Cross-type in Low Light Question

The light sensitivity of a AF point's line sensor, ignoring filters etc., is mainly governed by the size, i.e. light receiving area of its pixels - the bigger the pixels, the better the sensitivity, but the bigger the pixels the lower the inherent resolution of the AF line sensor.

Phase Detection AF is based on images taken from opposite sides of the camera main lens (exit pupil) projected by the Secondary Image Reforming optics onto said line sensors. With the subject in focus these images will be projected with a certain distance onto the line sensors, a distance referred to as "baseline". In the OOF state the image projections will be either closer together (front focus) or farther apart (rear focus) and hence the deviation from the "baseline" yield information about the amount of defocus as well as the direction.

That a line sensor is specified as F5.6 or F2.8 has to do with it's baseline, where a F2.8 sensor has approx the double baseline of a F5.6 line sensor. Back projection of the line sensors onto the main lens's exit pupil hence means that F2.8 line sensors get their images at a greater distance from the lens optical axis than do the F5.6 sensors. A F5.6 sensor will hence always be able to use a max aperture F2.8 lens whereas the opposite potentially would have the F2.8 line sensor pair projected outside exit pupil aperture of a lens with max aperture of F5.6..

The main enemy of PD-AF is vignetting, and even though the "back projection" of a F2.8line sensor might actually fit inside the exit pupil aperture of a lens having max aperture of say F4.0, using the sensor in this context might lead to vignetting of the "outer regions" of the images projected onto the line sensors, resulting in front focus.

Looking on vignetting data, you'll notice that most large aperture lenses are significantly vignetted wide open, one reason that say F1.4 baseline line sensors are not used. But large aperture lenses OTOH also mean more light, which works in the same way for AF sensors as for image sensors, i.e. faster exposure and less noise at a given shutter speed and light level.

To work out the projected image distance on the AF line sensors a certain amount of contrast (i.e. difference between the brightest and darkest part of the image seen by the line sensor) is needed, which can lead to problems with both too bright and too dark scenarios. To that comes that the latter scenario also has the problem that image noise will be added to the pixel data resulting in imprecise image distance determination (focus error)

Thus as such F5.6 line sensors are not more light sensitive than F2.8 sensors - if they use the same pixel size, but F2.8 sensors with their larger baseline have a better resolution in absolute terms, at the expense of a reduced OOF range handling capability (assuming the same line sensor length).

Along those lines, cross sensors are not more light sensitive than uni-directional sensors per se, but having the double sensor area available and the capability of reading contrast in more directions the chances of finding a sufficiently contrasty pattern at low light levels should be higher with cross point sensors.

How the 1/3 DOF accuracy with in the High Precision mode available with F2.8 AF points and F2.8 lenses or brighter is achieved , is a Canon secret. A qualified guess would however be, that since AF resolution as already calculated below the AF pixel pitch using linear or polynomial regression, the precision of this technique is enhanced by having more light available (read: higher contrast and better signal to noise ratio)...

~ hans ~
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Old 20th of February 2010 (Sat)   #9
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Default Re: AF Points: High Precision vs. Cross-type in Low Light Question

Thanks, Hans! That was extremely helpful in understanding how the sensors work. So the 1D Mark IV has more of the larger baseline sensors, so that allows it to be better in low light AF. Makes sense now.

-Mike
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Old 20th of February 2010 (Sat)   #10
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Default Re: AF Points: High Precision vs. Cross-type in Low Light Question

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Originally Posted by DarthMTS47 View Post
Thanks, Hans! That was extremely helpful in understanding how the sensors work. So the 1D Mark IV has more of the larger baseline sensors, so that allows it to be better in low light AF. Makes sense now.

-Mike
Actually the 1D series uses what's called an area sensor illustrated below:



As can be seen the distance (baseline) between the vertically sensitive (red) F2.8 blocks is about the twice that of the horizontally sensistive (blue) F5.6 blocks.

The blocks contain a number of rows or colums respectively, each subdivided into sections (line sensor pairs) corresponding to the 45 AF points.

The two central rows of the red blocks have larger pixels, and are thus more light sensitive but at the same time have a larger pitch and hence F4.0 rather than F2.8 like the rest of the rows.

This subdivision of longer columns into shorter sections is used in other Canon AF sensors as well (40D, 7D, 5D and more)

If you want to see how the 1dMklV AF sensor looks for real, consult the 1dMklV whitepaper on page 13 (and zoom in)..

~ hans ~
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Old 20th of February 2010 (Sat)   #11
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Default Re: AF Points: High Precision vs. Cross-type in Low Light Question

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Originally Posted by timbop View Post
something else that might be confusing you is the terminology of the "5.6 sensors" vs. "2.8". The 2.8 sensors are sensitive all the way up to 5.6; 5.6 is the minimal aperture at which any sensor can function other then the 1d's center sensor - which can work down to f/8.
Not so! The f/2.8 sensors only work with lenses of f/2.8 or wider. It absolutely won't work with an f/5.6 lens.

But reverse is true, though. The f/5.6 sensors will also work with f/2.8 lenses or any lens with an aperture of f/5.6 or wider.
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Last edited by PacAce : 20th of February 2010 (Sat) at 21:59.
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Old 21st of February 2010 (Sun)   #12
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Default Re: AF Points: High Precision vs. Cross-type in Low Light Question

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Not so! The f/2.8 sensors only work with lenses of f/2.8 or wider. It absolutely won't work with an f/5.6 lens.

But reverse is true, though. The f/5.6 sensors will also work with f/2.8 lenses or any lens with an aperture of f/5.6 or wider.
uh, no. I absolutely can use a 100-400 at 400mm (f/5.6) with any of my af points of any of my cameras
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Old 21st of February 2010 (Sun)   #13
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Default Re: AF Points: High Precision vs. Cross-type in Low Light Question

You aren't using the f2.8 sensitive part of that AF point, though when using slower lenses. On the 1D series, the f2.8 sensitive sensors become line points with f5.6 lenses or cross types with f2.8 lenses. On the 20D, all are f5.6 sensors except the center activates an extra X shaped f2.8 sensor when using faster lenses. On the 300D, all points are f5.6 only standard precision points.
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Old 21st of February 2010 (Sun)   #14
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Default Re: AF Points: High Precision vs. Cross-type in Low Light Question

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Originally Posted by DarthMTS47 View Post
Canon 5D Mark II - 9 AF points. Horizontal f/5.6 sensors. High-precision f/2.8 sensor center AF point.
Minor correction: That is probably the most common misconception about the 5D series. It does not share the same AF system you see on the EOS 20D or the Digital Rebels. The actual specs for the 5D are:
  • 15 total AF points
  • 9 selectable AF points
  • 1 Cross Type
  • 3 High Precision Points (with f2.8 lenses)
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Old 21st of February 2010 (Sun)   #15
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Default Re: AF Points: High Precision vs. Cross-type in Low Light Question

.. Perhaps a principle illustration will make things more clear:


From left to right, the camera main lens (exit pupil), shown for a large aperture, say F2.8 and a smaller aperture (dashed). Next the SIR optical system here reduced to one lens. To the right two a pairs of line sensors one pair with about twice the baseline of the other.

It should be fairly clear why a F5.6 baseline sensor will work on a F2.8 lens, but NOT the other way around..

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