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Understanding Auto Focus Sensors

With SPTs Canon Service Adjustment Software you can see and test each AF Sensor.  Watching the AF Sensors live output gives an easy test and understanding of how AF Focus Sensors work and why.  


AF Points and AF Sensors

  • AF Points are the AF brackes you see in the viewfinder.  

Looking at viewfinder image below, we can count 19 AF Points in the viewfinder for this camera.  These are the Auto Focus positions choosen by the photographer using their eye or Selector.  Different models use more or less AF Points .. depending on design.




  • AF Sensors are the Sensors under the mirror box used by the camera for Auto Focus.

We will explain in this article why this camera has 49 AF Sensors for those 19 AF Points.  Different models use more or less AF Sensors .. depending on design.



How Auto Focus Sensors Work

  • AF Sensors are electronic rangefinders.  
  • Each AF Sensor has two separate cells to create an electronic rangefinder.
  • Each AF Sensor is covered by a beam splitter that splits the image into two; one for each cell.

  • Each cell is photovoltaic.  Light produces a voltage.

Image contrast is needed to produce a wave.  The highlight creates the top of the wave and the shadows create the bottom of the wave.

  • Since there are two cells in each AF Sensors, two separate waves are created.


Types of AF Sensors


Now lets take a look at a camera using SPTs Canon Adjustment Software.  What we see is the 19 AF Points + 5 Sensor Groups.  The camera uses the different AF Sensors Groups in the same AF Points for focus targeting.   

  • Sensor Group #1 is the Horizontal Line Sensing Group, f5.6 for all sensors.
  • Sensor Group #2 is the Vertical Line Sensing Group, f5.6 for all sensors.
  • Sensor Group #3 is the Horizontal Line, Extreme Defocus Group for the center sensor.
  • Sensor Group #4 is the Diagonal LR ( Line) Group, f2.8.
  • Sensor Group #5 is the Diagonal RL ( / Line) Group, f2.8.


There are two or more AF Sensors on all AF Points, a total of seven in the center AF Point.  Typically cameras place the most AF Sensors in the center.  


For this camera there are a total of 47 different sensors used by 19 AF Points.  Different models use more or less AF sensors, AF Points and AF configurations.  Notice that the center AF Sensor has twin sensors in one group.  Other positions also twin sensors.


insert  70D AF sensor video .. with sensor waves on  .. under construction



The camera checks the focal length of the lens, the active AF sensor(s), identifies the type of target (Horizontal, Vertical, Diagonal) and the F/stop when it calculates focus.  This is the same math taught in Photography programs, without the hassle of doing the math yourself.



Phase Shift

Each AF Sensor has two individual cells covered by a beam splitter.  The beam spitter divides the image into two parts, one for each sensor.  The defocus split creates what is known as Phase Shift.  


SPTs Canon Service Adjustment software displays the voltage from each cell as a line, one red / one blue.  




Focus Direction

Its well know that when a lens is defocused highlights and shadows blur into each other.  The direction of the blur (highlight into dark and the opposite) depends on if the lens is focused too near (back focus) or too far (front focus).  


Because the AF Sensor is measuring a split image, the Phase Shifts right or left telling the camera which direction to move the lens.



Lens is Front Focused



Lens is Back Focused



The camera uses all the information mentioned and calculates the direction and rotation of lens.  After Auto Focus is finished, the camera rechecks focus by looking at the Phase Shift and does adjustments if necessary.





inter video .. lens focusing while watching phase shift .. under construction



We will cover other aspects of Auto Focus in other later Technical Notes.

All our Canon Service Adjustment Software offers Help files linked to that Service or Adjustment Function!