How Auto focusing works ?

Autofocus is so easy to use that the focus switch on some of your lenses may never have been moved to the manual position.

Most of the time autofocusing gives excellent results. It is faster than manual focusing, and often more accurate. However, autofocusing operates by a series of rules (algorithms). There may be times when these do not give the results you want.

EOS cameras use a highly sensitive line sensor for autofocus ranging. Called BASIS (BAse Stored Image Sensor), it consists of two 48-bit line sensors and associated amplifier circuitry.

The sensor is in the base of the camera. A sub-mirror behind the camera’s reflex mirror reflects light down to the sensor. This light is split by a small lens assembly to form two separate images. One image is formed on the first line sensor, the other on the second line sensor. If there is no deviation between the two images seen by the sensors, the lens is focused. However, if the spacing of the two images is not correct, a signal is sent to the lens motor to bring the subject into sharp focus.

Earlier cameras used a single AF sensor. This sensor was positioned to focus the lens on the subject in the centre of the viewfinder image.

However, there will be many circumstances where the main subject is not in the centre of the frame. This is why most EOS models have multiple focusing points. A number of AF sensors are positioned across the image area, each taking a reading from a different part of the subject. The camera analyses these readings and then decides which focusing point to activate. The lens is focused using the information from this active point.

Most EOS professional digital cameras use a 45-point autofocus sensor.

Black: Vertical line sensors operational with lenses having an f5.6 or greater aperture.

Blue: Cross-type sensors operational with lenses having an f2.8 or greater aperture 
Vertical line sensors operational with lenses having an f5.6 or greater aperture.

Red: Cross-type sensor operational with lenses having an f4 of greater aperture 
Vertical line sensor operational with lenses having an effective aperture of f8 or greater.

What you may notice looking at this is that of the seven cross type sensors, six of them (the blue ones) are only functional with a small number of lenses, mainly the fast prime lenses and the f2.8 zoom lenses. If you are not using one of those lenses, then those cross-type sensors perform no function in autofocusing your lens.

AF Microadjustment

AF Micro adjustment allows you to move the exact point of focus slightly forwards or backwards to ensure that the camera and lens are in perfect alignment. Because of the increase in resolution of camera sensors, any slight focus mis-alignment is more visible when reviewing images. Although the cameras and lenses are made to extremely high tolerances there is a tolerance range and, in some cases, the camera could be at one end of the range and the lens at the other. In this instance, you would notice the focus point is either in front, or behind where you thought it should be. Microadjustment allows you to correct for this.


The AF Microadjustment in the EOS-1D X is able to detect a lens' serial number so you can make an adjustment by each specific lens based on serial number. If the serial number of your lens isn't detected it's possible to register a serial number for a lens within the camera menu.Micro adjustment with zoom lenses is also more comprehensive. In the past it was only possible to register one microadjustment setting for each lens. However, with the EOS-1D X it's possible to make adjustments for both the wid eangle and telephoto settings of a zoom lens.

EOS 5D Mark III AF system

The EOS 5D Mark III takes many of the features of the EOS-1D X and brings them to the EOS 5D range. The focus system in the EOS 5D Mark III is exactly the same as that found in the EOS-1D X, with one difference – where the EOS-1D X can make use of the 100,000-pixel RGB AE sensor and dedicated DIGIC 4 processor for Colour and Face tracking (EOS iTR AF) the EOS 5D Mark III does not have the same AE sensor and therefore cannot perform colour or face tracking.

EOS-1D Mark IV AF system

The EOS-1D Mark IV features a high-speed, high-precision sensor with 45 user-selectable focus points of which 39 are cross-type. Compared to the EOS-1D Mark III which featured 19 cross-type AF points, the EOS-1D Mark IV offers more flexibility in composition, especially when tracking moving subjects.

As with all EOS models, the central AF point is the most sensitive. It has a cross-type sensor that is vertical line sensitive at f/2.8 to f/4 and horizontal line sensitive from f/5.6 to f/8. With any lens that features a maximum aperture of f/4 or faster, high precision cross-type AF is available. With a lens or lens-extender combination that has a maximum aperture of f/8 or faster, autofocus will still function at the central point using horizontal line detection.

The remaining 38 cross-type AF points are vertical line sensitive at f/2.8 and horizontal line sensitive at f/5.6. If the lens in use has a maximum aperture of f/2.8 or faster, high precision focusing with cross-type sensors will be possible. If the lens’ maximum aperture if f/5.6 or faster, AF will still function with horizontal line detection.

In an upgrade from previous EOS cameras, the following lenses and lens/extender combinations will also function with cross-type AF at all 39 cross-type AF points, even though the maximum aperture is f/4:

  • EF17-40mm f/4L USM
  • EF24-105mm f/4L IS USM
  • EF70-200mm f/2.8L IS USM + Extender EF1.4x II
  • EF200mm f/2L IS USM + Extender EF2x II
  • EF300mm f/2.8L IS USM + Extender EF1.4xII
  • EF400mm f/2.8L IS USM + Extender EF1.4x II

In automatic autofocus point selection all 45 points will be used and the camera will select the AF point automatically to suit the shooting conditions. In manual AF point selection you can manually select any of the 19 cross-type AF points. Additionally, by using CFn. III-10 you can choose to limit the number of AF points that are manually selectable from a choice of using all 45, 19, 11, 9 inner points or 9 outer points.