Canon EOS-1 - Manual (Page 9)

I. CAMERA FEATURES AND OPERATION 9
AF Performance According
to Subject Contrast &
Detail
AF Performance According
to Light Levels
When the camera is set for MFPS, the manually selected focusing point illuminates in
the viewfinder display when the shutter button is pressed halfway. This occurs in both
One-Shot AF and AI Servo AF. The illumination can be turned off or made brighter with
Custom Function 10.
The AF system tends to work slightly faster when the focusing point is manually selected
than during AFPS, but there is a trade-off between the wider coverage area of AFPS and
the comparatively narrow coverage area of MFPS. Choosing between the two is largely
based on subject matter, lens selection and personal preference.
The 45-point AF sensor used in current EOS-1 class cameras has been engineered to
perform as well as possible with low-contrast subjects. However, the sensor is part of a
passive AF system that requires certain levels of light and subject contrast or detail in
order to function at all. For example, the camera will not focus in complete darkness or
on a blank wall or sheet of paper unless it has the benefit of an AF-Assist beam to
project a striped pattern on the subject. (When you’re using Speedlite Transmitter
ST-E2 or an EX Speedlite, be sure to set the camera to One-Shot AF to activate the
AF-Assist beam.)
Low-contrast subjects are not limited to blank walls and sheets of paper. Typically, you
will find subject matter such as human skin and various types of single-color clothing to
be low in contrast as well. Additionally, the veiling glare sometimes encountered when
photographing backlit subjects can degrade subject contrast when it is being evaluated
by the camera’s AF sensor. In such cases, we suggest that you try to focus on the edge
of the subject rather than in the middle of it.
As a photographer you will not always have complete control over your shooting
position, lighting direction or the detail and contrast in your subject matter.
Nevertheless, you need to be aware that AF performance is affected by the contrast
levels presented to the focusing sensor.
Assuming adequate subject contrast, autofocusing speed in the EOS System is largely
dependent on the following factors:
• Light Levels: The darker it gets, the longer it takes for the AF system to lock on to the
subject, regardless of the lens in use.
• Maximum Aperture: In low light conditions, the faster the lens, the faster the AF,
generally speaking. For instance, in low light, an EF 35mm f/1.4L USM will autofocus
faster than an EF 28-135mm f/3.5-5.6 IS USM lens.
• Focal Length: Longer focal length lenses like the EF 70-200mm f/2.8L USM have a
much greater range of defocus than standard or wide-angle lenses. In other words,
when an image goes out of focus on a fast telephoto lens, it really goes out of focus.
This can cause delays or sometimes even failures in low-light AF. As a workaround,