I. CAMERA FEATURESAND OPERATION 9
AFPerformance According
to SubjectContrast &
Detail
AFPerformance According
to LightLevels
When the camera isset for MFPS, the manually selected focusing point illuminates in
the viewfinder displaywhen the shutter button is pressed halfway. This occurs in both
One-ShotAF and AI Servo AF. The illumination can be turned off or made brighter with
Custom Function 10.
The AFsystem tends to work slightly faster when the focusing point is manually selected
than during AFPS, butthere is a trade-off between the wider coverage area of AFPS and
the comparativelynarrow coverage area of MFPS. Choosing between the two is largely
based on subjectmatter, lens selection and personal preference.
The 45-pointAF sensor used in current EOS-1 class cameras has been engineered to
perform aswell as possible with low-contrast subjects. However, the sensor is part of a
passive AFsystem that requires certain levels of light and subject contrast or detail in
order to function atall. For example, the camera will not focus in complete darkness or
on a blankwall or sheet of paper unless it has the benefit of an AF-Assist beam to
projecta striped pattern on the subject. (When you’re using Speedlite Transmitter
ST-E2 or an EXSpeedlite, be sure to set the camera to One-Shot AF to activate the
AF-Assistbeam.)
Low-contrastsubjects are not limited to blank walls and sheets of paper. Typically, you
willfind subject matter such as human skin and various types of single-color clothing to
be low in contrastas well. Additionally, the veiling glare sometimes encountered when
photographing backlitsubjects can degrade subject contrast when it is being evaluated
bythe camera’s AF sensor. In such cases, we suggest that you try to focus on the edge
ofthe subject rather than in the middle of it.
Asa photographer you will not always have complete control over your shooting
position, lighting direction or the detailand contrast in your subject matter.
Nevertheless, you need to be aware thatAF performance is affected by the contrast
levelspresented to the focusing sensor.
Assuming adequate subjectcontrast, autofocusing speed in the EOS System islargely
dependenton the following factors:
•LightLevels: The darker it gets, the longer it takes for the AF system to lock on to the
subject, regardlessof the lens in use.
•Maximum Aperture:In low light conditions, the faster the lens, the faster the AF,
generallyspeaking. For instance, in low light, an EF 35mm f/1.4L USM will autofocus
faster than an EF28-135mm f/3.5-5.6 IS USM lens.
•FocalLength: Longer focal length lenses like the EF 70-200mm f/2.8L USM have a
much greater range ofdefocus than standard or wide-angle lenses. In other words,
when an image goesout of focus on a fast telephoto lens, it really goes out of focus.
Thiscan cause delays or sometimes even failures in low-light AF. As a workaround,