Canon EOS 50D Instruction Manual page 126

Using AF to Focus"*
Shooting conditions which can make focusing difficult:
• Low-contrast subjects such as the blue sky and solid-color surfaces.
• Subjects in low light.
• Stripes and other patterns where there is contrast only in the
horizontal direction.
• Under a light source whose brightness, color, or pattern keeps
changing.
• Night scenes or points of light.
• Under fluorescent lighting or when the image flickers.
• Extremely small subjects.
• Subjects strongly reflecting light.
• The AF point covers both a near and faraway subject (such as an
animal in a cage).
• Subjects which keep moving within the AF point and cannot keep
still due to camera shake or subject blur.
• A subject approaching or moving away from the camera.
• Autofocusing while the subject is way out of focus.
• Soft focus effect is applied with a soft focus lens.
• A special effects filter is used.
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