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AF (Auto Focus), focusing subjects automatically, is a convenient function with certain cameras. However, as a limitation of AF accuracy, the best focus point sometimes may shift forward and back a small amount.
The AF accuracy of the Canon digital SLR camera is judged with the same viewing prerequisite as regulated in film cameras: "AF accuracy is judged when print out the whole image and view it from a normal viewing distance."
When using digital cameras, it is easy to magnify or print out just a part of the image once downloaded to your computer. This can magnify small errors in AF that would otherwise not be apparent when being viewed at a more traditional print size.
You are seeing slight errors in AF magnified many times over when they would not normally be visible when the image is printed.
Even if small errors in AF are visible on the computer monitor, these errors may not be visible when the image is viewed as a full size print.
Tips for taking sharp and crisp images
For your information, below are a few tips to help you take sharp images:
1. Remember that AF may fail to focus certain subjects.
AF has difficulty in focusing certain subjects. In this case, try to focus on different point of the subject (i.e. an area with greater contrast), or use Manual Focus (MF).
2. Take advantage of deeper depth of field.
"Depth of field" is the range where acceptable focus achieved in front of and behind the point of focus. The depth of field is changed by both the aperture and focal length of the lens. Using deeper depth of field will help avoid getting out of focus images.
Stopping the aperture down (smaller aperture) makes the depth of field deeper.
If you use a zoom lens, move the zoom control towards wide-angle (shorter focal length) in order to get a deeper depth of field.
3. Shoot carefully without camera shake.
When the camera shakes, the image becomes blurred. Sometimes an image with camera shake may look like an out of focus one. Shoot carefully without camera shake. For further information, please refer to "
How do I prevent camera shake?" below.
4. Use a sharpening filter in image retouching software.
It is possible to correct the sharpness and contrast of the image after shooting by using the retouching software such as Adobe Photoshop Elements on your computer. However, you need to be careful about the correction because over-correction may cause a deterioration of image quality.
It is also possible to adjust the amount of the correction by changing settings of Picture Style of the camera before the image is shot and recorded as a JPEG images.
For more information, please refer to the instruction manuals that came with your camera.
5. Avoid magnifying and printing just an extremely small area.
Magnifying and printing an extremely small area multiplies slight inaccuracies of focus or the noise of pixels of the image.
It is recommended to crop a larger area when cropping the image.