Improved Cross Type Focus Points

One improvement that new Canon EOS 40D owners sometimes overlook is the upgrade to cross-type focus points at all nine focus zone positions (with lenses having that f/5.6 or larger maximum aperture), plus a center cross-type focus point that is extra sensitive when used with lenses having a maximum f/stop of f/2.8 or better. Why is this important? It helps to review exactly how the 40D determines focus.

The camera looks for contrast between adjacent pixels to determine relative sharp-ness—specifically, the transitions between those groups of pixels that determine the edges in a subject. At top in the extreme enlargement shown in Figure 5.10,

Figure 5.10

Focus sensors detect the increase in contrast in the edges of subjects, starting with a blurry image (top) and producing a sharp, contrasty image (bottom).

Figure 5.10

Focus sensors detect the increase in contrast in the edges of subjects, starting with a blurry image (top) and producing a sharp, contrasty image (bottom).

the transitions between pixels are soft and blurred. Even the boundary between the bright leaf and its dark background is smudged. When the image is brought into focus (bottom), the transitions are sharp and clear. You can see by the bright pixels around the edge of the leaf that it is backlit, and even some of the veins in the leaf at middle right show up. Although this example is a bit exaggerated so you can see the results on the printed page, it's easy to understand that when the EOS 40D detects maximum contrast in a subject being evaluated by the focus sensor, it is deemed to be in sharp focus.

The value of cross-type focus sensors, which can interpret contrast in both horizontal and vertical directions, can be seen in Figure 5.11. The two upper photos show a horizontal-type sensor evaluating a subject, which happens to be a piece of aged wood siding heavily creased with horizontal lines. At upper left, the sensor sees blurry lines, which become sharper when the wood is brought into focus.

Cross Type Focus Points Images
Figure 5.11 Horizontal (and vertical) focus sensors can interpret image contrast in only one direction (top), while cross-type sensors can evaluate contrast in both horizontal and vertical directions.

This type of subject is of average difficulty for a horizontal sensor: easier to interpret than an image with no pattern at all, and harder to focus than, say, vertical lines, which would stand out more clearly. You can see that a horizontal focus sensor does a good job but has some weaknesses. (A vertical-only focus sensor would have the same reduced performance with vertical lines and better focusing with horizontal lines.)

At the bottom of the figure you'll see the same subject being evaluated by a cross-type sensor. The horizontal lines are still more difficult to interpret with the horizontal arm of the cross, but they stand out in sharp contrast (even in the blurry version at lower left) and allow the camera to snap the image into focus easily, as you can see at lower right. In this example, both the horizontal and cross-type sensors were able to produce an equally sharp focus (upper and lower right), but the cross-type sensor probably focused the image a tad faster. And, in lower light levels, with subjects that were moving, or with subjects that have no pattern and less contrast to begin with, the cross-type sensor not only works faster but can focus subjects that a horizontal- or vertical-only sensor can't handle at all.

So, you can see that having nine cross-type focus sensors and one extra-sensitive center cross sensor (with faster lenses) is a definite advantage.

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