What is a cross point?

Leftyplayer · Aug 14, 2011

Camera information mentioned the number of AF points, but also the number of "cross points". What exactly is the difference between a regular old AF point vs. cross point -- or rather, AF points that are cross points. And would a camera with 39 AF points only one of which is a cross point be more or less desirable than one with 15 AF points that are all cross points. And do the number of AF points really matter that much when you tend to set only point in order to choose exactly where the focus should be? Is it only important when one sets them all and allows the camera to choose where to focus?

TIA for any enlightenment.

Kerbouchard · Aug 14, 2011

A cross point can detect contrast on both a horizontal and a vertical axis. For instance, a vertical focus sensor cannot really focus on a vertical object. It is looking for contrast between the top of it's point to the bottom. Same for a horizontal point, can't really focus on something that isn't changing from left to right.

A cross point can focus on poth horizontal and vertical lines of contrast. I would go with 15 cross points before 39 single points. Less clutter in the view finder and each of the focus points would be more useful.

The distribution of the points is another factor. I don't care if I can have 400 focus points near the center of the view finder. If there isn't a good distribution, the amount of focus points is more of a marketing ploy than actually useful.

Bitter Jeweler · Aug 14, 2011

Some focus points measure contrast horizontally, some only vertically, a cross points measure both ways.

Leftyplayer · Aug 14, 2011

THANK you! I've been wrecking my brain to understand this and know it's significant. I appreciate the quick and clear responses : )

Josh66 · Aug 15, 2011

Cross points are also generally better in low light - they might be able to lock focus when 'regular' points will fail (and just hunt for focus forever).

AUZambo · Aug 15, 2011

Since the original question has been resolved, I'm gonna slightly derail the thread.

Do any of you actually use the focusing points not in the center of the viewfinder? If I need to focus on an object that will be on the left or right side of the image I just center it and focus, then move the camera while holding the focus to snap the shot.

Is there a disadvantage to doing this over selecting the far left or right focusing point?

Kerbouchard · Aug 15, 2011

AUZambo said:
Since the original question has been resolved, I'm gonna slightly derail the thread.

Do any of you actually use the focusing points not in the center of the viewfinder? If I need to focus on an object that will be on the left or right side of the image I just center it and focus, then move the camera while holding the focus to snap the shot.

Is there a disadvantage to doing this over selecting the far left or right focusing point?

If you are using a very shallow depth of field, or the shot requires critical focus, there are times when the movement to recompose can actually move the focus plane far enough so that your shot will be out of focus.

Edited: Forgot to add, what is under your focus point also influences your flash exposure when in a TTL mode...another reason focus and recompose can get tricky.