Cross type focus points?

[Goldcoin79]

What are cross type focus points? When I compare cameras on compare websites some cameras have 9 cross type focus points and some have 1. I don't realy know what they are and need to know if I need more than one as the camera I am interested in getting has all I want but has 1 cross type focus point but I don't know if this will cause any issues or not.

your advice would be appreciated.

 

[Patrice]

Cross type focus points are better at phase detection in a greater variety of circumstances.

 

[KmH]

Understanding Camera Autofocus

 

[ph0enix]

You can get sharp photos regardless of how many cross type points your camera's AF system has. The key will be your technique. That's what beginners usually struggle with.

 

[KmH]

That plus understanding depth-of-field and how depth-of-field relates to focus.

 

[TCampbell]

A DSLR normally uses "phase detection" auto-focus. A prism splits the image so that anything not in focus will also be "out of phase".

Imagine taking a print, a cutting it two with a pair of scissors, then place the two halves of the image together so that they don't quite line up -- they're slid sideways. That's "out of phase". If you align the two halves then they are "in phase".

The phase detect sensor uses a prism which which splits and mis-aligns each point on the auto-focus system so that the image is out of phase UNLESS the focus distance is correct... then it will be in-phase.

The sensor is inspecting the pixels along the edge where the prism splits. If it notices a good match then it knows the image is focused at that point. The focusing system is extremely fast.

What the focus system is really trying to find is contrast. Suppose I photographing the top rail of a fence. As you follow the top rail from left to right, you would notice the fence takes a bit of a jog... the two halves wont line up. If you focus, it will line up. This alignment or misalignment is only visible to the focus sensor... not to you, The prisms are behind the reflex mirror, so the image YOU see in the viewfinder wont show this misalignment. If you had old (pre 1980's) camera with split-prism focusing then you would visually see this misalignment in your viewfinder.

While the system works well, it is possible to trick it... or at least make it struggle. For example... if instead of pointing to the fence rail, we point to the fence post (so now we have a vertical object instead of a horizontal object) it may not accurately track focus because the size of the focus sensor is limited and the fence post runs in the same direction as the split in the focus sensor. It really needs something that would provide contrast when running across the split... not along the split.

And THAT is where "cross type" points come in to play. A cross-type focus point has a prism that splits the light both horizontally AND vertically. Now it does not matter if your edges are horizontal OR vertical... the AF sensor will lock focus more accurately and faster.

Phase detect focus points need a lot of light to work correctly. They can generally work with any lens that can provide f/5.6 or lower while focusing (note that you can shoot at higher focal ratios because the focus is always done with the aperture blades completely retracted to their "wide open" position. They only stop down to the shooting aperture when you press the shutter button to take the shot.) Cross-type points and focus assist points may only come in to play when faster focal ratios are available (e.g. f/4 or f/2.8).

Once you learn HOW the focus system works, you know when you can have confidence that your camera is accurately locking focus faster and you also know how to help your camera if it's struggling to lock focus. If I'm taking a photo of very tall grades and reeds (lots of vertical stripes) I can help a vertical single-axis focus point lock focus by turning the camera SIDEWAYS (so the focus axis runs against the direction of the grasses) let it lock focus by half-pressing the shutter, then twist the camera back. Often you can find *something* to lock focus on without having to twist the camera.

I have a Canon 5D III (fantastic focus system) and a 5D II (a... not so fantastic focus system <ahem>). The 5D II only has a single center cross-type point. I didn't have very many missed focus shots because I knew how my focus system worked. I knew when I'd need to help it. If you're not in a hurry then they're all good focus systems.

If you're a pro sports photographer or wildlife photographer shooting action, then you don't have the luxury of time on your side... the focus system NEEDS to be fast. But that means you need f/2.8 glass to make sure you can take advantage of the focus system. It'd be tragic to buy a camera with all cross-type focus points for sports ... and then buy variable zoom lenses where f/5.6 is the best you can do wide-open and your high end focus system is back to struggling as though you don't have any cross-type points.