Whats crosstype autofocus points and how to use them?

[julianliu]

I heard DSLR uses cross type sensor focus points and they are supposed to be Better than regular focus points. I did some research and found the technics is confusing. Can someone explain it in plain English and how to use it? I usually just use one point focus point and am I using the crosstype focus point? I use Nikon DSLR. Thanks.

 

[robbins.photo]

In plain English? Eh.. maybe. I'd probably have a better go of it in Italian, but sure, here goes.

Ok, when your dealing with a focus point, what it does it a nut shell is to split the light coming in from each side of the lens - it's split into two different paths and projected onto tiny linear sensors, the light is analyzed for patterns of light and dark (contrast) and then that information is fed into the little computer in the camera (called the AFP, autofocus processor) which uses a formula, consults it's internal magic 8 ball and blam, calculates the correct lens setting to achieve focus, then it will usually take another reading, check to make sure if focus has improved according to it's contrast readings, and repeat if necessary.

A non cross sensor uses just two strips laid out horizontally, so that's all the data the AFP gets to work with - contrast readings from two horizontal strips. A cross-type focus point also has two more laid out vertically as well -which adds a lot more information and makes the whole thing both faster and more accurate - because now the AFP can consider information from both the horizontal and vertical strips. Generally makes the system faster because it has to go through fewer cycles adjusting the lens to proper focus.

 

[robbins.photo]

I heard DSLR uses cross type sensor focus points and they are supposed to be Better than regular focus points. I did some research and found the technics is confusing. Can someone explain it in plain English and how to use it? I usually just use one point focus point and am I using the crosstype focus point? I use Nikon DSLR. Thanks.

Sorry, missed the second part of the question, to which the answer would be.. umm.. maybe. Lol.

It depends on what type of Nikon camera body you have as to whether or not it is equipped with cross type focus points, in general if it is the cross types are usually clustered in the center of the camera's focus points. Most of the newer Nikon models have a good selection of cross type focus points, I'm pretty sure my D5200 has a total of nine. Really from a practical standpoint they generally don't make a huge difference until you start dealing with situations where your lighting isn't that great, in a case like that since you don't have enough light to really give you good enough contrast the cross type focus points will allow the lens to focus properly without spending a lot of time hunting since it has more info to work with than the standard focus point.

 

[TCampbell]

If you've ever looked through an old 35mm film SLR you may have noticed that the focus aid might have had a circle split in two halves -- referred to as "split prism focusing" -- this was a manual process. If I point the camera at a subject's face, one half will appear slightly higher than the other (or if it's horizontally oriented, one half will be shifted left and the other half shifted right.) "Phase Detect" auto-focus is an electronic variation on this idea.

Imagine taking a pair of scissors and cutting a photo into two halves (left and right) and slightly mis-aligned the two halves so that they do not match up. As you slide the two halves up or down, eventually at some point the two halves align. When phase detect focusing does this technique, it isn't on the "whole" image... it's just on a tiny spot within the image at the location of that particular focus point. But when the two halves of the phase "match" then it indicates that the lens is correctly focused for the distance of that particular object.

Now imagine that the photo is a white picket fence and that the focus point is aligned vertically. If you look just at a tiny spot around the focus point, you've got two halves of a "white" picket. The problem with this is that there's not enough contrast do detect if the image is correctly focused because you have two "halves" of a white spot. They appear to be in focus no matter how much we slide these halves up or down. This confuses the camera because the subject doesn't have enough contrast in that vertical orientation.

But imagine doing the very same thing... but cutting the image in half in the horizontal direction. Now it would be very clear that the "tops" of the pickets are not aligned with the "bottoms" of the pickets.

The "cross type" focus point actually has prisms that split the light in two axes simultaneously rather than just one or the other. This means it is increasingly likely that the camera can lock focus on objects that would have been challenging with a single axis system.

BTW... it's not necessarily accurate to say that DSLRs use these "cross type" points. Most probably have at least one cross-type point (if they do have a single cross type point it's usually the center point). Some only have one point and the rest of the points are single-axis points. Some have have all cross-type points... some have a mix of many cross type as well as many single-axis type points.

One more thing... the camera can actually check the "phase" on all the points at the same time. What's more, it knows if the focus is too close or too far to a point based on the direction of phase shift. That means that when you frame up a shot to focus, the camera can immediately tell which AF point is over a subject which is nearest to the camera AND if the camera's current focus distance is too near or too far AND it can even tell how much. Consequently, when the camera focuses, it tends to just snap the lens into focus on that focus point very quickly (assuming the subject has enough contrast and enough light).

It would be a true generalization to say that all DSLRs tend to use "phase detect" focusing systems... but there are both single-axis and double-axis (cross-type) variants of the implementation and not all DSLRs use the cross-type points (although most tend to have at least one cross-type point.)

 

[Designer]

..am I using the crosstype focus point? I use Nikon DSLR.

Which model of Nikon? If it looks like a little cross, then you have it. Not all of them have those points. The higher cost cameras do.

The entry level (like mine) do not have cross-type focus points. Mine has little rectangles to signify the focus points.

 

[KmH]

Cross-type AF points are able to detect the contrast caused by both horizontal and vertical edges. Cross-type AF points deliver better accuracy than regular AF points..
Regular AF points are single line sensors that detect the contrast caused by horizontal edges or by vertical edges depending on how they are oriented. Single line sensors oriented vertically detect horizontal edges. Single line sensors oriented horizontally detect vertical edges

Understanding Camera Autofocus

Nikon's 11 focus point Multi-CAM 1000 AF module has 1 cross-type, the middle one. (Entry-level - D80, D90, D3XXX, D5000, D5100)

Nikon's 39 AF point Multi-CAM 4800 DX AF module has 9 cross-type AF points arrayed in the middle area of the viewfinder. (Entry-level - D5200/5300, D7000/7100, D600/610)

Nikon's 51 AF point Multi-CAM 3500 DX/FX and Advanced Multi-CAM 3500 DX/FX AF modules have 15 cross-type AF points arrayed in the middle area of the viewfinder.

 

[tecboy]

I found this article. It is regarded to higher end canon models. Canon DLC: Article: EOS-1D X and EOS 5D Mark III "High-precision" and cross-type AF points

 

[julianliu]

Thanks everyone for answering my question. These helps a lot.
I use Nikon D7000 for now. So it has 9 cross type points and they in the center. But it seems these 9 focus points are no different than other regular focus points on the perimeter. Am I right?

 

[robbins.photo]

Thanks everyone for answering my question. These helps a lot.
I use Nikon D7000 for now. So it has 9 cross type points and they in the center. But it seems these 9 focus points are no different than other regular focus points on the perimeter. Am I right?

They will all appear in the viewfinder as squares (they do on my 5200's too), the 9 in the center are the ones that are cross type points. While there isn't a visible difference in the viewfinder the 9 points in the center will do the best job for you as far as focusing in low light or low contrast conditions such as mentioned above, where the horizontal contrast is such that it makes it difficult for the camera to lock focus using that alone. I don't find that happening often personally, normally the cross focus points for me really only seem to make a noticeable difference in low light since that's a more common shooting situation that I find myself in.

 

[tecboy]

I think sometimes less AF points can be helpful, easier, and faster to compose the subject with sharper focus. If you want to have the subject on the left, a cross-type AF point on the left will focus it when you press the shutter button half way. Once the focus is locked, you can still move your dslr to fine tune the composition. Maybe, the subject is too far on left, so you move the dslr to the right just a little bit. Remember, the AF will focus on the subject that is closer to your lens.

 

[Big Mike]

Thanks everyone for answering my question. These helps a lot.
I use Nikon D7000 for now. So it has 9 cross type points and they in the center. But it seems these 9 focus points are no different than other regular focus points on the perimeter. Am I right?

In terms of what you have to do, there is nothing different or special. It's just that the camera does have the ability to use those cross type sensors, so it has the ability to AF easier/quicker. It's sort of like having a car with a V6 rather than a 4 cylinder engine. You don't have to drive any differently, but when you step on it, there is more power at your disposal.

I don't know if anyone mentioned it, (or if it's even still something to be considered) but with some of my older cameras, the cross type AF sensor was only active/useful when using a lens with a max aperture of F2.8 or larger. So if I was using a typical kit lens, it wouldn't matter that the camera had a cross type sensor because it wouldn't work (or wouldn't work as well) with a max aperture of F3.5-5.6.

 

[julianliu]

In terms of what you have to do, there is nothing different or special. It's just that the camera does have the ability to use those cross type sensors, so it has the ability to AF easier/quicker. It's sort of like having a car with a V6 rather than a 4 cylinder engine. You don't have to drive any differently, but when you step on it, there is more power at your disposal. I don't know if anyone mentioned it, (or if it's even still something to be considered) but with some of my older cameras, the cross type AF sensor was only active/useful when using a lens with a max aperture of F2.8 or larger. So if I was using a typical kit lens, it wouldn't matter that the camera had a cross type sensor because it wouldn't work (or wouldn't work as well) with a max aperture of F3.5-5.6.

Thanks. I like your car engine analogy. But are you sure cross type focus does not work aperture smaller than F2.8? That would suck as most of my photos are not shot that wife open.

 

[astroNikon]

So is it better to keep your focus point(s) in the middle to use the cross types?
In sports I normally use focusing dynamic 9 pushed off to the side. Does this affect it's ability of "better" focusing ?

 

[Big Mike]

Thanks. I like your car engine analogy. But are you sure cross type focus does not work aperture smaller than F2.8? That would suck as most of my photos are not shot that wife open.

Focusing is always done before the lens stops down. So you are always focusing with the lens at it's maximum aperture. Therefore, a lens that has a larger maximum (F2.8), is getting more light (to focus with) than a lens with a maximum of F5.6.......even if you are shooting at F16 on both lenses.

 

[Tinderbox (UK)]

Some mirrowless camera have phase detection built on the sensor are these cross types or non-cross as i never see it mentioned in the specification.

John.