'Cross type focusing point' - make much difference to focusing accuracy? (1 Viewer)

Hello - wonder if some people might be able to give me some info on 'Cross type focusing points'. My EOS 500D has a 'f5.6 cross type focusing point at the centre' - does that imply that this focusing point only works as a cross type when my lens is at f5.6 (I use the Canon 400mm f5.6L)? What exactly is supposed to be the advantage of cross type focusing points? focusing speed? accuracy?
I only ever use the centre focusing point when photographing birds - I focus as precisely as I can on their eye - so what might be the advantage of a focusing system (say like the 40D) where all the focusing points are cross type?
regards Pete

In simple terms, cross-type AF points are more sensitive than non cross-type (vertical) AF points - they can detect contrast in both horizontal and vertical planes, whereas vertical AF points are much less sensitive to contrast in the horizontal plane.

A good read - scroll down to "Number and Type of AF Points".

The reference to the aperture indicates the maximum aperture at which a sensor will function at full sensitivity, so your 400mm will allow the cross-type sensor to work properly. If you were using (say) an old mirror lens - these usually have a max aperture of f/8 - you wouldn't get the full benefit of the cross-type sensor - it'd revert to operating in vertical mode only.

Note that even if you were to stop your lens down to f/8, the AF sensor will still work at full sensitivity, because regardless of the aperture you might dial in, a camera will always meter and AF with the lens at its max aperture (f/5.6 here), stopping it down only as the shutter is activated.

There are some extra-sensitive AF sensors which are in a "double cross" configuration - like a Union Jack, really - and which will only function with lenses having a maximum aperture of f/2.8. Same logic applies.

A camera with all cross-type AF sensors will simply allow you to get a better AF lock with the peripheral points, more reliably (and generally more quickly), than you'd get from a camera with only a central cross-type AF point, and vertical AF sensors everywhere else.

I can confirm that I noticed a clear improvement in the sensitivity of the peripheral AF points when I moved from the 30D (same configuration as the 500D) to the 40D, and having 19 cross-type AF points in the 7D is a joy...

Cross type focusing means that it uses two planes of contrast to focus (see my attempt on paint beneath), basically autofocus works by breaks in the continuum of contrast, so if a subject is in the same orientation as the plane of contrast the lens will hunt. A cross type sensor can use both planes so as long as there is a difference in contrast on one plane it should lock on.

Hope that helps

Ryan

Firstly - thank you both for such helpful replies - Keith your explanaton has really clarified things - I was concerned that stopping my lens down to say f8 meant that I would lose the cross type functionality but now I understand that I do not - which is good. Also - the tutorial was a good read - helped a lot. Ryan - your little graphic is nothing short of a masterpiece! summed up the concept perfectly.
Just a couple of other questions (sorry!) - Keith you say that having the 19 cross type sensors in the 7D is a joy but may I ask in what kind of situation do you use anything other than the centre sensor? I only ever use the centre sensor in bird photography - I can see that if there was a small central cluster of cross type sensors that might really be useful but the really peripheral ones - do you ever use those when shooting birds?
Also - I only ever use 'one shot' auto focus - would 'servo (continuous) autofocus be better even when shooting non-flying birds (after all they are in continuous motion even when perched)?
I am asking these questions because most of my pictures fail because the bird is just not sharp enough. Now I know that there are a lot of parameters which control 'sharpness' but obviously focus is the most important. I suspect that the main reason is that I am not close enough to the birds and I am thus having to crop too heavily but anything which I could do to optimise focusing would perhaps help, regards and thanks again Pete

wilfredsdad said:

Just a couple of other questions (sorry!) - Keith you say that having the 19 cross type sensors in the 7D is a joy but may I ask in what kind of situation do you use anything other than the centre sensor? I only ever use the centre sensor in bird photography - I can see that if there was a small central cluster of cross type sensors that might really be useful but the really peripheral ones - do you ever use those when shooting birds?

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I use the peripheral sensors all the time, Pete - for composition.

It's easy to demonstrate than to explain: in each of the attached images, which are as near as makes no difference full frame, the active AF point is (more or less) on the bird's eye.

wilfredsdad said:

Also - I only ever use 'one shot' auto focus - would 'servo (continuous) autofocus be better even when shooting non-flying birds (after all they are in continuous motion even when perched)?
I am asking these questions because most of my pictures fail because the bird is just not sharp enough. Now I know that there are a lot of parameters which control 'sharpness' but obviously focus is the most important. I suspect that the main reason is that I am not close enough to the birds and I am thus having to crop too heavily but anything which I could do to optimise focusing would perhaps help, regards and thanks again Pete

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I use AI Servo exclusively, Pete - I've literally never used any other mode, regardless of what I'm shooting, and it seems to work out. I just think that AI Servo better suits my shooting style (I only ever shoot handheld) and the things I photograph.

Hello keith - thank you again for your very helpful reply - very useful paractical information - I shall go over to servo focusing and see how I get on - and I shall try using different focusing points - see how I get on there. One of the reasons for that question is because I am hoping to upgrade my camera and it's a choice between the 550D and the 7D - obviously the 7D is a far superior camera in broad terms but it's a lot mor money - I just needed to know whether the more sophisticated focusing system is actually of any real benefit in bird photography. I think from what you have said (and a lot more other BirdForum members) the 7D is the way to go (maybe as well by the time I have saved up the dosh it may be coming down in price a little).
Having said all of that, I can see from your photos that you are obviously getting a lot closer to the birds than I am managing, I am experimenting with portable screens and camo - seems to be helping (all tho I have on a couple of occasions terrified the natives when I have emerged from the undergrowth draped in camo netting and looking like the SAS!), kindest regards Pete

wilfredsdad said:

Firstly -
may I ask in what kind of situation do you use anything other than the centre sensor? I only ever use the centre sensor in bird photography

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Apart from for composition as Keith has said you sometimes have to use one of the outer points in order to get the whole bird in the frame (with focus on the eye). I often find this when shooting in the garden, if you use the centre point then, say, the birds tail or legs could be cut off if you use the centre point.

I also use AI Servo mode just about all the time for birds - one thing I find is that if you move from one stationary target to another nearby then you sometimes need to 'bump' the focus in order to pick the bird up - I use back button focusing and bumping the focus becomes second nature to me when using the back button. You also sometimes need to do this for flyers when you momentarily lose the bird.

Roy C said:

Apart from for composition as Keith has said you sometimes have to use one of the outer points in order to get the whole bird in the frame (with focus on the eye).

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Aye, this is very true (although still "composition" in my book). The attached is an example.

Keith Reeder said:

Aye, this is very true (although still "composition" in my book). The attached is an example.

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Yep, still compo but a bit different from just using it to give you nice 'rule of thirds' type compo where the bird is not near to filling the frame. Gave you an opportunity to post the example though Keith

Thanks again Roy and Keith - very helpful and much appreciated - 'back button focusing' 'scuse my ignorance but what exactly is that - regards Pete

Keith Reeder said:

http://www.learn.usa.canon.com/resources/articles/2011/backbutton_af_article.shtml

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I have read the above article on back button focussing with some interest.However on my 7D there appears to be no custom function that you can set that will totally separate the shutter button from AF focussing using a back button,although the article says that this is possible on most Canon cameras and lists the various options on one of the custom function numbers.
Any further thoughts?
Tom Lawson.

Definitely doable on the 7D, Tom - if nobody gets back to you before this evening, I'll write again when I get home,

Tom Lawson said:

I have read the above article on back button focussing with some interest.However on my 7D there appears to be no custom function that you can set that will totally separate the shutter button from AF focussing using a back button,although the article says that this is possible on most Canon cameras and lists the various options on one of the custom function numbers.
Any further thoughts?
Tom Lawson.

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Tom, on the 7D it is custom function C.Fn IV -1.Its the second one down marked AF-ON that I use. You have four choices with this button, I use 'Metering and AF start'. You also need to set C.Fn IV-1 so that the shutter button is not 'Metering and AF start'. I think you can also assign other buttons for this as well but AF-ON suits me fine.

Roy C said:

Tom, on the 7D it is custom function C.Fn IV -1.Its the second one down marked AF-ON that I use. You have four choices with this button, I use 'Metering and AF start'. You also need to set C.Fn IV-1 so that the shutter button is not 'Metering and AF start'. I think you can also assign other buttons for this as well but AF-ON suits me fine.

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Roy,many thanks for the information.I learn something new every day! I should have looked more closely at the details of the custom functions in the instruction book.I really do like the 7D,and got some really nice pictures with it using the 100-400 lens on a recent birding trip to Ethiopia.Much improved on my previous efforts with the 50D.
Tom.