What does AIServo do?

[TonyW]

Even after reading the Canon documentation I don't understand fully what AIServo does. On single point AF, I understand it just maintains focus on the place to which it is pointed. But otherwise, it does much more than that. Can someone explain or point me to some clear and complete description?

The camera is a 5diii. I assume that Nikon cameras have something similar.

[Colin Southern]

Hi Tony,

One-shot AF will focus and take the shot; it's the most accurate if the subject to camera distance isn't changing, but you'll get an OOF shot if it is.

Servo mode constantly re-focuses and even tries to predict focus (eg a speed boat coming towards you). You normally need to acquire AF with the middle AF point first, and then depending on the AF options set, it'll try to track the object with other AF points (eg birds in flight).

Does that help?

[Glenn NK]

Even after reading the Canon documentation I don't understand fully what AIServo does. On single point AF, I understand it just maintains focus on the place to which it is pointed. But otherwise, it does much more than that. Can someone explain or point me to some clear and complete description?

The camera is a 5diii. I assume that Nikon cameras have something similar.

Interesting last comment - the 5DIII has an excellent focusing system (much improved over my 5DII).

And yes, the manual is a bit confusing for sure. Colin's post covers it well (don't know why the manual isn't a bit clearer).

I have two bodies (30D and 5DII) - both are always set to AI Servo. In addition, I use the "*" button for focusing which for moving objects is very useful (I just hold the * button in so the camera keeps re-focusing and press the shutter button when I'm ready). You can find the setting in the menus under Custom Functions - I'm not familiar with the 5DIII menus - check the manual.

Glenn

[rpcrowe]

Some very interesting information pertaining to BIF photography - including focusing on moving objects is contained in this lengthy and comprehensive tutorial. I am linking to this website because of its involved explanation of acquiring and keeping focus on moving birds (which are IMO just about the most difficult subjects to keep in focus).
http://www.digitalbirdphotography.com/contents.html

The portions of this tutorial to which I specifically direct your attention in answer to the posting is:
2.6 Autofocus
6.5 Autofocus Modes
6.7 Selecting Autofocus Points
8.10 Photographing Birds in Flight

This is a lengthy and all-encompassing tutorial and I haven't yet digested all of it. However the images shown as examples are truly magnificent.

The only problem that I can see with this website is that you cannot link to specific chapters but need to use the next and previous arrows (in this case bird's beaks) to navigate around the website...

[Colin Southern]

Also, since the 5D3 has virtually the same AF as the 1D X, you might find this AF PDF of interest ...

https://drive.google.com/file/d/0B2g...it?usp=sharing

[dje]

Tony

I came across this link some time ago when I was looking up something or other. It is called "Canon 5D MkIII AF Setting Guide Book". It seems to have some good stuff in it.

It may well be similar to the 1DX guide Colin refers to above. I don't know, I don't move in such exalted circles, I'm only a 600D man

Dave

[TonyW]

Colin, Glenn, Richard and Dave, thanks for the replies and references. I have looked at these quickly. It looks as though the 1D and 5D have the same system and in the two documents even the pictures are the same. My immediate application for this is capturing birds in flight, where I have been dismally unsuccessful so far. Admittedly, I have tried in difficult circumstances where the birds have been not very large and have flitted about quickly.

A summary of what I have gleaned so far is:

When set to single point or single point spot AF, AIServo just keeps the focus on that point. If the subject moves towards or away from the camera, focus is maintained or, if the camera moves, the focus is changed to the new point. You have to keep the chosen focus point on the subject to keep it in focus. I have experimented with this and this seems right.

If more than one focus point is used (AF point expansion or zone AF) and focus is established on the subject at the centre point, then as the subject moves other focus points take over and focus on the subject is maintained. There are three parameters which can be set to determine the sensitivity of the tracking of the subject and how it switches between focus points.

In practice, for me, the catch is having to get the initial focus. With a bird flying around in all directions, I haven't got fast enough reactions to get it centred.

Cheers,
Tony

[Colin Southern]

Colin, Glenn, Richard and Dave, thanks for the replies and references. I have looked at these quickly. It looks as though the 1D and 5D have the same system and in the two documents even the pictures are the same. My immediate application for this is capturing birds in flight, where I have been dismally unsuccessful so far. Admittedly, I have tried in difficult circumstances where the birds have been not very large and have flitted about quickly.

A summary of what I have gleaned so far is:

When set to single point or single point spot AF, AIServo just keeps the focus on that point. If the subject moves towards or away from the camera, focus is maintained or, if the camera moves, the focus is changed to the new point. You have to keep the chosen focus point on the subject to keep it in focus. I have experimented with this and this seems right.

If more than one focus point is used (AF point expansion or zone AF) and focus is established on the subject at the centre point, then as the subject moves other focus points take over and focus on the subject is maintained. There are three parameters which can be set to determine the sensitivity of the tracking of the subject and how it switches between focus points.

In practice, for me, the catch is having to get the initial focus. With a bird flying around in all directions, I haven't got fast enough reactions to get it centred.

Cheers,
Tony

Hi Tony,

Yep - spot-on.

Stick with scenario II, but also accept that although the camera will do it's best to keep something on the bird, at the end of the day, it's not a military-grade radar-guided anti-ICBM type system. In practice it works well with a bird against a plain background, but if the background is "busy" then it can and does get it wrong. Off memory I think mine is set to "Case 5" with enhancements.

To get the initial focus I zoom out - lock on to the bird - then zoom in (using my thumb) then start shooting. I think that any BIF is always going to be "spray and pray" to a degree.