Does AI Servo work while the shutter is open?

[swishbrade]

Hi all. I have an idea for a frame and was wondering if the sifus of Canon on this forum can advise before I proceed to experimenting it on the field.

I have an idea of a shot of a child running towards the cam, using a 2 or 3 sec shutter speed and rear flash sync to get the child's last position in sharp focus.

Question is, will the AI servo keep the child in focus while the shutter is open? Or does it maintain the same focus point as when the shutter initially opens?

Thanks for your help. I don't have much experience with using the AI Servo function.

[RyanG]

AI Servo tracks the movement of your subject. It doesn't light up any AF points IIRC.

I only use AI Servo for panning but not always.

[Dream Merchant]

Technically, NO. There is NO actual focusing done on a moving subject during AI servo during an exposure i.e., while the shutter is open.

What the AI servo does, IIRC, is COMPUTE and PREDICT the movement of the subject based on the information it gathers during the half to one second (recommended BTW) half-shutter press to focus.

The 'tracking' and re-computation takes place at
'blindingly fast' speeds, depending on your camera model's AF system, personal set-up/customisation (if available) as well as loads of other things like subject contrast and both speed and angle of movement and how much beer and pizza is left in the buffer.

That's the simple explanation. And kidding about the beer and pizza.

I'm guessing, if your subject movement is constant and non-erratic, of sufficient contrast and EV, and you give the system enough information to work on, AI servo may be able to accurately predict where the correct focus of the subject may be at the end of the exposure. Also depends on which body you are using.

Good luck on your experiment! Please share the results of your findings if it's not inconvenient or P&C.

CHEERS!

[swishbrade]

Hi guys,

Thanks so much for your comments and sorry for the late reply. I had just come home from a full day and could only just get to my PC.

DM, can always count on you for a well thought out analysis and I will definitely post my results of my little experiment when I get to it this coming weekend. I hope to run some little tests with my son at home before getting the opportunity to try it outdoors though, so stay tuned

Thanks again.

[Dream Merchant]

Originally Posted by swishbrade Hi guys, Thanks so much for your comments and sorry for the late reply. I had just come home from a full day and could only just get to my PC. DM, can always count on you for a well thought out analysis and I will definitely post my results of my little experiment when I get to it this coming weekend. I hope to run some little tests with my son at home before getting the opportunity to try it outdoors though, so stay tuned Thanks again.

This is from my hazy recollection of reading the external technical papers on Canon's 1 series AF. There are slight ooperational differences between models, and they tend to be very specific. I do not know which model you will be using, but the base principles of how AI Servo works should be the same. I hope I remember correctly the details on how exactly AI Servo works.

[swishbrade]

Originally Posted by Dream Merchant This is from my hazy recollection of reading the external technical papers on Canon's 1 series AF. There are slight ooperational differences between models, and they tend to be very specific. I do not know which model you will be using, but the base principles of how AI Servo works should be the same. I hope I remember correctly the details on how exactly AI Servo works.

Hi DM,

Unfortunately, I am using the model with the slowest AF speed...the 5D...classic!

I had experimented with some shots last weekend while my son was zooming towards me on a flying fox rig and I am happy to report that the 5D is still able to keep up. I estimate the speed of his approach to be only about 3m/sec which is approximately 11km/h. Not too lightning fast, and I remember reading that the AI servo function works best when subjects are coming towards the camera.

The fact that I looked at the pics and they look no different than if he had just sat there, led me to find a shot which could give the viewer of the speed which he was moving at.

I did notice that the 24-70L did not produce as many keeper shots than when I was using the 85f/1.8. Are primes known to AF faster than than zooms?

Cheers

John

[Dream Merchant]

Originally Posted by swishbrade Hi DM, Unfortunately, I am using the model with the slowest AF speed...the 5D...classic! I had experimented with some shots last weekend while my son was zooming towards me on a flying fox rig and I am happy to report that the 5D is still able to keep up. I estimate the speed of his approach to be only about 3m/sec which is approximately 11km/h. Not too lightning fast, and I remember reading that the AI servo function works best when subjects are coming towards the camera. The fact that I looked at the pics and they look no different than if he had just sat there, led me to find a shot which could give the viewer of the speed which he was moving at. I did notice that the 24-70L did not produce as many keeper shots than when I was using the 85f/1.8. Are primes known to AF faster than than zooms? Cheers John

Some primes do seem to focus faster than a similar (not same) max f zoom lens. Less cams and glass and stuff to move. It depends on a specific lens and the motor used in that lens. The ring-type USM, IIRC, offers the fastest focusing. Please check, as I said, I'm recalling this from stuff I read a long time ago.

And congrats on your successful shots Awishbrade!

Edit: If it makes you feel better, the 5D classic seems to keep up very well in high-contrast situations. ;-)