# What is the maximum velocity possible for autofocus (focusing speed) on a Canon 5D Mark II

This question is very specifically worded, because I lack the knowledge to formulate it in general and also proper terms. I am an amateur, so don't hesitate to improve this question with better wording.

I am interested in optimizing my position (point-of-view) when shooting (thriathlon) bike races. I have found that the autofocus works better (not so surprisingly) when the riders are passing by at wider angles in contrast to when coming up from sharper angles. In other words, shooting from the opposite side of the street works better than from the side where the riders pass by more close. I am pointing at a single rider at a time, half-pressing the trigger, then turning myself with the rider, until they are in a position I want to actually shoot. This is most of the time somewhere where they are nearest.

Some pictures are very sharp, other out of focus. The autofocus will need to keep up with the dwindling distance, and I am interested what is the max speed that a rider can have, relative to my position, while the continuous autofocus still works.

I also tried manual focus, with a guessed average distance, but this was very unreliable, as not all riders passed at the very same distance of course.

I am shooting with a Canon 100-300mm f/4.5-5.6 lens, at approx 100mm when on the same street side, and 200mm when from the opposite side, at the moment when the riders actually pass by. I have a Canon 5D Mark II. I used the AI-Servo mode, the single active focus point being the one that is most to the "back" of the rider.

What is the minimum or optimum distance (how can I calculate it) to the rider, that my autofocus still works reliably, given a steady hand.

You are also welcome to challenge my technique:

• maybe a one-shot autofocus would be better?
• Should I position myself at a slight turn?
• I could try a more inclining spot of course, with slower speeds, but this question was with a flat terrain in mind.
• Would a better lens work better in general (i.e. a larger aperture)
• I myself have recently also had an unsatisfactory experience with my camera's autofocus performance for sports. It is definitely worth bearing in mind that autofocus technology has advanced quite a bit, I would say, since the Canon 5DII was released. Nowadays it's possible to get cameras with much better subject identification and tracking capability built-in. Commented Jul 7 at 13:30
• Thanks @osullic for this bit. I am however not currently able to invest in a newer camera at the level required. I could spend a bit on used lenses though. Commented Jul 7 at 13:48
• You should probably consider trying to maximise depth of field. One way of achieving that is by reducing aperture as much as possible. This however usually means increasing exposure time, which probably isn't the best option for fast moving targets. Of course a flash would help, but I'm pretty sure the cyclists (and other people present) wouldn't like that (and you may be too far for that anyway). You probably get better results on very sunny days compared to cloudy ones, don't you? Commented Jul 8 at 23:59
• Some of the best sports photographers I know (who have been published in the NY Times and ESPN Magazine) struggle more with getting clean panning shots than anything else they do. It's HARD! Even if AF is perfect, unless the panning is also perfect there's going to be some blur. Commented Jul 21 at 9:37
• Not really an answer so I'm putting it in a comment here: What you expect is beyond the capabilities of the 5D Mark II's AF system. I shot with a 5D2 as my primary camera for about 4 years, yet did most of my sports telephoto work on a 7D specifically because it had a more advanced and more configurable AF system. When the 7D Mark II came out in 2014, it actually worked the way the 7D was supposed to. The 5D Mark III was also a massive improvement in AF performance over the 5D2. Commented Jul 21 at 9:44

The lens you are using is has a ring type USM focus motor... these are very fast.

The latest top of the line cameras do 120 AF calculations per second; the 5DII does not, although I do not know the actual rate.

In most situations phase detection AF does not predict *focus distance. Focus tracking predicts the next location of the subject (2D) but not the distance. The required focus shift is then determined by the phase offset of the AF images at that location. When the shutter is activated AF is then locked and shifted slightly based upon the focus distance change rate history, in order to account for distance change during shutter delay.

(*Some Nikons have a "3D focus" mode that does add in distance prediction in certain situations.)

The point of that is that AF is almost always slightly behind in distance calculation/determination; no matter how fast it is. So as things become more demanding with shorter distances/higher relative speeds you need to make room for more error. You can do that by reducing the relative speed (longer distance, shorter FL, slower subject), and by increasing the depth of focus (smaller aperture). Note that reducing the FL and increasing the subject distance will not make a difference if you crop in post to compensate.

Additionally, you probably want to use AF point expansion which allows the camera to use surrounding focus points to assist in tracking... IDT the 5DII has zone AF. I would also select a focus point towards the front of the cyclist, not the back. Note that AF point expansion tends to focus on the nearest point. Also, only the center focus point is of the cross type which is generally more accurate.

Finally, panning only fully assists with focus and motion on one axis; where the two paths line up. So optimally your images need to be timed to be taken in that zone. Panning also cannot compensate for simultaneous motion in multiple directions; e.g. a cyclist pumping up and down. It is very hard to differentiate small amounts of motion blur vs slight defocus, but I think motion blur is affecting both example images.

If the cyclist will pass at a known distance, it is also possible to use manual *"zone focus" instead in order to eliminate AF variability entirely. When combined with timing the shot appropriately, zone focus eliminates all variables of error.

*zone focus is simply manual pre-focus with an adequate depth of field allowance.

• Just a comment, but I think nowadays, people's expectations with regard to focus/sharpness is (unreasonably?) extremely high. Zone focus with a telephoto lens may not cut it, even with the "adequate depth of field allowance". Commented Jul 7 at 17:38
• @osullic, fair point... the ability to pixel peep at 200% zoom has certainly created higher demands. Commented Jul 7 at 18:47
• @Marcel pre-focussing manually might work better on a bend where you can predict the cyclists' line better (as they'll all want to take the same racing line). If you're on the inside of the bend their path is going round you as you pan which would also help the AF. Commented Jul 8 at 8:24
• and definitely try to focus on the front. Even in shots like this, getting what's visible of the face in the best focus is good; the DOF should then be enough for the arms. Especially when the rider is out of the saddle or on the rivet as in your 2nd image, there's additional movement near their hips. If the whole bike is rocking from side to side, i.e. in and out of focus you might need to get lucky, but the thighs will be going up and down pretty fast at high cadence Commented Jul 8 at 8:30
• @Marcel, AF point area expansion is C.Fn III -7... it adds 6 invisible "helper" focus points to the selected visible focus point. Automatic also enables the central helper points (p.80) Commented Jul 8 at 11:04

Disclaimer: I never used Canon and I never tracked high speed objects.

1. AF sensor is an image sensor albeit having low resolution. If you track moving target with camera the background starts moving and gets smudges on the AF sensor therefore making it easier for camera to decide what to focus on.

2. I do not expect larger aperture lens to work better because brightness is far from being the limitation IMO (and because DoF is already too small).

3. AF motor speed depends on how far the target is and (as you pointed out) at which angle it's moving relatively to the eyesight. If a bicycle is riding towards you at constant speed the motor will have to work faster over time. There's very simple test you can do: ask somebody to ride at constant speed towards you and do slow burst of photos. You will then see with your own eyes the limitation of AF motor (the bigger the speed the further away the rider on the last sharp photo will be). I bet it will be significantly harder to find this information online because there's no standard testing methodology for this.

You did not explicitly ask about anything other than AF and your pictures do not have good resolution but IMO you should shorten your exposure (the scarf looks smudged in first example and the print on the shirt looks smudged in the second example even though it's apparently inside sharp field) and close down the aperture (the driver does not even fully fit inside depth of field on second example). This will inevitably increase noise but you need to compromise to get sharp images.

• The smudged look may stem from the unfortunate fact that this lens has some amount of fungus. I may be able to invest in a better (used) lens, but first I want to get my technique right. Commented Jul 7 at 21:40
• @Marcel fungus might make image soft but that's definitely not what I am talking about here. Fungus cannot smudge image or shorten the sharp field unless there's a metric kilo of it inside. Commented Jul 7 at 22:50
• Regarding 2, autofocus is done with an open aperture, so the maximal aperture plays a role, not the aperture that is used in the photo. Commented Jul 8 at 11:33
• @CarstenS it does not when there's abundance of light. Commented Jul 8 at 16:13
• I agree, I only find the part in parentheses a bit misleading. Commented Jul 8 at 16:16