auto focus speed, body vs. lens

[mfreg]

I'm just curious since I only own (1) camera body, the Xsi, What has more effect on the aquasition (sp?) of focus, the lens or the camera body itself. I realise that there will be a big difference in focusing speed between say a 50mm prime and 100-400mm zoom, but will a higher end body increase the autofocus speed. I also realise that conditions will also affect the ability of the camera to focus, but all things being equal will a 50D or a MKII aquire focus faster than my 450D ?....like I said, just curious.

mike

[acedrew]

The body will have more of an effect depending on the amount of light available. Raw speed of the focusing motor is a function of the lens. My 100-400mm actually focuses faster than most of my other lenses, due to the fact is it's a JR. version of Canon's legendary sports lenses.


Andrew Rodgers
Perfected Perspectives -- Photography by Andrew Rodgers | Andrew Rodgers (acedrew) on Twitter | Login | Facebook

[OsmosisStudios]

It depends. Like everything else photography related.

Faster lenses (larger apertures) let more light to the AF sensor. The speed at that point, especially in Canon, is the speed of the motor in the lens - USM or not - and the gearing of the motor to the focus mechanism (Macros are geared for precision, not speed!).

In good light, it's generally going to be a moot point. In BAD light, a faster lens will mean better AF.

[mfreg]

so, "all things being equal" autofocus speed is a function of the lens, not , the camera body. What about aquisition, Common sense ( if there is such a thing :-) ) would tell me a body with more autofocus points would AQUIRE focus more quickly, but what about sensors ?....do sensors make a difference in focus aquisition ?

The reason for my question. I hope to someday actually have time for this hobby and I really enjoy photographing birds, especially "in flight". I currently have the 450D body and a 100-400L series, focusing on a flying bird in a nightmare, I don't know if going to a faster prime would help or not ( if I could afford one ), so I was curious if in the future I might consider a new body.

thanks for your thoughts
regards,
mike

[OsmosisStudios]

The AF sensor system does play a role, but it's generally not that much of a huge difference unless youre using continuous focus on a moving subject.

Your comparison is far too open: there are so many factors that affect focusing acquisition and speed. Light, subject, movement, aperture, etc.

[acedrew]

The newer better (7D, 1D MkIV) have sensors that are larger, and in being larger, they are more sensitive to light. working better in the f4.5-5.6 of the 100-400L. So yes, bodies do play a part, and there is some new tech in this area. I'm sure some of it wil trickle down to the lower end cameras in the near future.

Andrew Rodgers
Perfected Perspectives -- Photography by Andrew Rodgers | Andrew Rodgers (acedrew) on Twitter | Login | Facebook

[OsmosisStudios]

Originally Posted by acedrew

The newer better (7D, 1D MkIV) have sensors that are larger, and in being larger, they are more sensitive to light. working better in the f4.5-5.6 of the 100-400L. So yes, bodies do play a part, and there is some new tech in this area. I'm sure some of it wil trickle down to the lower end cameras in the near future.

Andrew Rodgers
Perfected Perspectives -- Photography by Andrew Rodgers | Andrew Rodgers (acedrew) on Twitter | Login | Facebook

Admittedly, but it will be no better than another body with an f2.8 lens or, better yet, one of those lovely f/1.2 primes!

As I said, there are too many factors to simply say "this lens will focus faster than this other lens", empirically, across the board.

[Doug Pardee]

Originally Posted by mfreg

What has more effect on the aquasition (sp?) of focus, the lens or the camera body itself.

On the acquisition of focus? That's an entirely different question from "speed", which is what the title of the posting said.

The body is in charge of determining what to focus on, tracking the target (if applicable), measuring the misfocus, and (if applicable) projecting what the misfocus will be at the time that the shutter will be open. The 1-series and the 7D have a dedicated processor just for handling AF target selection and tracking.

The only effects that the lens have on this part of the process are:

1. in dim light, a slow lens might not provide a bright enough image for the AF sensors where a faster lens might;
2. on most Canon DSLRs, at least one AF point (the center) can measure the misfocus with 3 times the precision when the lens is f/2.8 or faster; this gives a more accurate focus.

All of the above is optical/electronic and occurs at computer speeds. You won't notice a difference in speed from the body, but you will notice better target acquisition and tracking on a 1-series body than on a Digital Rebel.

The actual process of adjusting the focus is entirely done by the lens. The speed at which the lens elements are shifted to achieve focus depends on the type of AF motor—for Canon brand lenses there are four possibilities: AFD, micromotor, USM micromotor, or ring USM—and to a lesser extent on the mechanical process of shifting the focus elements inside the lens.

One other item: if the light is dim and the AF sensors aren't getting enough light to make out an image, AF assist lighting comes into play. With the popup flash, this takes a noticeable amount of time. With hotshoe-mounted external flash units (or an ST-E2), the IR beam assist is much quicker.

[mfreg]

Thank you so much, sorry if the thread title is misleading I didn't know exactly how to word it since I'm kind of asking (2) seperate questions. I'm assuming aquasition is better with the 1 bodies because they have more than the (9) autofocus points that the Xsi has, what about the 7D, I'm not familiar with it.

thanks again, that was more of the "all things being equal" answer I was looking for.
thanks to all for your help.

regards,
mike

[TerryMC]

Body AF system determines focus lock. If there is no focus lock then even the fastest lens motor will just hunts faster.

I think most AF sensor only good up to f/2.8. There is no improvement in going from f/2.8 to f/1.2.