Does 50D focus better or faster with f2.8 lenses?

Let me preface this with I have had my 50D only a few months and I previously had a 20D. I took up photography when my daughter took started cheerleading at 8 yrs old. Since I had to sit through Youth Football games and Cheer Competitions I needed something to keep me from poking my eyes out from boredom.

Now to the meat of the question... I have been shooting a lot of indoor competitions for 3 years now and always shoot with a 70-200/f2.8L. In the past I have avoided shooting wide open because of the lens being ever so slightly softer. In the past I have tried to keep it a f3.2 or f3.5 to keep the shutter speed at least 1/250.

This weekend the lighting in the San Jose convention center was horrid so I had to shoot ISO1600, 1/250, f2.8 and was still probably a half stop underexposed. The 50D has never performed better. Very few shot were discarded due to missed focus, and I am sure some of those can be attributed to my technique and the general speed that I was switching targets. Even a couple weeks ago, in much better lighting, when I shot ISO800, 1/320, f4 I did not get as consistent results as I did this last weekend.

I always use shoot manual with custom white balance, center focus point. Focus distances range from probably 4 meters to 20 meters and I use One Shot on the * button. Hit * and rattle off a burst, go to next target and repeat.

So, at f2.8 does the 50D focus faster or more accurately?

Actually, if you set your aperture to f16, you still be focussing at f2.8...

De 50D does AF better due to his crosstype AF-sensors, which are activated using f2.8 or faster lenses,

The camera focuses with any lens at its largest aperture. With a f/2.8 it will enable the extra sensitivity of the center focal point on the 50D, regardless of the actual aperture of the resulting image. In other words, you get the benefit of the better AF with any lens f/2.8 or faster, regardless of the desired aperture.

So as the others have indicated, your 50D does focus better with a lens who's maximum aperture is f/2.8 or wider. (What you actually take the picture at, such as f/4.0, 5.6, 8.0, etc is irrelevant). But what does this actually mean. For lenses that are slower than f/2.8 (i.e f/4.0, f/5.6), AF accuracy is guaranteed to be somewhere within the DOF of the lens. That means that if your DOF is 4 inches, using the chosen aperture value and shooting difference, then your lens will achieve focus somewhere within that 4 inch space. So if you put your AF point on your subject, the focus area will be within a 4 inch window of your AF. It could mean that the focus is 2 inches in front of your AF point, or maybe 2 inches behind it, or best case, directly on the AF point.

When you use a f/2.8 or faster lens, the additional light allows the camera to increase the accuracy to 1/3 of the DOF. So using the scenario above with a 4inch DOF, a f/2.8 lens will increase your focal accuracy to 1.33 inches from the AF. In this scenario, with a fast lens, your 50D should focus within .67" inch of your AF point (forward or backwards). The additional light that fast lenses provide help enable the AF sensors to make smarter decisions, as they can "see" more.

The 1D's have more than one special f/2.8 AF sensor but the rest have only one in the center.

On the other hand, a camera like the 7D has 19 cross-type sensors and a 50D has nine cross-type sensors at f/5.6.
A 1D Mark IV has zero.

The 1D IV has 39 cross type sensors for f/2.8 but you're right, for f/5.6 it has none. The 1D is a pro camera meant for fast pro lenses. It does have an f/8 sensitive center AF point (horizontal only but better than nothing).

So, since I was shooting with an f2.8 lens at differnt aperatures there should have been no difference between hitting focus at f4 and f2.8. It must have come down to technique. When I was shooting f4 I was standing and when shooting f2.8 (getting better focus accuracy) I was kneeling on one knee.

hpulley wrote in post #9471247
The 1D is a pro camera meant for fast pro lenses.

True, and as long as you have all at least f/2.8 lenses, there's no problem at all. But at f/5.6, or even f/4, it doesn't have much of an advantage over the 7D.

apersson850 wrote in post #9472462
True, and as long as you have all at least f/2.8 lenses, there's no problem at all. But at f/5.6, or even f/4, it doesn't have much of an advantage over the 7D.

Horizontal-line only sensors are not useless. People disparage the Rebels as only having one useful AF sensor but this is not really the case. The 45 AF sensors on the 1D, even if only horizonal line sensitive due to a slow lens, still provide more coverage and thus composition advantages but you're right that the 1D is best served by lenses to match it. Most sports pros who use a 1D body will also use the big expensive f/2.8 lenses to get good shutter speeds anyways.

The center point of the 1D is also good for AF to f/8 so there is an advantage there when using teleconverters. You have to goof around with tape and suffer very slow AF on the other bodies to get AF at all at f/8.

I agree with everything you say, but I have the feeling that far from everyone know about it. That's why I posted.

And how is handled the f/1.2 on such sensors?

mshill wrote in post #9472292
So, since I was shooting with an f2.8 lens at differnt aperatures there should have been no difference between hitting focus at f4 and f2.8. It must have come down to technique. When I was shooting f4 I was standing and when shooting f2.8 (getting better focus accuracy) I was kneeling on one knee.

Camera shake when standing, even with the slightly higher shutter speed to try and compensate. Time to get the IS version - I had the same problem before i.e. shutter speeds not quite high enough to compensate for my own technique. IS can help, even at 1/250th or 1/320ths

Chris_R wrote in post #9473484
And how is handled the f/1.2 on such sensors?

Same as f/2.8.

hpulley wrote in post #9474335
Same as f/2.8.

Okey, just realized that the meaning of "f/2.8 or faster" huh.