50D focusing trouble

Underachiever

Ive been having trouble getting used to my new 50D which i upgraded from my 400D. I shoot bands mostly, so lots of low light conditions and I usually shoot Al Servo with auto focus points and I was finding that more an more often the camera was focusing on the 'wrong' part of the shot. So i started choosing my focal points and im not having much more luck IEven when I pick the focal point it seems to be focusing on the nearest object rather than the focal point. ie focal point set to top drummers face top centre point an the cymbal in the top right area is in focus because its closer. Is there some weird setting that I dont know about that does this or is it something that im just going to have to get used to? I never had this with any of my other canons before an its starting to drive me nuts

thanks guys

cmason

AI Servo is selecting for the most movement, or movement toward the camera. It will change focus depending on movement. Of course, if you have it on a single focus point, this should be reduced. If you have it on multifocus pts, it will select the one it thinks is correct. Great for sports, but maybe not so hot for what you shoot. Perhaps you should try AI Focus instead

ziggy53

As "cmason" said, AI-Servo is best suited for rapid and predictable motion.

AI Focus simply tries to select automatically between One-Shot and AI-Servo. It does not choose correctly in many cases.

One-Shot mode and center single AF point selection will work best, especially if using lenses of f2.8 constant aperture or better. Lenses slower than f2.8 will definitely not work well to focus in low light levels.

racer

I use the 50d, and I dont have any trouble, it sounds like it is in your technique. I have found that, Al Servo is good for tracking moving objects, like flying birds or moving cars, objects that are moving in a smooth motion from far to up close. If you are using Al Servo on relatively stationary people who are moving around some, I have found that focus will jump back and forth between objects that are in your focus point area. Try this instead, manual select the center AF point, select "One Shot" AF, chose your meter area (I would use spot meter), lock exposure, now focus on the face, still holding the button half way, recompose and shoot. Might take some practice, but you shouldnt have to worry about focus

Now if the person is running back and forth on the stage, you could try Al Servo, but be sure to track them from a distance so you have a better chance that the focus is tracking the movement, and shoot off a burst so you have a better chance of one of the shots being in focus, but with a concert, I would just wait for the person to stand still.
Also, remember that if your shooting in low light, the camera will have a much harder time trying to focus (and might hunt for focus), because the contrast is low

Underachiever

thanks guys, just cant see how i shoot as i always have and i cant seem to get it right wit this particular cam. I use al servo because as racer said they tend to move back and forth a lot, maybe one shot will do it. problem is with a gig you dont get much chance to change settings, its not having a problem focusing, the bits its focused on are spot on, just not where i want heh. I hope I crack it soon, its making me miss my old 400D

Howzit

ziggy53 wrote: »

As "cmason" said, AI-Servo is best suited for rapid and predictable motion.

AI Focus simply tries to select automatically between One-Shot and AI-Servo. It does not choose correctly in many cases.

One-Shot mode and center single AF point selection will work best, especially if using lenses of f2.8 constant aperture or better. Lenses slower than f2.8 will definitely not work well to focus in low light levels.

B&H gives a good description of this: http://www.video.bhphotovideo.com/?fr_story=023e7711fd0a11717179bc933084e559429d297d&rf=rss

Underachiever

I kinda get it and I know their different cameras but how am i getting such different results between cameras with the same settings? (i feel like im being really dumb now sorry)

ziggy53

Underachiever wrote: »

I kinda get it and I know their different cameras but how am i getting such different results between cameras with the same settings? (i feel like im being really dumb now sorry)

The short answer is that the 2 cameras you are comparing have considerably different AF systems and each will respond differently (to a degree) to the same circumstances.

Also, there is no camera made that will always reliable AF. Professional photographers always talk about a "percentage of keepers", either high or low, when discussing different cameras and different systems.

Your previous Canon 400D/XTi also had less pixel density, meaning that if you review images at 100% you are looking at a higher magnification on the 50D images.

Anyway, please try my suggestions of using a fast aperture lens of f2.8 or faster, and for unpredictable motion, or static subjects, I do believe that "One Shot" mode with center poinjt selected only will give you the best results. AI-Servo is called "predictive AF" and it requires that you sample the subject's motion for just a bit while the camera tries to figure out where prime focus should be at the time of exposure. With unpredictable subject motion it may plot the opposite of correct AF.

Underachiever

thanks i already use f2.8 or faster lenses, i'll try it in one shot

jdryan3

ziggy53 wrote: »

AI-Servo is called "predictive AF" and it requires that you sample the subject's motion for just a bit while the camera tries to figure out where prime focus should be at the time of exposure. With unpredictable subject motion it may plot the opposite of correct AF.

Absolutely true on the 50D re: predictive when using AI Servo. If shooting AI Servo/multi-shot rarely is the first shot spot on, usually #2 or #3. Since you are using AI Servo, have you put separated the focus button using the Custom Function so you can hold and track, separating it from the shutter button? It makes a big difference.