"dum" autofocus question

[jmoody]

Thought I was becoming quite the advanced amateur hotshot photog only to

realize that I am still very much of a newbie (sometimes even feels like I'm

going backwards).

 

Anyway, here's the deal. Recently upgraded from drebel to the 40d and I'm

having what "feels" like a focusing issue (sorry, no samples at the moment). On

my rebel, I would use "all af points" at once unless trying to pseudo-spot

meter a scene and then I'd choose a single point.

 

On this camera, I'm finding that tripod shots at f11 or so using all the

points, often seem to be choosing some random part of the scene (usually front

left) to focus on (will test with other lenses soon).

 

Question is... if I choose a center-point (or some other single point) to be

sure of my focus with, am I also effectively spot metering for exposure at that

point as well? In other words, if I leave the camera on "evaluative metering",

and use a single af point for a darker area of the scene, will I over expose

the rest of the scene as opposed to using all 9 points?

 

My drebel did not have evaluative/partial/spot metering, so I always just

assumed that the af points more or less were responsible for metering as well.

 

The camera's still pretty new (duh), and obviously a big jump from the drebel,

so I'm hoping this is largely user-error and not some other dastardly failure

in my lens or camera

thanks in advance and please be gentle

[elliot1]

Your best choice is to select the focus point that is directly over the subject matter you want in focus.

 

Your next best choice is to use the center point, positioning it over the subject matter you want in focus, then locking the focus, then recompose the shot in the viewfinder and take your picture.

 

You must select spot meter to spot meter (I believe there are 3 metering modes with your camera and they are user selectable - check your manual or perhaps someone else can comment - I don't own the 40D).

[lester_wareham]

Yes, I only normally use a single point, normally the centre one, but move to a different point if the subject is not central. It is important to be careful of using the centre fp and doing a focus-and-recompose, particularly with lenses with very limited depth of field.

 

Other are probably more expert in shooting moving subjects than I, but I think the time to use all the focus points is if shooting a moving subject using AI Servo, as the AF system uses the surrounding points if the the subject moves off the centre one.

 

Remember, there are no dumb questions, only dumb answers. ;)

[mendel_leisk]

You're mixing metering and focussing, there's no connection. FWIW, I prefer to use the center focus point exclusively, almost always have focus assigned to the * button (by setting custom function 4 to 1).

[jmoody]

so far so good (was nervous about having to recalibrate my 24-105). So, since there are "no dumb questions" (here we go :-) why would anyone prefer to use all 9 points at once? On my drebel, I found no issue with using all points at once, focusing with a half press and then recomposing.

With this current camera, I've only tried the center point while following a moving subject (and servo), but I'll start trying it out more often on stationary (landscape for example) subjects to guarantee my focus.

[jim_simon3]

generally Canon cameras will by default evaluate the scene weighting the scene by using the focus point. it is not the same as using spot or partial metering, just a different center of evaluating. Realizing that this isn't always the way to set the exposure, the camera has an Exposure Lock that stays on for, I think, 6 seconds. Aim the camera at the part of the scene you want to take the exposure off of, press the exposure lock button, then recompose the shot and focus where you want. Then take the photo.

[jimstrutz]

I use the custom function that enables direct AF point selection with the stubby little joystick. It takes a bit of practice to activate the points on the diagonal with the first poke, but it's worth doing. This way any exposure biasing takes place over your subjects, and you don't waste time doing the focus-recompose dance. Most of the time, while you are setting up for the shot, you can see what area is going to hold the center of attention so you can preselect that AF point. I have gotten in the habit of re-centering my AF point choice after each shot just in case I get surprised.

[sdsalyer]

Try using the custom function that seperates focus and exposure. Where one is set to the shutter button half-press (exposure lock) and the other to the * button on the back of the camera (focus lock). This is what I prefer to use, rather than having both on the shutter button.

[jmoody]

Thank you all so much. I can see that my learning curve has just come up an a rather steep grade, but this should at least keep me out of trouble and out of the local gangs for a while :-)

 

I seriously appreciate all the info and the absence of being flamed with the typical "read the manual" (which I will do more of, of course). Wish me luck

Jeff