Focal Points?

Cason

I have a Rebel XT and I am trying to figure out the benefits of having all focal points turned on. I know I can toggle through each one and they will light up. When all 7 are lit up, how come when I push the shutter half way down to compose my shot all do not light up. Sometimes 1 or 3 or 4 will light up. I thought all 7 would be used. :scratch

If I am doing a close head shot in low light and have the aperture set to 2.2, I want to have the whole face in focus.

Any ideas?

claudermilk

The ones currently in use light up.

For your narrow DOF situation, I'd manually select the focus point--and use the center one if at all possible since that't the most sensistive one. I virtually always have my 20D set to manual selection & 90% of the time it's on the center point.

Cason

If I want to do a landscape shot, I should be using all focal points when the aperture is increased?

Shay Stephens

The focus points are not independant, in that it will focus on a point far away and close up. The points light up to indicate the focal plane it is choosing to focus on.

I find the multiple focus points to be rather unpredictable, and opt instead to use the center focus point. It is by the far the most sensitive and fastest responding of all the points available. So I move that point and focus on the thing I want to focus on, then recompose the camera to the desired framing and finish taking the picture. It can be a little tricky learning to do that, but well worth the effort because you gain more control over the focus determination.

Khaos

Shay, I agree and do the same, but since I only shoot with a 20D and the 1 DS MKII has something like 45 focal points, do the pros really use them or do they go with the center and all those points are wasted? Are they more predictable?

Shay Stephens

Khaos wrote:

Shay, I agree and do the same, but since I only shoot with a 20D and the 1 DS MKII has something like 45 focal points, do the pros really use them or do they go with the center and all those points are wasted? Are they more predictable?

Well I can only speak for myself, but no matter how many fous points a camera has, there is still no way to predict where the camera will decide to focus. With the center point or a manually selected point, there is little unpredictability in where the focus will be. And as mentioned, the center point is more sensitive, and works in lower light typically than any of the other points.

If I have enough light to use them, I will select one outside point I want to use so as to keep my framing the way I want it, but I never let the camera choose the focus point, I never have good success with that.

That may change in the future as the camera start to recognize faces and are able to pick them out from the rest of the frame and focus on them, but until then, I am going to be selecting the point myself.

Cason

Shay Stephens wrote:

...So I move that point and focus on the thing I want to focus on, then recompose the camera to the desired framing and finish taking the picture.

So, when you do a tight face composition with something around f\2.5, you select the middle focal point which would probably be on the nose. Then you point the middle focal point between the eyes press the shutter half way down and then go back to the nose and press all the way down to get your shots.

Would be nice if you could choose all the focal points and all will be used.

Shay Stephens

RTP wrote:

So, when you do a tight face composition with something around f\2.5, you select the middle focal point which would probably be on the nose. Then you point the middle focal point between the eyes press the shutter half way down and then go back to the nose and press all the way down to get your shots.

Yes, that is the jist of it.

Would be nice if you could choose all the focal points and all will be used.

Impossible. If all points were used, you have a lens that does not exist because you can't have multiple focus planes (e.g. the eyes as one plane and the tip of the nose as another plane). You can only have one focus plane (eyes or nose, but not both).

wxwax

RTP wrote:

So, when you do a tight face composition with something around f\2.5, you select the middle focal point which would probably be on the nose. Then you point the middle focal point between the eyes press the shutter half way down and then go back to the nose and press all the way down to get your shots.

Yes.

However... if your depth of field is that narrow, focus-recompose can be dangerous. When you recompose, you're shifting your lens away from the place where everything was in focus. Usually, this doesn't matter. But if you're in close and/or you have a really narrow depth of field, something as simple as recomposing the shot might throw-off your focus.

I focus-recompose as well. But I know that in some situations, it might lead to trouble.

cmason

wxwax wrote:

I focus-recompose as well. But I know that in some situations, it might lead to trouble.

http://visual-vacations.com/Photography/focus-recompose_sucks.htm

photogmomma

Don't forget, (I'm assuming this is true for the 20D, too, but I have a 10D) you have a dial on top that has a setting called A-Dep. The way this works is that it looks at what you ahve in your viewfinder and determines the best aperature to use. I believe it looks at 2 or more focal points to determine this so if you really want to have multiple things at different distances in focus and you aren't sure of the proper f-stop, this might be an option. (I've only used it once and it annoyed me. ! But it might be a way for you to see what the camera suggests and then use that in manual mode....)

I, too, use only one focus point - and 99% of the time it's in the center and I reposition when necessary. It's MUCH more predictable and easier to use, for me....

ETA: Hmmm, after reading about "Focus-recompose" in the link above, I'm going to try resetting my focus point to one of the side ones.... That might just help me. Thanks for the link, cmason.

LiquidAir

RTP wrote:

I have a Rebel XT and I am trying to figure out the benefits of having all focal points turned on. I know I can toggle through each one and they will light up. When all 7 are lit up, how come when I push the shutter half way down to compose my shot all do not light up. Sometimes 1 or 3 or 4 will light up. I thought all 7 would be used.

If I am doing a close head shot in low light and have the aperture set to 2.2, I want to have the whole face in focus.

Any ideas?

Generally, if you have all the focus points enabled, the camera focuses on the point that is closest to you. For instance, if you have resonable depth of field and 3 of your focus points fall on a person in the foreground and 4 fall on a tree in the background, the camera will focus on the person and 3 will light up. That is the theory anyhow, and sometimes it actually works out that way.

If your subject is not moving, generally I consider using the center focus point and recomposing to be the most reliable. However, you can have minor focus problems using that approach with wide apetures if you are close to the subject. That said, on anything short of a top of the line camera (1-series in the Canon line) the off center focus points are not accurate enough to save you in that situation. For me, focus and recompose almost always works for still subjects. When I am having trouble with it a tripod with a good ball head is usually the right answer because it keeps the camera from shifting when I recompose.

When your subject is moving, the whole game changes. Focus and recompose is not an option. Using an off center focus point, AI servo and tracking your subject is one choice here but I recommend stopping down to f/8 or smaller because focus won't be too accurate. If your subject is erratic and you are having a hard time tracking it then turning on all the focus points and leaving it to the camera may be your best choice.

erich6

photogmomma wrote:

Don't forget, (I'm assuming this is true for the 20D, too, but I have a 10D) you have a dial on top that has a setting called A-Dep. The way this works is that it looks at what you ahve in your viewfinder and determines the best aperature to use. I believe it looks at 2 or more focal points to determine this so if you really want to have multiple things at different distances in focus and you aren't sure of the proper f-stop, this might be an option. (I've only used it once and it annoyed me. ! But it might be a way for you to see what the camera suggests and then use that in manual mode....)

I, too, use only one focus point - and 99% of the time it's in the center and I reposition when necessary. It's MUCH more predictable and easier to use, for me....

ETA: Hmmm, after reading about "Focus-recompose" in the link above, I'm going to try resetting my focus point to one of the side ones.... That might just help me. Thanks for the link, cmason.

If you are doing a portrait you can use A-dep mode and then move in with your camera until all the focus points are just insde the subject's face. The A-dep mode uses ALL focal points and finds the best plane and aperture to focus on so that all points are in focus (not necessarily at BEST focus but in focus nontheless...at least within the depth of focus of the lens).

That said, the best thing to do is to experiment with your camera until you get a sense of what aperture you want for your distance to the subject to get the depth-of-field you need. Once you know that, you can just frame the subject and select the focus point that's closest to the point in the subject you want to be in best focus (usually the eyes for a portrait).

It's true the center focus point is more accurate than the others. It is also a "cross-sensor" which means you'll get focus detection in both horizontal and vertical directions. Other sensors are arranged either horizontally or vertically which means that if you have an edge you are trying to focus on and is not lined up perpendicular to the sensor the camera will not be able to focus.

Erich

erich6

wxwax wrote:

Yes.

However... if your depth of field is that narrow, focus-recompose can be dangerous. When you recompose, you're shifting your lens away from the place where everything was in focus. Usually, this doesn't matter. But if you're in close and/or you have a really narrow depth of field, something as simple as recomposing the shot might throw-off your focus.

I focus-recompose as well. But I know that in some situations, it might lead to trouble.

Yup. Anytime the lateral distance between the place you focused at and where you recomposed to significantly changes the distance from subject to the film/focal plane (the focus distance) this will be a problem. Significant depends on your depth-of-field...the less you have the more sensitive you are to changes in focus distance.

Erich