Focus points

TiaS · Jun 20, 2010

I have the Canon Rebel XS and it has the option to choose from 7 focus points or to allow the camera to choose the focus points. The auto focus didn't always choose the correct subject and so I have been choosing my focus points manually for the last while. I have often wished that I could choose 2 or 3 focus points at a time, but of course the camera only lets you choose one manually.

Case and point, today I was taking a photo of the dog pulling a kid in the pond. Cloudy but fairly bright day. Went for an ISO of 200, and a shutter speed of 100 per second and an aperture of 5.4. I put my focus point on the kid and than took another photo with the focus point on the dog. When I got home I had only one in focus, as I expected that I would... either it was only the kid or only the dog that was sharply in focus.

I am thinking that to fix this I need to make my aperture narrower. However to do so, I need to decrease my shutter speed which is often not an option with the outdoor lighting or bump the ISO which increases the noise. I have ordered an external flash will allow me to be able to use higher shutter speeds and also narrower apertures, and still have good exposure. Please correct me if I am incorrect in anything I am saying or if you have tips for me on how to solve this issue.

Fedaykin · Jun 20, 2010

When shooting moving objects and especially at an aperture of 4.5 or similar, use AI Servo instead of one shot. Use you center focus point as it is the most accurate.

Big Mike · Jun 21, 2010

I just typed this out for another thread, so it's not 100% applicable here, but most of it is.

You can physically only focus a lens on a single spot (distance).

When you use the automatic setting (all points) the camera uses the points that it thinks you want, but any of the points that also happen to be at the same distance, will also light up. Some people think this means that the camera is focusing on all those points (which I guess it is) but that's more of a coincidence that a feature of the camera. It can only focus at one distance at a time.

So when shooting a group photo, you can focus on anyone in the group, but you need to be aware of your DOF, which is a result of your aperture, your focal length and the distance to the subjects. If you have enough DOF to cover the whole group (which is probably what you want), then you are going to be OK. If you are using a shallow DOF, then you will have to choose where you focus, with greater care.
The way DOF works, is that it's depth is 2/3 past the focus point and 1/3 back toward the camera. So if you are shooting a group, ideally you want to focus about 1/3 into the group (distance from front to back). Of course, you can also help by arranging the group to control how deep they are from front to back. If you have a shallow DOF, you probably want everyone in the group to be on the same plane.

KmH · Jun 21, 2010

The amount of shutter speed you need to stop motion depends on the angle the motion is to the plane of the image sensor.

When the motion is parallel to the image sensor plane the most shutter speed is needed to stop motion.

When the motion is perpendicular to the image sensor plane the least shutter speed is needed to stop motion.

Motion on a diagonal to the image sensor plane falls between those 2 extremes.

TiaS · Jun 21, 2010

Thanks. useful info

Petraio Prime · Jun 21, 2010

TiaS said:
I have the Canon Rebel XS and it has the option to choose from 7 focus points or to allow the camera to choose the focus points. The auto focus didn't always choose the correct subject and so I have been choosing my focus points manually for the last while. I have often wished that I could choose 2 or 3 focus points at a time, but of course the camera only lets you choose one manually.

Case and point, today I was taking a photo of the dog pulling a kid in the pond. Cloudy but fairly bright day. Went for an ISO of 200, and a shutter speed of 100 per second and an aperture of 5.4. I put my focus point on the kid and than took another photo with the focus point on the dog. When I got home I had only one in focus, as I expected that I would... either it was only the kid or only the dog that was sharply in focus.

I am thinking that to fix this I need to make my aperture narrower. However to do so, I need to decrease my shutter speed which is often not an option with the outdoor lighting or bump the ISO which increases the noise. I have ordered an external flash will allow me to be able to use higher shutter speeds and also narrower apertures, and still have good exposure. Please correct me if I am incorrect in anything I am saying or if you have tips for me on how to solve this issue.

Why are you doing this? Get a manual camera.

AlexL · Jun 21, 2010

KmH said:
The amount of shutter speed you need to stop motion depends on the angle the motion is to the plane of the image sensor.

When the motion is parallel to the image sensor plane the most shutter speed is needed to stop motion.

When the motion is perpendicular to the image sensor plane the least shutter speed is needed to stop motion.

Motion on a diagonal to the image sensor plane falls between those 2 extremes.

KmH, do you teach classes? lol

TiaS · Jun 21, 2010

Why are you doing this? Get a manual camera.

I am not sure what you mean? My camera has manual mode, obviously, and I use that mode much of the time. Can you please explain what it is that you think I am doing wrong? I am open to suggestions to improve my technique.