to focus lock or not???

[SaraSue]

Forgive me if I don't explain this in technical terms or if I start to ramble.

Ok, so I use a Canon Rebel XTi. There are 9 "little dots" for my focus points. I would like to think I have a pretty good understanding of aperture. I understand the smaller the aperture(bigger number) the more depth of field in focus. So you would have a bigger plane of space in focus. So when photographing a group is it better to select 1 focus point, lock that focus point, and recompose the shot with a smaller aperture and get a clear photo that way, or is it better use more focus points active and get a clear shot that way? I hope that makes sense. I haven't shot enough groups/families to really have a good understanding of which is better.

I'm not sure if this is a technical question or a preference question but any comments are greatly appreciated!!!! If this has already been answered somewhere please point me in the right direction. Thanks again!

[RecurrentNerve]

Spot focussing generally gives you the most control. I'm not sure why you would 'recompose the shot with a smaller aperture' though. You would use spot focussing to make sure the camera focusses on the correct point - the classic example is focussing on the wall behind two people standing next to each other.

[Skoropada]

Originally Posted by SaraSue

Forgive me if I don't explain this in technical terms or if I start to ramble.

Ok, so I use a Canon Rebel XTi. There are 9 "little dots" for my focus points. I would like to think I have a pretty good understanding of aperture. I understand the smaller the aperture(bigger number) the more depth of field in focus. So you would have a bigger plane of space in focus. So when photographing a group is it better to select 1 focus point, lock that focus point, and recompose the shot with a smaller aperture and get a clear photo that way, or is it better use more focus points active and get a clear shot that way? I hope that makes sense. I haven't shot enough groups/families to really have a good understanding of which is better.

I'm not sure if this is a technical question or a preference question but any comments are greatly appreciated!!!! If this has already been answered somewhere please point me in the right direction. Thanks again!

The 9 little dots are the spot auto focusig indicators, that means that the camera will "look" at the spot you have selected and focus on that.

Regarding the aperture setting, reducing the aperture (bigger f number) will give you more Depth of Field, meaning that a larger slice on the subject wil be in acceptable focus.

I'm no expert but I believe that if you are shooting a group of people you should use a small enough aperture so everyone is in focus, unless you are aiming for something different.

[SaraSue]

thanks for all your comments. I guess I still haven't found the answer I was looking for. I guess what I am asking is
Is aperture applied to the picture as a whole, or to each auto focus point? If it was each focus point since this is a 3d world it would make most of the picture in focus even if the people were not all in a line. Maybe I need to just practice with both to better answer my question. Thanks

[Eastree]

Originally Posted by SaraSue

thanks for all your comments. I guess I still haven't found the answer I was looking for. I guess what I am asking is
Is aperture applied to the picture as a whole, or to each auto focus point? If it was each focus point since this is a 3d world it would make most of the picture in focus even if the people were not all in a line. Maybe I need to just practice with both to better answer my question. Thanks

The lens is only focused at one distance. If more than one focus point blinks (indicating focus), it means both of those spots are very close to the same distance from the lens (at least the camera believes so).

Aperture affects the depth of field at a given distance of focus.

Ultimately it's a very one-dimensional property: depth from the lens.

[Rob Necciai]

Don't confuse focus points and aperture. The aperture is indeed "applied to the picture as a whole". It is, in fact, the size of the hole letting light into the camera.

The focus points are used to aid in the autofocus operation. Choosing an auto focus point just tells your camera where (in the area of the image) to do its autofocus magic. It really has nothing to do with aperture. Keep in mind that, although your camera has multiple focus points (used for varying purposes), your camera has only one lens and therefore has a SINGLE focus.

For the group shot, use a fairly small aperture (such as f11 to f16). Focus on the middle row. If there are only two rows, focus on the front row. You should be fine.

[SaraSue]

Once again, thank you for the comments! I'll get this eventually!!


and thank you Eastree, that makes a lot more sense. Now I understand just because the different focus points are blinking it is because they are relatively the same distance. I layed awake in bed last night trying to think this through. haha, lame I know but it was one of those things. I kept thinking the points could reach all different distances but this makes more sense. Now I understand why focus lock would be better with a smaller aperture(bigger number).

Thanks again everyone!

Originally Posted by Eastree

The lens is only focused at one distance. If more than one focus point blinks (indicating focus), it means both of those spots are very close to the same distance from the lens (at least the camera believes so).

Aperture affects the depth of field at a given distance of focus.

Ultimately it's a very one-dimensional property: depth from the lens.

[inkista]

One more thing to note.

Your lens is always wide open at its maximum aperture when you're composing--the lens's iris is only closed down to the aperture setting you use just before the picture is taken. This gives both you and the camera's autofocus/autoexposure systems the most light to "see" by as you work on composing/metering/focusing the image.

This means whatever DoF you see through the viewfinder at any given time is less than what you're going to have in the shot if you're not shooting wide open. This is why Canon provided you with a DoF preview button. When you hit that button, the aperture is stopped down to what you've set, and while the view will be dimmer (the smaller hole lets in less light), the depth of field will be accurate, and you can judge focus.

I don't think you necessarily need to be using f/11 or f/16 just to get a group of people in focus--it will depend on how far away you are, and where you're placing your focus. Be aware that closing down your aperture to something extremely small may have you losing sharpness to diffraction effect. I'd say start with the more "middle of the road" apertures of f/4-f/8 and see if you need to adjust from there.

[SaraSue]

thanks inkista for the tips. DoF preview button?? huh? I might need to look that up, I'm not sure I know what button you talking about.

[RecurrentNerve]

The DoF preview button lets you, erm, preview the depth of field before you take the shot. It's more of a leftover from film cameras; nowadays they are a little redundant as you can simply take the shot and check the DoF on the screen, which is generally better because the DoF Preview closes the aperture, making the viewfinder image pretty dim.

On your Rebel, I believe the DoF preview button is underneath the lens release on the front of the body.