Focus points vs DoF

[htid]

Hi,

I'm a bit confused as to how the focus points work. If I set dof to f4, then aim at say a bird which is sitting in the middle focus point, why do other focus points need to lock on? The background is blurry which is dictated by the aperture, so what relevance do the focus points other than the one on the bird have?

Thanks

 

[Original Poster]

Dependent on which camera you are using you can adjust how many and which focus points are actually used. I know this is the case on Canon bodies, I presume the same applies with Nikon.

 

[Big Andy]

Different images use different focus points, you may want to focus on the forground in another image so would select a focus point that covered that area. You can choose which point to use as to which area you want sharpest.

 

[knightrider]

If you want your AF to only use one focus point, then you must set this manually on your camera, otherwise it will try its best to focus all points...

 

[towershot]

If say,you want the bird on the edge of the frame and not in the middle,this is when you shift the focus point.
Othewise you will have to focus with the bird in the middle ,then lock focus before reframing .

 

[Big Andy]

If left with all focus points turned on a camera (mine anyway) will usualy focus on the nearest part of the image covered by a focus point. If you check your manual it will tell how to select the point you want.

 

[htid]

If you want your AF to only use one focus point, then you must set this manually on your camera, otherwise it will try its best to focus all points...

Yes I actually do use specific focus points so of course in the example above I would use just the central one, so this is more of a "how does it work" than "what technique should I use". Its just that at f4, the camera can't focus on the bird in the foreground and the background so does it render some of them unused in that case?

 

[htid]

If left with all focus points turned on a camera (mine anyway) will usualy focus on the nearest part of the image covered by a focus point. If you check your manual it will tell how to select the point you want.

Sorry, hadn't seen your post when I posted, but thats what I was getting at and that answers my question

 

[BigRuss]

I think the simple answer to your question is they don't

If i'm taking pics of birds, i always set the AF selection to manual and depending where you want to compose the bird in the shot choose the single af point accordingly, or you can just set it to the middle one focus on the bird with a half press then recompose your shot while holding half down.

I find if I leave it on auto AF, you can easily miss-focus on twigs surrounding the bird.

In fact , why there is an auto af setting i haven't figured out yet...cos I for one never use it.

 

[htid]

I think the simple answer to your question is they don't

If i'm taking pics of birds, i always set the AF selection to manual and depending where you want to compose the bird in the shot choose the single af point accordingly, or you can just set it to the middle one focus on the bird with a half press then recompose your shot while holding half down.

I find if I leave it on auto AF, you can easily miss-focus on twigs surrounding the bird.

In fact , why there is an auto af setting i haven't figured out yet...cos I for one never use it.

but af when using one focus point is useful isn't it? Or do you actually manually focus on a single point? If so, doesn't that mean you miss a lot of shots as those birds are rather fast!

 

[BigRuss]

but af when using one focus point is useful isn't it? Or do you actually manually focus on a single point? If so, doesn't that mean you miss a lot of shots as those birds are rather fast!

Sorry my bad as the young 'uns say lol

I meant auto focus point selection, not auto focus it's self

 

[htid]

So another point then. I don't do landscapes, but from my understanding you'd use something like f16. This would mean all the scene is in focus, again making focus points redundant, or am I missing something?

 

[Jelster]

So another point then. I don't do landscapes, but from my understanding you'd use something like f16. This would mean all the scene is in focus, again making focus points redundant, or am I missing something?

There's being "in focus" and there's being sharp....

Think about this one.

A group of 5 or 6 people 3-4 metres away. You would need a cluster of focus points to make sure you had them all (or at least most of them) in focus, but you would probably set at about f8, maybe f5.6, depending on light etc.

The reason you have multiple focus points is because there are times when you need one, or a cluster, depending on your subject and the environment in which you are shooting.

I shoot mainly portraits & wildlife, so I normally only have one active focus point. That said, I may like to have a cluster of AF points locked if I was going to shoot a particular animal with its surroundings.

You need multiple AF points to give you the option of how you use them.

Steve