Nine Point Automatic Focus in 20D

In the EOS-1, Digital SLR, D1 etc. white paper (the systems probably work the same in the 20D?) it says something like .…there is a tradeoff between the wider coverage area of automatic multipoint focus and the comparatively narrow coverage area of manual selecting a single point…. My questions are:

1. What exactly does happen when multiple points are automatically selected (I had assumed that they were simply at equal distances from the focal plane, as I had assumed that there can be only one distance “in focus?” Can different distances as represented by different points on the AF actually be focused upon (I highly doubt it).

2. This automatic choice seems to be an annoyance as much as a help: I point the camera, and points peripheral to the subject are often selected automatically. What do you do about this—set up a single center point? Isn’t just using the center point the best except when on a tripod or when very close, and one doesn’t want to aim, focus, and recompose?

3. So, when do you use the automatic multipoint focus, and when do you shut it off? What are its pros and cons

kdoc

Kdoc said:

1. What exactly does happen when multiple points are automatically
selected (I had assumed that they were simply at equal distances
from the focal plane, as I had assumed that there can be only one
distance “in focus?” Can different distances as represented by
different points on the AF actually be focused upon (I highly doubt
it).

Click to expand...

I believe I read camera automatically selects the focus point mostly according to contrast.

Kdoc said:

2. This automatic choice seems to be an annoyance as much as a
help: I point the camera, and points peripheral to the subject are
often selected automatically. What do you do about this—set up a
single center point? Isn’t just using the center point the best
except when on a tripod or when very close, and one doesn’t want to
aim, focus, and recompose?

Click to expand...

I use single center point just about always. I don't think a tripod would make any difference, I focus on what I want to be in focus.

Kdoc said:

3. So, when do you use the automatic multipoint focus, and when do
you shut it off? What are its pros and cons

Click to expand...

Where is Doug Kerr when he is needed. In the meantime, I will substitute the short version. You might want to use it when in AI Servo mode, but I still use center focus on that too. I know there are situations where it might be advantageous for example when shooting fast sports.

I really haven't upgraded my shooting techniques to take full advantage of multiple focus points, I pretty much stick to what I have been doing for years and years.

Regards,
Mike

--
New Gallery (in development stage) http://wnyphoto.com
Photography is just one of my hobbies

Kdoc said:

1. What exactly does happen when multiple points are automatically
selected (I had assumed that they were simply at equal distances
from the focal plane, as I had assumed that there can be only one
distance “in focus?” Can different distances as represented by
different points on the AF actually be focused upon (I highly doubt
it).

Click to expand...

I believe your assumption is correct, it simply selects all focus points it deems to be in the focus plane. Haven't paid much attention to it as I never use the automatic focus point selection in one-shot mode.

Kdoc said:

2. This automatic choice seems to be an annoyance as much as a
help: I point the camera, and points peripheral to the subject are
often selected automatically. What do you do about this—set up a
single center point? Isn’t just using the center point the best
except when on a tripod or when very close, and one doesn’t want to
aim, focus, and recompose?

Click to expand...

Using a single focus point of your choice is definitely the best bet. The center point is the most accurate and works with lower light and/or less contrasty subjects than the others, and I guess for many of us the focus-and-recompose technique is the quickest and easiest after years of practice on all kinds of AF cameras. However when shooting at very close distances and large apertures (hence short DOF), it might be better to pick a more suitable focus point and avoid recomposing to make sure the focus ends up exactly where you want it.

Kdoc said:

3. So, when do you use the automatic multipoint focus, and when do
you shut it off? What are its pros and cons

Click to expand...

The only real use for it in my opinion is in the AI Servo mode, where the AF system initially picks up the subject using the center point and then keeps tracking it using all the points (so you don't lose focus if the subject moves away from the center of the frame -- nice for framing moving subjects off-center). Other than that, it's always off for me.

Btw, I like to set custom function 13 (AF point selection method) to 1 (multi-controller direct), that way I can use the joystick to directly select a focus point on those rare occassions when I want to use something other than the center point.

Niku

Niku Toivola said:

Niku Toivola said:

3. So, when do you use the automatic multipoint focus, and when do
you shut it off? What are its pros and cons

Click to expand...

The only real use for it in my opinion is in the AI Servo mode,
where the AF system initially picks up the subject using the center
point and then keeps tracking it using all the points (so you don't
lose focus if the subject moves away from the center of the frame
-- nice for framing moving subjects off-center). Other than that,
it's always off for me.

Click to expand...

A-DEP (Automatic Depth-of-Field) also needs several focus points to have any effect. The whole point of A-DEP is to measure several points to make sure e.g. a large group of people are all in focus.

Finno

Finno Furre said:

A-DEP (Automatic Depth-of-Field) also needs several focus points to
have any effect. The whole point of A-DEP is to measure several
points to make sure e.g. a large group of people are all in focus.

Click to expand...

I believe A-DEP automatically changes it anyway, so why would it need to be changed to accomodate that mode. It would be the same as the auto modes, they are chosen by the camera and not the person using the camera.

Regards,
Mike

--
New Gallery (in development stage) http://wnyphoto.com
Photography is just one of my hobbies

You are probably right, Mike. (haven't tested it). And boy do I like your bike-gallery!

Anyway, I just wanted to mention a situation where several, if not all, AF points actually make sense.

Finno

Wild-Mike said:

Finno Furre said:

A-DEP (Automatic Depth-of-Field) also needs several focus points to
have any effect. The whole point of A-DEP is to measure several
points to make sure e.g. a large group of people are all in focus.

Click to expand...

I believe A-DEP automatically changes it anyway, so why would it
need to be changed to accomodate that mode. It would be the same as
the auto modes, they are chosen by the camera and not the person
using the camera.

Regards,
Mike

--
New Gallery (in development stage) http://wnyphoto.com
Photography is just one of my hobbies

Click to expand...

All of what you all said makes sense to me and seems to be in agreement to what I understand from the various sources. But I keep thinking we're missing something. After all, Canon isn't stupid, and went to a lot of trouble to make that autofocus work on 9 points. Why would they want a system which would automatically choose 2, 3 or 4 points automatically, often not the point you wanted most?

kdoc

Here, I'm asking about when you're in One Shot mode?

Kdoc said:

All of what you all said makes sense to me and seems to be in
agreement to what I understand from the various sources. But I keep
thinking we're missing something. After all, Canon isn't stupid,
and went to a lot of trouble to make that autofocus work on 9
points. Why would they want a system which would automatically
choose 2, 3 or 4 points automatically, often not the point you
wanted most?

kdoc

Click to expand...

Finno Furre said:

You are probably right, Mike. (haven't tested it). And boy do I
like your bike-gallery!

Click to expand...

Thanks, I take it you are into bikes. Here are a few shots of some things I did to the bike this winter.
http://tkis.com/mike/bike.html

I also put on some progressive airtail shocks which you can't see in the photos. Now I can lower and raise it easily without affecting the ride quality.

Finno Furre said:

Anyway, I just wanted to mention a situation where several, if not
all, AF points actually make sense.

Click to expand...

That works

Regards,
Mike

--
New Gallery (in development stage) http://wnyphoto.com
Photography is just one of my hobbies

Kdoc said:

All of what you all said makes sense to me and seems to be in
agreement to what I understand from the various sources. But I keep
thinking we're missing something. After all, Canon isn't stupid,
and went to a lot of trouble to make that autofocus work on 9
points. Why would they want a system which would automatically
choose 2, 3 or 4 points automatically, often not the point you
wanted most?

Click to expand...

I beleive versatility is the answer.

Regards,
Mike

--
New Gallery (in development stage) http://wnyphoto.com
Photography is just one of my hobbies

Does any one have the link to the EOS-1 "White Paper" that talks about multipoint autofocus"

Thanks
Joe

http://www.photoworkshop.com/canon/EOS_Digital.pdf

jrokovich said:

Does any one have the link to the EOS-1 "White Paper" that talks
about multipoint autofocus"

Thanks
Joe

Click to expand...

Kdoc said:

After all, Canon isn't stupid,
and went to a lot of trouble to make that autofocus work on 9
points. Why would they want a system which would automatically
choose 2, 3 or 4 points automatically, often not the point you
wanted most?

Click to expand...

Certainly one reason is to accommodate true point-and-shooters, so to speak. We have all seen those shots with blurry uncle Jim and aunt Jane in the foreground and the mountain range 50 km away in very sharp focus in the background, because the shooter didn't know about focus-and-recompose and the center focus point just happened to lock on the bit of mountain visible between uncle and aunt...

I honestly don't know if this reason alone makes developing multipoint autofocus systems worth the expense, though. Perhaps another reason is that since multiple focus points do have their uses in some situations (such as AI servo) so they need to be there anyway, it's not too much extra trouble to also provide the automatic focus point selection functionality.

Niku