Autofocus system question

I have a Canon 30D and when I select a single autofocus sensor, the performance is much poorer than when all of the sensors are selected. Presumably this is because the camera ignores the information from the other sensors?

My question is, is there any way to select a single autofocus point to be prioritised, but still have all of the autofocus sensors working in order to help get the best performance. If not, do any other Canon cameras have an autofocus system that works in this way?

A slr has only 1 sensor.
I think u mean af point.
There's no such thing in your 30D and also no such thing in any slr.
It's either u choose single point,all af point,zone or spot. That's in a 7d.
I think in your 30d only single point and all af points. Nothing more nothing less.

Sorry, yes, I should have written "point" not "sensor".

Thanks for your reply. So are you saying that in the 7D you can do what I'm asking about: selecting one autofocus point to be prioritised, so that part of the subject is in focus, but still have all of the other autofocus points operational?

Hi Sam - I have to agree with pjmyles on this one.

When you select ALL of he AF points (not sensors) you are really asking for a compromise focus situation. I believe the body averages them all in some fashion, and then comes up with a happy medium that will give you decent or good focus across the range of the field of view. This can often be a very good thing. It can also have unpredictable and very disappointing results.

I use Center AF most of the time, and can select that on my old 5Dc with the jogging button on the back of the body. But you may be facing another issue really - the optimal conditions, or lack thereof, that will result in fast, easy and accurate AF, with almost any of the AF points.

AF works best in well illuminated situations. If the lighting is flat, dim, and with little or no contrast in the scene, it is often difficult for the AF system to get a firm focus lock on the subject. Also, underexposed or low contrast subjects will appear to be soft or out of focus. Could these issues be a problem perhaps? - Stu

SamHH wrote in post #10877656
Sorry, yes, I should have written "point" not "sensor".

Thanks for your reply. So are you saying that in the 7D you can do what I'm asking about: selecting one autofocus point to be prioritised, so that part of the subject is in focus, but still have all of the other autofocus points operational?

No - when you select one focus point, you are telling the camera to use that one and ignore the others.

The 7D doesn't allow you to select one focus point but have all the others operational, but it does have an option to use a single focus point with expansion, so that three or four focus points (depending on the location of the selected main focus point) around the selected point may also be used.

From http://www.usa.canon.c​om …rticleAct&artic​leID=3049:
"AF Point Expansion allows the user to manually choose any one AF point to be the primary point he or she wants to use to focus on their subjects. Again, it can be the center point, or any off-center point. But now, additional surrounding points are active, and if the primary point for any reason loses sight of the subject, or can’t find sufficient detail, the surrounding AF points are immediately called-in to assist in focusing upon the subject."

stsva wrote in post #10877710
The 7D does have an option to use a single focus point with expansion, so that three or four focus points (depending on the location of the selected main focus point) around the selected point may also be used.

From http://www.usa.canon.c​om …rticleAct&artic​leID=3049:
"AF Point Expansion allows the user to manually choose any one AF point to be the primary point he or she wants to use to focus on their subjects. Again, it can be the center point, or any off-center point. But now, additional surrounding points are active, and if the primary point for any reason loses sight of the subject, or can’t find sufficient detail, the surrounding AF points are immediately called-in to assist in focusing upon the subject."

Ah - I did not know that and stand corrected. You learn something new everyday..... thanks for clarifying it.

So if I understand this correctly, you choose your main active AF point and if for any reason it fails to get a lock on the target, then other nearby AF points become active and take over - correct? But..... it does not act in conjuntion with the first AF point, providing a focus compromise or averaging of the two, right?

SamHH wrote in post #10877656
So are you saying that in the 7D you can do what I'm asking about: selecting one autofocus point to be prioritised, so that part of the subject is in focus, but still have all of the other autofocus points operational?

Sort of....

The 7D also has AF point expansion. mode.

Basically, you are using only one AF point, but if that point can not achieve focus, one of the 4 other adjacent points will focus instead.

You can also move this point around. It doesn't have to be the center point.

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SamHH wrote in post #10877534
I have a Canon 30D and when I select a single autofocus sensor, the performance is much poorer than when all of the sensors are selected. Presumably this is because the camera ignores the information from the other sensors?

My question is, is there any way to select a single autofocus point to be prioritised, but still have all of the autofocus sensors working in order to help get the best performance. If not, do any other Canon cameras have an autofocus system that works in this way?

No, there isn't on the 30D. However, the 7D and the 1D-series cameras have a feature called AF point expansion which allows you to manually select one AF point and the camera will use the manually AF point along with the adjacent AF point for focusing. All other AF points are ignored. In One Shot AF mode, the selected AF point and the adjacent AF points are given equal priority in acquiring focus. In AI Servo AF mode, focus is first acquired with the manually selected AF point and then the expansion AF points are used, if necessary, to maintain focus on the subject.

That describes pretty well what happens when using Servo AF with one point with expansion on a 7D.
Which is exactly what also happens on a 30D, if you use all points in Servo AF, just you don't expand with 2,3 or 4 points, but with all 8 of them.

In One Shot AF, it's different, though.
If you use all points, there is no averaging going on. Instead, the camera evaluates all points, and checks which of them it can find any contrast to focus on at all. For those, it checks which has found something that's closest to the camera. It will then focus on that. Finally, it will check if this brings other focus points into focus as well, and eventually flash the primary and those possible secondary points in the viewfinder, to tell you where it considers your image to be in focus. This is the same for the 7D and 30D.

But if you use single point with expansion on the 7D, you tell it to try to focus with the center point. The camera will always do that, if there's something with enough contrast under that point. It doesn't matter if there are things at closer distance under the expansion points.
Only if the selected point can't find anything to focus on, will it use the auxiliary points, to see if it has better luck there. If it has, it will tell you in the viewfinder which of the expansion points was used.

If you use zone AF on the 7D, it works just like with all points in automatic selection, just with fewer of them.

For a 30D, also realize that the center point has two focusing elements, perpendicular to each other. It can thus focus on a line or edge regardless of whether it's horizontal or vertical. But the other 8 points are linear, either horizontal or vertical. If the contrast you are trying to focus on is in the wrong orientation, the camera may not see it.

Some newer cameras, like the 40D and 7D, have all cross-type focus points. That increases the probability that the camera can find something to focus on.

msowsun wrote in post #10877810
Sort of....

The 7D also has AF point expansion. mode.


Basically, you are using only one AF point, but if that point can not achieve focus, one of the 4 other adjacent points will focus instead.

You can also move this point around. It doesn't have to be the center point.

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Yes, that's pretty much what I was thinking of. Is the 7D the only one that can do this or are there any others?

SamHH wrote in post #10877534
I have a Canon 30D and when I select a single autofocus sensor, the performance is much poorer than when all of the sensors are selected. Presumably this is because the camera ignores the information from the other sensors?
?

How is the performance poorer? Is it slower, or are you getting a lot of images that are out of focus? None of these situations are normal when using a single focus point. There shouldn't be any difference.

Most people only use a single point. It ensures that what you are shooting is always in focus. With multiple points, a high contrast background can 'steal' focus from your subject.

You might try A-DEP. It's the closest to what you seek.. It uses all focus points. It finds the nearest and farest object in the scene and calculates a depth of field that will get as much as possible in focus... The disadvantage to this is that you can't control the depth of field.

Jim_T wrote in post #10877862
How is the performance poorer? Is it slower, or are you getting a lot of images that are out of focus? None of these situations are normal when using a single focus point. There shouldn't be any difference.

Most people only use a single point. It ensures that what you are shooting is always in focus. With multiple points, a high contrast background can 'steal' focus from your subject.

You might try A-DEP. It's the closest to what you seek.. It uses all focus points. It finds the nearest and farest object in the scene and calculates a depth of field that will get as much as possible in focus... The disadvantage to this is that you can't control the depth of field.

Poorer in the sense that it hunts a lot and sometimes gives up trying to focus altogether, even if the lens was very nearly correctly focussed before you pressed the shutter release. You then switch to all nine autofocus points and it focusses almost immediately, but not necessarily on the part of the subject you wanted to be in focus. As someone suggested above, it tends mainly to be a problem in poor light.

Thank you for all the information guys.

PacAce wrote in post #10877840
In One Shot AF mode, the selected AF point and the adjacent AF points are given equal priority in acquiring focus.

Sure? My experience says that if there is something the selected point can focus on, the camera will not consider the expansion points. Only if the selected point finds nothing, it will look at the other points.

SamHH wrote in post #10877849
Yes, that's pretty much what I was thinking of. Is the 7D the only one that can do this or are there any others?

The 5D/5D Mark II can surround the center point with six assist points, but they are only available for Servo AF.

Apart from that, it's the 1D series, which over the years, and thus models, have had a number of different such solutions.