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  #1  
Old June 22nd, 2008, 07:45 AM
doug anderson doug anderson is offline
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Default 51 Focus Points, Automatic (?)

I've been shooting on full auto focus with all 51 points active and am somewhat puzzled at how it works. The focus point indicator moves around in the viewfinder and seems to be looking for the right point, especially in low light. In bright light I don't see it at all. I compose my pictures, as usual, and the subject seems to be in focus most of the time. How does it know what I'm shooting, especially in a busy composition?
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Old June 22nd, 2008, 07:57 AM
Cem_Usakligil Cem_Usakligil is offline
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Quote:
Originally Posted by doug anderson View Post
I've been shooting on full auto focus with all 51 points active and am somewhat puzzled at how it works. The focus point indicator moves around in the viewfinder and seems to be looking for the right point, especially in low light. In bright light I don't see it at all. I compose my pictures, as usual, and the subject seems to be in focus most of the time. How does it know what I'm shooting, especially in a busy composition?
Hi Doug,

As far as I know, there are certain algorithms which are applied in the camera in order to choose the most likely point of focus in a given composition. But this will not work in all situations.

If you are shooting P-mode, the camera might play with your aperture value to create enough depth of field in order to satisfy multiple focal points in the composition.

So the short answer is that if you want to be in control for 100%, you cannot depend on the camera making the choice for you. For most practical purposes, however, there is no harm in using the camera intelligence. As you apparently have discovered it for yourself :-).

FWIW, I assign the focal point myself and focus and sometimes recompose. Although I try to avoid recomposing by choosing the most nearby focal point.


Cheers.
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  #3  
Old June 22nd, 2008, 08:17 AM
Asher Kelman Asher Kelman is offline
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Doug,

Have to found the dials/controls that move around thew focus points? In Canon one presses one button (back upper right, and then rotates one wheel to get the point where you want it. Once you get that skill, you are all set. Otherwise the camera looks for contrast differences or phase shift, I believe to choose the most likely focus point you are pointing towards.

Asher
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Old June 22nd, 2008, 09:18 AM
James Newman James Newman is offline
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What settings do you have active as far as your Autofocus is concerned? I am curious because with the settings the way I have them, my focus point does not jump around. It stays on the middle focus point initially and then I can change it manually with the multi selector button on back of the camera if needed. It can be a little hard to see when there is a lot of light but you can adjust the brightness of the point in your menu settings to make it a little easier to see.
James
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Old June 22nd, 2008, 09:29 AM
Cem_Usakligil Cem_Usakligil is offline
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Originally Posted by James Newman View Post
What settings do you have active as far as your Autofocus is concerned? I am curious because with the settings the way I have them, my focus point does not jump around. It stays on the middle focus point initially and then I can change it manually with the multi selector button on back of the camera if needed. It can be a little hard to see when there is a lot of light but you can adjust the brightness of the point in your menu settings to make it a little easier to see.
James
Hi James,

That is the setting Asher and I have referred to. Which is the manual assignment of the focal point to be used by the camera. The setting referred to by Doug is the one whereby all points are active and the camera chooses which one(s) to use.
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Old June 22nd, 2008, 05:33 PM
Edward Bussa Edward Bussa is offline
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Doug,

I have the D300 and here is what I know:

1. The only time I've experienced the dance you mention, is when a) the menu is set to 51 point (3D-tracking) and b) the back switch is set to the middle position (in this case "3D").

2. If you use 51 point (3D-tracking) with the back switch in the 1st or top position (Auto), there are algorithms that do high performance scene recognition, part of which is face-detection-like features base on the color of skin.
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Old June 23rd, 2008, 06:44 PM
doug anderson doug anderson is offline
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Dear Folks: Thanks for the info. I have only recently started to use this feature, since I turned it on accidentally. Prior to that, I had been manually setting each focus point according to where I wanted to place the subject. I guess only time will tell which is better, but I don't have any fuzzy shots yet.

Cheers,

Doug
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