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DSLR Focus Points


Binnacle

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What are the advantages of a multipoint focus system?
I have a Nikon D60 which has three AF-area modes -
1. Closest subject
2. Dynamic area
3. Single point.
I mainly use single point and can more or less work out the depth of field by selecting the appropriate aperture.
A lot of new cameras have eleven or more focus points , how do these work? Do they average out the scene and set the correct aperture or what do they do?
I will shortly be looking to change my camera and this is one feature that confuses me.

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hssutton

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First things first, Focus Points do not control or set the aperture, so have no control over DOF.
All they do is set the focus.

The DOF is controlled exclusively by the aperture, which is set either manually or automatically depending on what mode you have set.

The benefit of multi focus points is that you can move/select where you want the main point of focus to be. This does away with prefocussing and then re-composing.

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Binnacle

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Thanks for your detailed explanation .

I think what confused me was thinking back to a film SLR I owned which had a feature that allowed you to focus on a near object and then again on a far object , the camera would select a setting to keep everything inbetween in focus.
So ,looking forward to multi focus ,it's just something to get used to and hopefully enjoy.

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ventanas

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My advice for what it's worth - never, never use Closest Subject. You have no control at all over where the camera focuses.
Dynamic is ok, but if the camera "sees" movement the focus will shift. OK if that's what you want.

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donki

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Correct me if I am wrong but I would say the focus points do affect depth of field. If you are taking a potrait and say select a single focus point over the subject's eye this will where the image will be clearly in focus. If using a large aperture say F1.8 the depth of field will be set around this focus point. Of course the size of aperture effects if a DOF is shallow or long but the focus points effect where the depth of field is placed, please correct me if I am wrong but When I am shotting that certainly seems to be the way of it.

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cycoze

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Where you focus will indeed determine where the "in focus" part of the image will be...

What hssutton has said is correct in that DOF is controlled by the aperture, which is set either manually or automatically.

Some cameras, a few Canons for example do have an A-DEP setting 'auto depth of field' which lets the camera set the aperture to cover a certain area for the DOF (which comes under automatically as pointed out by hssutton), which is what I believe is what Binnacle was asking about in the first place.

Canon have taken that off some newer models and used a DEP (DOF preview) button instead, this allows you to quickly check the DOF through the view finder when you have manually changed the aperture and ultimately the DOF.

Again as already said the focussing points can be manually selected say to focus on an object on the rule of thirds point so that is in focus, using a shallow DOF would then blur out the foreground and background to a degree.

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