I've read a lot but could not find a clear explanation. As I understand, a lens can only focus physically on one point since the focus depends on the physical position of the lens vis-a-vis the sensor. So what is the purpose of these 49 focus points my camera has? Does the camera take an average of the points to determine the final single focus? In that case, I don't see why 49 points would be any better than 5 or 10 points.
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2\$\begingroup\$ Possible duplicate of Can more than one focus points be selected? \$\endgroup\$– mattdmApr 25, 2018 at 15:27
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2\$\begingroup\$ Possible duplicate of What is the advantage of the huge number of AF points? \$\endgroup\$– inkistaApr 25, 2018 at 18:33
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\$\begingroup\$ Related: Autofocus points in Mirrorless Cameras and What is the effect of the number of cross-type focus points on sharp focus? which both discuss the number of focus points as well as the difference between CDAF (mirrorless/Live View) and PDAF (DSLR only). \$\endgroup\$– Michael CApr 26, 2018 at 4:31
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\$\begingroup\$ I think it's worth mentioning that there are many different ways for cameras to use autofocus points, and higher-end cameras typically let you choose from several different options for this. (The "autofocus mode" is a primary setting related to this.) A careful perusal of the focusing section of your camera's manual will help understand what your particular camera is doing in any particular mode. \$\endgroup\$– cjsMar 25, 2022 at 5:39
1 Answer
You're right about the physical / optical limitation. There is only one plane of focus, and you can't use multiple focus-system focus points to get around that.
The advantage is simply that you can choose what to focus on without changing the framing or otherwise recomposing. If the thing you want in focus is over in a corner, you can select the corresponding focus point, and there you go.
Additionally, modern advanced autofocus systems generally have modes where they track subject motion. This is super-useful for sports, wildlife — or just your kids at the playground. More focus points (and ideally a wide spread of them) make this possible.
Modes where the camera allows more than one focus point to be active do one of three things (depending on the specific camera and settings):
- They grab one of the focus points as determined by the camera's processor. Probably the one that's most in focus already, or the most contrasty — but it could be more complicated than that.
- They try to optimize depth of field so that multiple points are in as much focus as possible.
- They use the multiple points as part of a tracking mode, as above.
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\$\begingroup\$ So you're saying a single focus point is tied to a specific portion of the image- if it had just one focus point, it physically could not focus on the sides, for example. But what about when it uses multiple focus points at once to focus, is it taking the average of these? In other words, what's the point of using more than one focus point at the same time, if a lens can physically focus on one only? \$\endgroup\$– MarcoApr 25, 2018 at 16:09
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\$\begingroup\$ by the way, are there focus points on a camera with a touchscreen (any help?) or are they there to indicate that the some places cannot be (auto)focused? \$\endgroup\$ Apr 25, 2018 at 17:16
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\$\begingroup\$ @user152435 That sounds like it might be a separate question. Some cameras with touchscreens let you select focus points in that way — is that what you mean? \$\endgroup\$– mattdmApr 25, 2018 at 17:33
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\$\begingroup\$ @user152435 A touchscreen can be used as an interface to allow manual selection of which focus point to use. For example, when photographing a computer component on my desk I'll set up the framing first and then touch the screen to indicate on which part of the component I want the focal plane to be set, and the camera will do its autofocus using that point. \$\endgroup\$– cjsMar 25, 2022 at 5:37