Auto-focus points in digital cameras confuse me. Explain them?
April 30, 2017 8:48 AM   Subscribe

I don't get the point of auto-focus points, how they work, and how the capability to handle more of them makes a camera better than other cameras. Please enlighten me.

A camera exposure is focused on only one distance, yes? If so, is the idea of many auto-focus points that the camera has a lot more focusing distances to choose from, so it's more likely to use the best one? Or is it that these cameras actually take many separate exposures nearly instantaneously, at all the many focusing distances, and splices an image together so that every element in it is in focus?

I've tried to get multiple supposedly camera-savvy electronics store employees explain this to me, and none of them seem to understand the concept better than I do. They basically just assert that more auto-focus points = better camera.
posted by Mechitar to Technology (6 answers total) 4 users marked this as a favorite
 
A high number of focus points simply means the camera lets you choose the focus point more flexibly. If you have only nine focus points then that limits somewhat how you can compose the photo so that the object you want to be in focus is actually in focus, compared to a camera that has 75.

(You can focus and then recompose and take the picture, but that takes time and you might miss the shot, depending on what you're trying to shoot.)

When you let the camera choose the focus point on its own, it is more likely that the subject of interest will be close to one of them if there are more of them.
posted by kindall at 9:10 AM on April 30, 2017


There's two ways to auto-focus. Contrast detection is used by many mirrorless cameras (phones, for instance) and just look at the sensor data to see if stuff's sharp. With that approach it can autofocus anywhere in the frame and there are no autofocus points (or everywhere is an autofocus point).

The other approach is phase-detect autofocus, commonly used in SLRs. A partially silvered mirror redirects some light to special sensors that determine focus based on properties of the light itself, rather than blurriness. This allows AF to be faster, and allow it to focus on stuff without much detail or contrast. But the special sensors aren't just everywhere in the frame, and wherever they are is the only places that the camera can tell if the image is in focus or not. Those places are the autofocus points, and they are the only parts of the picture the camera can autofocus on. More will also help the camera track moving subject and keep them in focus
posted by aubilenon at 9:34 AM on April 30, 2017 [3 favorites]


RE: your question about one exposure, one photograph, yes, cameras only capture one exposure with one focal depth. The exception to that is a light-field camera, of which Lytro is the only commercial example, I believe.
posted by misterbrandt at 10:43 AM on April 30, 2017


  A camera exposure is focused on only one distance, yes?

In theory, yes. In practice, due to finite sensor/film size, lens apertures being small, and the human eye being able to accept slightly out of focus as in focus, there are a range of distances that will still be “in focus”.

Regular camera lenses render everything from the focus plane to infinity as in focus (explained reasonably well here: Depth of Field in Photography Defined: the Basics | B&H Explora). If you have a simple multi-point focus sensor that checks the distance across the frame in a grid of locations (how different cameras do this is explained well by other posters), most of the time you'll get a decent picture if the camera only focuses on the closest point.

This gets past the problem that early auto-focus cameras had of only focusing on a centre spot. If you're taking a picture of two friends standing side by side by the Grand Canyon, the centre spot focus will pick the landscape behind the subjects. The result will be a nice sharp canyon with two blurry muppets in front of it.

Phone cameras still obey the basic laws of physical focus, but their small sensors have result in a huge depth of field, making selective focus quite hard with phone cams and small-sensor digital cameras. Phone cameras do have fairly epic data processing capabilities, however, so some of them simulate depth of field through selective digital blurring.

So your camera takes just one picture, but has done lots of sampling and calculations across the frame before you finish pressing the shutter button.
posted by scruss at 1:44 PM on April 30, 2017


A lot of simplifications follow ...
A camera exposure is focused on only one distance, yes?
Yes, the lens can only be focused at one point (Lytro, etc. being the exception). However, as scruss mentions:
In theory, yes. In practice, due to finite sensor/film size, lens apertures being small, and the human eye being able to accept slightly out of focus as in focus, there are a range of distances that will still be “in focus”.
... it's not like just that one distance (say 5 feet) is in focus and everything else isn't. Depending on film (or sensor) size and lens aperture, the depth of field will vary. You can calculate DoF here.
If so, is the idea of many auto-focus points that the camera has a lot more focusing distances to choose from, so it's more likely to use the best one?
More focusing distance is a feature some lenses offer. Every (or nearly every) lens can focus out to infinity, it's just a matter of how close they can focus. For example, Canon's 50mm f/1.4 lens can focus down to 18 inches, which means it can focus on anything from 18 inches away to the moon, sun and stars. This can be done manually (by adjusting the focus ring on the lens itself) or by letting the camera focus the lens for you based on its autofocus sensors.

So if the autofocus sensors don't give you a greater focal range, what are they for? As you (and scruss in his/her Grand Canyon example) mention, it does let you use a better one if you've got more. Every photo doesn't need the dead center subject in focus. More AF sensors let you not only use different ones (you can pick), but also let the camera track motion as well as the subject moves across the frame, like the Canon 5D III does. And not only can you benefit from more than a few AF sensors, there are even different types of AF sensors.
Or is it that these cameras actually take many separate exposures nearly instantaneously, at all the many focusing distances, and splices an image together so that every element in it is in focus?
Generally no, but the Lytro camera mentioned above and the Light L16 camera might be up your alley if this is what you're looking for.

Are all of these things necessary? No. I like the photos I take with my film rangefinder camera and often find myself scale focusing and shooting from the hip.
posted by Brian Puccio at 8:58 PM on April 30, 2017


The benefit of more AF points on higher-end cameras is that the camera's software has more data when trying, for example, to track a moving target. Just how complex this can get is apparent from Canon's guide to AI Servo Mode Autotracking on its EOS 1D and 5D cameras, which allow photographers to have different AF tracking setups depending on how fast and in what manner the subject is moving.

I have an EOS 70D and about the only time I ever do anything with AF is to switch to a single AF point if I want to be sure a particular part of the image is in focus. Even then I do lock-and-compose most of the time.
posted by Major Clanger at 5:27 AM on May 1, 2017


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