What is the point of Auto-Focus points?

Question

If a camera has a touchscreen, like the T4i, what is the advantage of using AF points vs touching the screen where you want to focus?

Why don't all cameras have 50+ AF points, instead of only the more expensive ones? Is it expensive to have them in a camera?

Answer 1

Lots of focus points allow the camera to automatically select the area which it thinks you want the camera to focus on. In theory, this allows you to frame your shot without worrying about selecting the proper area to focus on. If you're doing people shots and you always want to focus on the closest person, this function works well if you're using live view with face recognition.



If using the viewfinder, lots of focus points means the camera has a better chance of choosing the proper area to focus on. Yes, focus points are expensive to implement when using the viewfinder, so the better cameras will have more. It's actually physical structures which form the focus points when using the viewfinder. I find that 3 focus points is no fun, 11 focus points works well, and 39+ focus points is overkill for me. However, I usually find myself using single point focus, lock my focus and then frame my shot. Having the camera automatically choose my focus point usually will end up focusing on the wrong thing, especially when shooting animals with narrow depth of field and studio shooting of industrial objects. I have my camera set up in a bizarre way where the focus control is associated with one of the special function buttons which allows me to continuously track focus or lock focus.



Another reason for having different number of focus points is product differentiation. You don't want to put all sorts of good stuff in the low-end products so it will force folks to buy the more expensive product if they want the extra features. For some, it's enough to make them buy the better product which means more money and better profit margin.



Some cameras allow you to track your subject if it moves. This, in theory, is great for tracking focus of moving critters, including children. I have found this to be a gimmick so far since I like shooting wild critters against a complicated background. Perhaps this works better in a back yard setting.



If you're getting a DSLR, try using the viewfinder for a month straight. You just may find yourself liking it better than live view. If you're not used to using a viewfinder, it will take some time to become accustomed to it. There are situations where live view is handy to use, but I find it very hard to use live view since you are fighting with ambient light on the LCD, slow update rate (makes tracking and photographing fast moving critters really hard), horribly slow focus (Sony is the exception), really bad battery life (important to me since I like to go out in the bushes for a couple weeks where there's no power or go on holiday and not bring a charger), hard to frame images, difficult to do manual focus, hard to track fast moving subjects due to bad holding ergonomics, and hard to hold the camera steady at arms length. There are more evils regarding live view, but I think I've beaten the subject to death.



On manual focus, since digital SLR focus screens are unsuitable for manual focus, most people use the auto focus functionality and may even use single point focus like I do. In the old days, we had split and micro prism focus screens which worked great for manual focus. BTW, you'll be hard pressed to find a pro using live view. Pros need to operate efficiently which live view is not. The only time I use live view for stills is if I need critical focus which is usually studio shooting of inanimate objects. Live view uses contrast focusing which is very precise and horribly slow, especially shooting action shots. Phase focusing, which the viewfinder uses, is much faster, but is not as precise in certain cases.



See http://graphics.stanford.edu/courses/cs178/applets/autofocusPD.html for a nice demonstration on phase focus. You'll need to wait for the demonstration applet to load which will show you how phase focus works.

Answer 2

Hahaha!



I also don't get a gazillion focus points. I only need one - at dead center. I focus there then move to frame. I understand others frame and focus at the same time. The technology's there. They chose to use it. During my time, it wasn't there. I learned to do it differently. It doesn't matter how you shoot. What matters is you get the shot.



That's the reason I have no use for Liveview. I don't mind lying down on the grass or stooping down and aim by line of sight. I always get my shot no matter what.



Which drives me to the last issue. Expensive cameras. I would religiously send my thousand and one gratitudes to anyone who sends me a Hasselblad but for the time being, I'll get by with cheap secondhand dSLR's with disabled multi-AF points and untouched LiveView buttons. Software? Programmer speaking here. The more points there are, the more programming code grows exponentially, the slower the camera becomes. The fix is to install a faster microprocessor in the camera driving up the cost further. Then again, with cheap cameras, why would you want to do that. How can you even cram 64 focus points on a tiny digital sensor?

Answer 3

Ok, autofocus points can be thought of like the points of a frid on which the details can be captured best. Think of when you are using autofocus on a moving target- as it moves, the autofocus point changes giving better pictures- the more autofocus points, the better the tracking of the moving target. However, making those points of sensitivity and the the software to track and utilise them, costs money- so to keep the price down in cheaper cameras, there are fewer autofocus points.



Also, not all autofocus points are equal. you will also see that generally the specification includes how many "cross type" points there are. Whats the difference? Normal autofocus points generally only track horizontally, cross type points vertically and horizontally making them more sensitive and better able to track in low light.



Why have all these points and not just a central one? First is the moving tracking with autofocus I used as an example above. The second is the issue of having very tight focus. As the number of pixels and sensitivity of sensors increase, they become more sensitive to movement and focus. This means that even the slight movement to recompose the picture by moving the camera after focussing in the center can result in losing sharpness. As the pixel count goes up, so the issue becomes even more evident- so on the new Nikon D800 with a 36MP sensor, even a slight shift can result in horrible blurring on your picture. Just one of those things of changing technoogy, a change fo habit from using the central point to using multiple points.



Personally I rarely take pictures of moving subjects- but generally us AF-S (autofocus, single point) and select the point most useful for the picture. In the beginning it takes a while, but you get used to altering focus points, even with the camera held up to your eyes

Answer 4

You're using one of the best consumer level cameras there is and you use the screen for a view

finder? Oh my, no wonder you don't understand your camera at all.



Obviously you just don't get it and won't until you get some education is these cameras. I'm sorry but I can't explain it to you here - not enough room or patience.