Why is it better to have more focus points?

thechumpen · 03-15-2013, 05:39 AM

I see a few people talking about upcoming DSLR releases and saying they want more focus points. Why is this important? In what scenario to people use anything other than centre? I always use centre focus to focus on what I want and then recompose the shot. I figure I must be missing something so am keen to understand how people use other focus points.

Is it e.g. if you know you're going to want your focus point in the bottom right for the next few shots then you set it that way? Or is it for use where the camera selects the focus point for you? I've never really understood why I would want to use that as it doesn't always focus on what you want.

From all the requests for more points I'm guessing there must be some good reason for that that I am not understanding so am hoping to learn a bit and expand my technique.

JinDesu · 03-15-2013, 05:54 AM - **4 Likes**

There's two items to consider - focus points and focus point size. More focus points means the focus point size must be smaller.

Let's start with the number of focus points - the reason for this is focus tracking. See examples below:

The first image is a Olympus E-3 with 11 points. The second is the 1D Mk3 with 40 something points. See the density difference? If you are tracking a bird with one point on the E-3, and it moves into one of the empty spaces in the AF grid, your E-3 will stop tracking it. It might not even be able to start tracking again when the bird reaches the next AF point. This is partly the processing requirements (to recognize the same subject). The 1D Mk3 doesn't have as much "empty space". Thus - it will track better.

This is most applicable to tracking - users of single point AF won't notice the difference between these two (except for focus point size issues). Without a large number of focus points, Predictive AF is not going to be reliable.

As for AF point size - the k-5 is notorious for having "large" focus points. A large focus sensor can be problematic because the single sensor can "see" multiple objects and will select one of those objects to focus onto - for example, if you are taking picture of a bird who is somewhat far away, the focus sensor might focus on the bird.. or something it also sees in the foreground (branches) or background (branches). It doesn't know what YOU want to focus on, so it'll just guess. Smaller focus points means you have finer control.

Smaller focus points have it's own issues, because if you don't have a good density of them, you have to use focus and recompose. For example, in the E-3 image - if those points were small, then if I wanted to take a picture of someone's face where the eyes are not at the center of the frame, there is no focus point I could select to focus on the eyes. I would have to focus and recompose.

In summary - quantity of focus points matter most when dealing with tracking. When dealing with single point autofocus, quantity of focus points doesn't really matter. Focus point size must also be taken into consideration.

Na Horuk · 03-15-2013, 06:39 AM

Well, I guess you don't "need" 60 focus points. What I would like to see is smaller AF points and more strategically placed AF points. But I certainly wouldn't argue against adding a few.
But what some brands are doing with AF points is a bit absurd, in my opinion. For example when they have a bunch of them, but they are all clustered in the middle.

thechumpen · 03-15-2013, 06:47 AM

Very useful description, thank you. That makes complete sense now. I haven't started doing moving objects yet which is probably why I've never thought of it like this.

audiobomber · 03-15-2013, 06:48 AM

I use center focus point & recompose for pretty much everything except birds in flight. I would like better tracking and that means more focus points.

normhead · 03-15-2013, 07:31 AM

There is an inherent assumption that more traking points means faster more accurate focusing, but that forgets that more focus points means more time spent on processing by the focusing processor.

Obviously , as with all things... at some point more points means slower less accurate focusing, as you've simply overwhelmed the processing capability of the microchip driving the focusing system. At what that point that happens is at least to me completely unknown. For all I know 11 points may be optimal. Or it could be 39, or it could be as many as you can squeeze in.

It gets a little tiring the endless parade of comments insisting more is better without reference to actual real world tests proving it's true. That it is true, that more is better is an assumption.. not based on any facts I've seen. But as usual, I'm always open to any info anyone might choose to post.

JinDesu · 03-15-2013, 07:59 AM

It is certainly true that the processing needs to be on par in order to utilize the numerous points. I sincerely believe that the top end AF systems like the 1Dx (61 points) can keep up with all 61 points when tracking. However, the "gaps" in the E-3 grid is clear - if an object ends up in those gaps, it cannot be focused on - so 11 points might be too few. The 45 points of the 1D Mk3 seems plenty dense, so maybe that's enough and we don't need more than that.

If anyone with much more knowledge could come in and advise on why we need more than a certain amount/what's the optimal amount of AF points, that'd be great. Personally, I think somewhere between 20 and 30 points is sufficient for AF tracking if we keep our subjects within the middle portion of the viewfinder.

ofer4 · 03-15-2013, 08:00 AM

I can't speak for tracking (although improved performance here would be nice). But even for those of us who mainly use center-point focus, more points would be nice. As mentioned above, higher point density also typically means smaller points. I've encountered many situations in which my K-r misses focus mainly because the center point is rather enormous. In fact, I would conjecture that at least some of the front/back focus complaints discussed on this forum are due simply to the size of the center point.

And norm, I agree that we don't want to be bogged down by processing time. A good implementation of "more points" should, indeed, be balanced by increased processing power. I get the impression--but would love to have more specifics from others--that part of Canon's better autofocus comes from more computational horsepower.