What's the advantage of many autofocus points rather than fewer spread wider across the field? [duplicate]

Question

I have a D3200 with 11 focusing points:

The newer and more expensive bodies have even more focus points like e.g. the D610:

I mostly work in Single-point AF mode, which means I choose one of the 11 points, move the one point over my subject, and focus.

Isn't it more comfortable to have fewer focus points wider spread over the whole field instead of like 50 in a small area in the center?

• when I want to focus something in the left corner, there is no focus point and I have to set the focus with a different focus point and move my subject into the position with locked focus
• the more points I have, the more difficult to handle is the Single-point AF mode by switching between all of these points

Am I misunderstanding something about focusing?

Even in Dynamic-area AF mode or 3D-tracking which uses all of my 11 points, it won't be able to focus something in the corner.

Answer 1

This is a bit of an apples and oranges comparison, because the D610 is a full frame camera.

isn't it more comfortable to have less focus points wider spread over the whole field instead of like 50 in a small area in the center?

There are two different things here:

1. number of af points
2. spread of af points

Now here's the thing: the af points of the D610 are not in a smaller area in the center, the overall image is just bigger. Having more focusing points doesn't mean that they are in a smaller area. In general, the overall area where the focus points are located is the same for the equivalent field of view.

For comparison, I looked up what top of the line crop bodies Nikon offers and found a AF point distribution image of a D7100, which has 51 af points.

I added them all together into one image. The D3200 is dark blue, the D610 is red and the D7100 is green.

As you can see, the D7100 and D3200 have a very similar area of af point distribution. The D610 appears to have a smaller area, but again, it is a full frame camera. I added a scaled image of the D610 and made it more opaque. The scale factor was 1.5, which is roughly the crop factor of Nikon bodies. The scaled D610 area is roughly the same as the one of the D3200 and D7100.

conclusion

• The number of AF points can be increased, without increasing the area of the spread. Spreading the points further would require lenses with a more open aperture. That's why the additional AF points are cramped into the same area.
• If using a full frame camera, the additional sensor real estate makes the area of the AF points smaller. If switching to a full frame camera, this should be taken under consideration.

Answer 2

Depends on what you're shooting, but usually, yes, it is much better to have more focus points.

There are cameras with just one focusing point (eg. Hasselblad) but usually, especially when you shoot action, you need more points because:

• focus in center & recompose is prone to error, blur and the delay which the movement of your hand introduces can make you miss the 'decisive moment'. Been there done that.

• you need many points to successfully track a moving subject. Yes, I understand you that 11 points aren't enough to track - however the situation changes radically when you have eg. ~65 points like in Canon's professional models 1D X, 5D3 and 7D2. (I don't know very well the Nikon side, but I suppose that a similar AF engine exists in their top models).

• prefocusing (or similar techniques). You need to (pre)focus exactly in 'that' point of the frame without moving the camera (recomposing), but you don't have the luxury to manual focus. It happens enough time in sports, performance, photojournalism etc. Having too few points means that you will NOT have an AF point in 'that' point, thing which could be very frustrating.

• there are other AF modes than Single-Point AF. These modes are there for a reason and they help sometimes a lot in certain situations (low light/contrast, moving subjects etc.)

Perhaps I'm a spoiled child, but I felt a lot this difference in an occasion when I was forced to shoot with a model from 'Rebel' line (Canon 600D IIRC). Usually I shoot with Canon 5D3. The difference in practice is much bigger than in theory.

As an aside, at least some cameras (personally I know Canon 5D3 & 1D X) have the possibility to disable some AF points, so there is a reason in your question.