Well that explains it. I missed that in all the reading I've done in the last few days. But thinking about it, it makes sense.
So if I have it set to ONE, and I see it show just one on the color display. Why do I see a screen full of red in the viewfinder. I can't see the points in the VF, it's just a red cast that comes up for a moment.
I used to have a d40x. That had three points. I could see those and see when I'd select each. Of course they were larger. But with these I just see a red light that comes on, not individual points.
I've enabled the showing of the focus points in the image review. That auto does a darn good job, much of the time. But then there are some that are not where I'd put the focus.
I suspect the red cast you are seeing in the VF may have something to do with your visual impairment since that doesn't describe what I see in the VF. (It's also possible that your VF is defective.) When you change the AF point or half-press the shutter button, there is a brief red illumination of the selected focus point(s) and, if enabled, viewfinder grid lines. If this is troublesome for you, you can turn it off via CSM item a4. Then the focus points will always be the small black rectangles.
You are right that the focus points are smaller than the ones on your D40x. After all, there are 51 of them!
When you half-press the shutter button, in any AF-area mode except AUTO, the display just keeps showing the selected focus point. In AUTO AF-area mode, the display will briefly show the focus points where the subject is (as decided by the camera), then show the single point that is the one chosen by the camera as the one to use.
In the 3D and AUTO AF-area modes, the displayed focus point will change as the camera tracks the subject. That's not true of the other AF-area modes.
In every mode except AUTO, you can move the selected point around. (In 3D mode, it's just a starting point as the camera will shift points to track the subject.)
One other note... if you hold down the AF button, the viewfinder display changes to show the focus points that will be used (in 9-, 21- or 51-point dynamic modes) or it will form a "3D" symbol in 3D mode or an ellipse in AUTO mode.
The single point is not limited to the center -- you can pick other locations -- but the center is often the easiest one to use.
One of the reasons that the dynamic mode (9, 21, 51) only shows the one point in the center of your group is that this is the one point that you are supposed to keep on your subject to help the camera know which object is your moving subject. The bigger the group you choose, the less accurate you can be, subject to the guesses by the camera.
One of the other reasons to show only the one point is that showing all of them would clutter up the display, which is assumed (rightly or wrongly) to be undesirable in most situations.
---------+---------+---------+---------+---------+ Joseph K Seattle, WA, USA