Let’s assume that you have a camera which claims that it has 51 focus points.
You attach a 300 f/2.8 on it and you can select any of the 51 focus points. So far so good. Next you go to shoot some birds and choose to a lens like the 500 f/4.
Now it seems that you are allowed to select only 30 focus points. Furthermore, you go for a 500mm with a 1.4 TC and you find only 9 points selectable now. I might not be exact with thee numbers for your camera but you will definitely notice a reduction in the number of focus points.
But why is this happening?
Well simply put, not all AF points in your camera are equal and you need to check for this when purchasing one.
When the camera manufacturer makes a camera with 51 autofocus points, it is quite possible that all of them will work only with a very wide aperture (f/2.8 in the above example). As the maximum aperture of you lens starts to reduce, so does the ability of the autofocus points to detect contrast and the camera manufacturers disable the points that will not be able to acquire focus at those lower aperture values.
Why is that some of the AF points can function and others can’t. That depends on how sensitive the AF sensor behind each of these points is and also how much light actually can reach that sensor point. There are a couple of types of AF sensors in the market, for simplicity sake lets consider just two of them.
- Cross Type
- Non-Cross Type
The cross type AF sensors which are able to detect contrast in horizontal and vertical direction. They are generally the AF points that are active for lower max apertures but the horizontal AF sensors points and the vertical AF sensor points are the first ones to turn off.
There are more details about how under lower Aperture lenses the cross type points change to unidirectional points as well because of lack of light but that’s for a later post. For now, just do not panic if on some lenses you do not see all the focus points. It is by design and you just need to be aware of it.
I have found that as long as you understand the limitations of your equipment, you will be able to appreciate and use it better as well.