The FujiFilm HS-10 has a 'spot' focus function. This is called AF Centre.
I was reading something recently which was speculating about how big this 'spot' may be in terms of percentage of frame covered. I thought the speculation was a little over speculated. In other words, a wild guess.
Anyway, I have always found that when using AF Centre, there is very little leeway if you depart from the cross-hairs.
So, thought I would do a little controlled test.
Note : Clicking on any image will take you through to a larger example
The image at left is with the front cotton reel top centred in the cross hairs. I have drawn a rough box outline of the dimensions which the cross-hairs and AF Centre box occupy in the frame.
I also have the grid lines turned on in Photoshop. The area occupied by the AF Centre box is approximately the same size as one of the grid boxes.
The image at right simply shows the cotton reel now moved outside the AF Centre box area.
Same as above except enlarged to show the box a little more clearly.
This time at 100% magnification and with the box removed. I have also applied some aggressive sharpening to make things a little more clear (pun intended).
Keep in mind that the cotton reel is within the AF Centre box boundaries, left image, and just outside the boundaries, right image.
What I am seeing is that the in-camera AF Centre 'box' display is pretty much what you see, is what you get.
As I mentioned earlier, the box dimensions are approximately the same as the dimensions of one 'box' in the grid display. If my math is correct, this equates to 1/36 of the frame area - which equals 2.78%.
I have found no literature which mentions the actual percentage. Allowing for some leeway, the area is probably somewhere between 2.5% and 3%.
I always knew it was tight !
This is similar to the image above, but this time I have set the AF Area box to the centre of frame. The result is the same as using AF Centre.
Again, this has had some aggressive sharpening to show the effect more clearly.
Images 5 through 8
This is an update added next day
I posted my results on an internet forum and it generated some debate. Even attacks from another blog.
This ranged from - My incompetence in testing, making s**t up, the 'spot' area moves at will, the 'spot' area is centre weighted, the 'spot' area will vary with focal length/aperture, Fuji calls it AF Centre therefore I am lying by calling it 'spot', etc., etc.
Anyway, I was out today and shot several sequences where I had a central object and then zoomed in/out and took consecutive images. Result was the same every time.
IF you get the cross hairs on the subject, the camera will retain focus on that subject, irrespective of focal length.
Not rocket science, is it ?
Incidentally, I did the exact same sequence with my S100 FS, using AF Centre and the results were identical.
Anyway, this is just one of the sequences, and is indicative of all. The other sequences included subjects near/far and varying sizes.
These images are not post processed, not even sharpened.
Images 9 through 12
Same as above series but this time focusing on the pole at centre.