How do the AF systems of each work? I hear the hexar's is legendary. Is it actually better? If it is than why didn't Fuji just copy the IR idea?
I have used both cameras, and while the X100 in many ways quite obviously has been inspired by the Hexar AF, their AF systems are radically different:
The Hexar AF uses an active IR focusing system that requires practically no ambient light to work. The camera emits two IR light beams that are made to intersect at the desired subject distance. This works very well and extremely fast, as long as you don't try to focus on mirrors or through glass panes. The Hexar's VF only displays a cross as AF target, and can be off if you try to focus on very small targets.
The X100 uses a contrast detect AF system that is pretty fast for contrast detect standards, but a little, but not annoyingly slow when compared to an SLR/DSLR phase detect AF system. If you're used to AF SLRs/DSLRs, you will have to adapt to the X100's different AF operating principle: The camera will not focus on edges, but on surface structures found in its AF window.
It is therefore a good idea to make sure that the AF window is filled entirely by your target, and does not contain any background, or else you might get flaky AF performance. The AF window in the X100's optical viewfinder cannot be resized, the window in the camera's electronical viewfinder does offer a resizing option. Thus, if you need to focus on small objects, the EVF is the better option. The OVF's AF window is quite sufficient for measuring on people's faces on short to medium distances, however.
If you adhere to these rules, then the camera delivers good and precise AF performance, even in very low light. Actually, the X100 can focus equally well in low-light situations as a camera that uses phase-detect AF. Under no-light conditions, both AF systems will start to fail at approximately the same luminance levels.
Another inherent consequence of the X100's contrast detect AF system is the fact that the camera cannot focus well on moving objects. Unlike with phase detect AF systems, the X100's AF system cannot tell if it is focusing on a point closer or further away from the actual target's focusing distance, so it cannot work preemptively. If you're shooting moving objects, you'll either have to close down your aperture somewhat to get some "safety DOF" or change to manual focusing mode and use zone focusing. The X100's focus assist function for manual focus, however makes this a very user-friendly option.
By the way, is it just me or do others find the AF is almost impossible when you use the optical finder in the X100, but far better when you use the EVF?
I initially had the same impression, until I found out that this is due to parallax effects that are becoming more serious as the focusing target is moving closer to the camera.
Luckily, Fuji has introduced a new feature with the last firmware update: When you use AF in the optical viewfinder, the AF measurement window will be shifted to reflect viewfinder parallax as a function of focus distance. That is, with the shifted AF window, you can do another focus measurement to really nail it in a second run. This has been a huge improvement, and it effectively did away with just about 95% of any AF problems I was experiencing.
I am very impressed with this camera, and I immensely enjoy using it.
