Friday, November 12, 2010

F300EXR Review – Part 26 – Auto Focus Issue

I’ve been meaning to explore this discovery for quite some time, and I finally got off my butt this evening to do it. I noticed a few months ago that the AF Center focus mode sometimes was completely unable to find focus on a small object running through the center of the frame, even when there was lots and lots of contrast and it was at full zoom.

Well, today I set the tripod up at the door end of my porch, facing a string of Christmas lights hanging off the far end of the porch (yet one more task for the weekend) and tried shooting the string.

Here is that scene at 24mm …


Well, it locked on immediately, so I figured maybe the contrast was way too high.


By the way, note that Fuji has fixed the F80’s wonky IS when mounted on tripod … the image is perfectly sharp, just like the F70 when mounted on tripod. Thank you Fuji.

Anyway, I decided to shift to one weed stem poking up abive the railing. The contrast is much lower and the target is much smaller.


So it nailed the focus pretty easily. I poked and prodded and it was nailing focus with no problems. But then I tried it by focusing on the mirror to the left of the stem and then focusing on the stem again.


So the car worked with no problems. Then shift the camera slightly right to overlap the stem and you get … the same image. Over and over and over … the AF is clearly shortcutted to be happy with the car in focus instead of the stem. Heck … we know that they use a funky arrangement of AF phase detect sensor pixels, and perhaps none were lying on the stem. The phase detect was likely active, as the camera makes a special buzz sound when focusing that way (as near as I can tell.)

I then decided to shoot another image with the self timer and the stem snapped into focus! No buzz this time and you could see the AF traverse the whole distance (which is still very fast in EXR cams.) I recreated the test several times … focus on mirror … shift to stem and half press … bzzzzt and focus stays on mirror / car … set self timer and boom! … focus is achieved …

Ok … so changing modes like that forces the cam to focus the whole range. Got it.

So when you are trying to capture focus on a bird or squirrel on a fence or a wire, just set the 2s self timer and the cam will lock focus.

But is there a better way?

I’m glad you asked … because I had found another fix for the issue way back when and have used it successfully for video, which is notoriously bad at AF on these Fujis. That is to set AF mode to “Multi”, which on a Fuji means to find the highest contrast subject closest to the center of the frame.

I set that mode and ran the test again … no focus failures! The cam always ran through the whole range and found the subject.

And it just struck me why … the phase detect is based on pixels in fixed positions on the sensor, which pretty much means that the phase detect works only in one position on the sensor, and that would be the center focus position. That’s why we hear the buzz only in AF Center mode.

But in other modes, the old contrast method prevails and it is deadly accurate and always runs through the whole range. So this is another, possibly better, fix for you to nail the focus on small items near the center.

So this test strongly implies that the phase detect AF needs work before it is reliable …

Here is an example of AF Multi avoiding the distraction of a person walking right through the frame while focusing and nailing the best subject anyway … good job there Fuji.



Jeffo said...

Hi Kim, regarding on Autofocus problems with the F300exr, can you try AF center mode with macro enabled. Does it help with the autofocus problem instead of the 2 second timer solution? I had the same problems and it worked as a "fix"

Kim Letkeman said...

Jeffo: Macro mode allows you to focus at about half the normal distance, and without using it you cannot get focus lock at all. But this subject was 7 or 8 feet from the camera, so there was no need for macro mode, and in fact it may have prevented focus lock ... macro mode is limited as to how far away it can lock focus. I don;t think this was the specific problem I was seeing.

I just ran the test and the behavior did not change. Shifting the camera slightly to focu on a closer subject (even a fairly big one with a very high contrast to the background subject) would cause the AF to perform its short form of focus and leave it on the background.