I assume you mean single point focus or do you mean spot metering?
Z8/9 Auto Focus (C) w/Sub Detection 🐦+👁️=🧐
- Thread starter Larry S.
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No, no changing modes that fast
All I mean is that quite a bit of the success for a diving bird lies in the photographer keeping it in the same place in the frame as much as possible 
ajrmd
Well-known member
Again, for a vertically diving bird, I've been surprised how well the Z8 follows the bird and depending on the splash, it can maintain focus through the dive, through the splash, and then during the recovery on the surface (think pelicans). Where I am encountering the unexplained AF behavior, namely sudden loss of af before the strike, is on birds who dive through the catch, namely Osprey and Eagles. The AF doesn't appear to be grabbing a more contrasty or distracting element, rather it appears to pulse oof for a couple frames and then recovers back on after the strike (see the earlier images I posted). Again, I haven't seen this oddity with my Canon or Sony rigs and I am beginning to wonder if it is a lens issue, a lens/body communication issue, or something else.No, no changing modes that fast
Today, I was shooting osprey close to the water (unfortunately, no strike sequences) and didn't witness this phenomenon. Maybe something was fixed in FW 2.1?
ajrmd
Well-known member
My observations are with the Z8 and I am unable to comment regarding the Z9's performance in these situations.
You own a Z9?
Looking at those shots, it's almost like the camera is anticipating the bird getting closer and then correcting when it doesn't happen. Almost like an error with predictive tracking.
Also in this case the bird if far enough away that depth of field has the whole bird eye and all in focus. I used single point focus for many BIF in my DSLR days. Only started using group for some with the D4S so thousands before that. Single point is still effective for me when needed with Z9.
Makes sense, but why would the camera do predictive adjustments when it can calculate AF at 120fps?
Cameron T
Well-known member
Because it has to move the AF to where it thinks the subject will be, otherwise it'll constantly be behind?
I assume subject detection bird was turned on ... in Z9 and I assume Z8 the sequence is body, head, eye ... it happens so fast I frequently miss it. But if there are contrast issues ie. the bright ness of the white on the bird it can have issues. I would guess the histogram would show the right side climbing the wall. Also the strike point is behind the wave at the moment of impact. The vagaries of subject detection can be a mystery.
I do not have a Z8 but were there any notes in what updates were made with 2.1 ... was 2.0 when bird was added as a subject detection choice in the Z8? Also I and many with more expertise than I have noticed that almost every firmware update with the Z9 has included AF improvements and sometimes they are not even mentioned in Nikon info on the update.
And at that point the target fish is behind a good sized wave with reflection of the Osprey on top of the wave.
tomcat
Member
Why wouldn't Dynamic area AF mode work here? If you stay focused on the sitting bird, then focus should not jump away. I use dynamic area AF mode all the time for basketball to ensure that the camera stays focused on a particular player when there is a gaggle of players around the ball. Just wondering...It was really weird. I’ve had the sub-det box bounce around a bit when a bird turns its’ head and the camera is hunting focus. It locks on odd stuff occasionally and gets confused. Here’s another example that occurred a couple weeks ago. I was locked down on the female osprey’s eye as the male came to the nest. She dropped her head behind a branch and the yellow sub-det box bounced around and locked on a white round object to the right. No problem here as both were in the same focus plane but the reflection issue is another matter. It might be best just to cut off the sub-det….
View attachment 87279
Why behind? Isn’t 120 calculations / adjustments per second more then enough to use the live data and apply it to 20fps?
Cameron T
Well-known member
Because the lens has to move elements to focus.
If the camera sees something in the past (which it does, even if it's only very very slightly) it might be oof. It has to move the elements to keep up in real time and know where the subject is going to move them correctly. Thus, it has to predict it. I'm not sure how to explain this to you.
Yes, that might work just fine. I had recently encountered this anomaly of the subject detection locking on the eye and jumping forward to the reflected image which was not in the focus box. The upside of this is the f/6.3 with the Z600PF has a broader depth of field than a f/2.8 or 4 so slight focus errors are not as critical. Your suggestion has merit and I will give that a try. I’ve used the dynamic focus area before with very good results. The new stuff keeps coming with the firmware and I like to see if it’s useful.
ajrmd
Well-known member
Appreciate the thoughts, though short of some sort of lens/camera communication/operational issue, I tend to side with Steve's explanation involving predictive AF. Why? This AF error is repeatable and situational. The panning technique appears appropriate from the context that the subject remains large in the frame and is relatively in the same position, thereby minimizing user disturbance in the AF point. Also, if you look closely at the sequence - and my apologies for not posting more images though I am trying to be respectful of the forum - the AF plane doesn't appear to move to the bird's reflection in the wave, rather it is several feet in front of the bird where it remains for several frames and then it resets back onto the subject. One of the challenging aspects of deconstructing these sequences is the lack of information of the true AF point. The reliability of the AF point displayed in NxStudio remains debatable, however from the sequence presented, it doesn't appear to deviate significantly.
As a consequence, I tried to dissect the situation further by looking at the images AF information in exiftool and have included a portion of it here to spur further conversation. To recall, the sequence was comprised of 70-100 images shot continuously as the osprey was swooping in for a catch. I labeled the image taken immediately before the camera lost AF as "one" and provided several sequential oof images, labeled "two" through "four", and skipped the additional oof images until the camera regained AF several frames later in image "nine". The summary of information is listed in the table below:
| Image one | Image two | Image three | Image four | Image nine |
Focus Distance | 49.17 | 49.17 | 49.17 | 49.17 | 49.17 |
Lens Position | 1241 | 1302 | 1326 | 1326 | 1278 |
AF Area X position | 3699 | 3946 | 3989 | 4279 | 4452 |
Focus position H | 2L of Center | 1L of Center | 1L of Center | 2D of Center | 1R of Center |
AF Area Y position | 3355 | 3129 | 3000 | 3226 | 3549 |
Focus position V | 3D of Center | 2D of Center | 1D of Center | Center | 3D of Center |
AF Area Width | 344 | 494 | 494 | 430 | 430 |
AF Area Height | 344 | 494 | 494 | 430 | 430 |
Focus Result | Focus | Focus | Focus | Focus | Out of Focus |
Looking at the data, there are several interesting observations. First, the focus distance is recorded as the same in all of the frames. I'm not certain whether this is a function of exif tool, a recording issue in the exif, or an indication of something more ominous, i.e. the camera/lens is not responding/recording properly. What I find more revealing is the lens position information. When the bird was in focus, the lens position was indicating "1241", it then jumped significantly to another position for the OOF frames and then returned to a reasonable compromise in frame "nine". The AF area X/Y information is difficult to interpret, although the focus position H/V didn't change that much suggesting that the subject was kept relatively centered in the frame. What I find more interesting is that the recorded "focus result" does not correspond to the actual image. The OOF images are regarded by the camera as being in "focus" while ironically, all of the images (not just image "nine") where the camera regains AF are regarded as "out of focus".
While one can look at the tabulated results and make some observations, I am left pondering as to what is really occurring. Is the camera and/or lens malfunctioning or miscommunicating? Is there a problem in the tracking software? I wish that I could reach out to a Sony engineer for further exploration.
….didn’t think I was the only one to experience this….thank!
I’m getting it, I just try to get a better deepper understanding.
Basically, The bottleneck is the motor not able to move the focus element quick enough? Hmm, the amount of movement is so tiny. With 120 calculations per second, Whats the physical limit the AF motor can move the elements in tiny increments per second?
Cameron T
Well-known member
No, the element speed wasn't the problem in OPs example. The problem is the camera was focused in the wrong place. Where that fault lies is the question we need to know first, and then can go from there.
I agree @Steve's thoughts on predictive AF seem quite plausible.
Yes Nikon support and one Nikon engineer in person in the past that the focus point representation in NX Studio and previous versions was not always exact and especially in Auto Area AF.
Of course I could not see all of the images from the film strip but 4 jpg looks to be on the front of the wave and because of the compression of the long focal length lens it appears to be making the wave look taller than it was and possibly closer to the birds strike point than it was. So as you said it could have been quite a bit further in front. The wave and long lens compression does add another element to the equation how predictive AF would relate to that is well beyond my technical kin.
I do not know what this software is that you used and were it is getting it's data?
Since the Osprey is not in a straight down head first dive like the urban bird below (Z9 with ftz adapted 600 f/4 E shortly after I got my first Z9 before I got Z800). It is a puzzle why it has the focus distance the same when it is measuring with supposed .00 precision.
What is meant by lens position?
As you said a lot from this software, NX Studio questionable recording and rendition of the focal point still leave you with a puzzle. As you noted you have no way of knowing at this point what is the software, a camera, lens issue or what?
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This is a classic case of a busy background and a subject that is hard to identify. The camera will often struggle with this kind of scene since the bird is hard to recognize as a bird, and the background has distracting high contrast detail. I've run into this scenario in the past. When I have run into this issue, it was a specific subject and background causing the problem, and changing backgrounds made a big difference. That may not be possible - but could be an alternative with a busy background like this.
While you and I know this is a bird, it's really only the bright part of the bird on the top of the wings and the back of the bird's head that is used as a target. That's a shape to the camera, but not a bird shape. A silhouette of a dark bird and a clean background would be easy - but the shape is tough to recognize in the lighting available.
The key is whether the camera ever acquires focus, or does it achieve focus and then lose focus. If you are not achieving focus at all, I would try to use a smaller AF area. I would also make sure I was pre-focusing between the subject and the camera so the camera is moving from mid to far distance of focus. I don't want to allow any hunting from far to near. Wide small should do the trick in terms of focus area if Wide Large is not working. If you continue to struggle with this angle, I'd try turning off bird subject detection and using Animal subject detection rather than have a profile the camera does not recognize as a bird. If the camera is losing focus on the subject and picking up the background, I'd adjust tracking and make tracking stickier - or slower to change subjects.
apakett
Member
Much of the discussion has been loosing focus as a bird dives and the head momentarily goes underwater. Would raising the ”Focus Tracking with Lock-On” number help? Note that the setting is ignored when in 3D focus mode.
One hundred and twenty calculations per second may seem like a lot, but consider the speed our subjects might be approaching. Not even getting into airplanes and racing cars it's easy for a bird to be flying at us at 50 kilometers per hour (approximately 31 mph) and that's on the slow side as Ospreys have been clocked at 80 mph, Pronghorn Antelope can run at 60 mph and a Peregrin Falcon in a dive at close to 240 mph but dives aren't generally right towards us. Still, speeds can be fast even for live animals and a lot faster for other photo subjects.
At 50 kph coming directly at us the subject is moving nearly 11.5 cm in 1/120 of a second or roughly 4.5 inches between each AF calculation. That might not seem like much movement but when viewed through a long lens that's easily enough to be out of focus by the time the shutter is released based on the most recent AF calculation. So for many years the camera manufacturers have used predictive AF tracking algorithms to try to stay slightly ahead of subject movement to deliver as many crisp well focused images as possible for fast moving subjects. Even at 120 calculations per second there are many subjects that move fast enough that we'd want predictive AF tracking.
But yes, moving the mass of focusing elements is a small part of the question but not the whole question as even with instantaneous mechanical focus adjustment 120 calculations per second can't keep up with the fastest photography subjects if they're coming right at us.