Shooting moving subject (walking) at f/1.8 - poor hit rate in indoor light

Is it a plain impossible task? I'm shooting with D800 and 85mm f/1.8 (yes, not the fastest lens I know) in a small room with a subject about 4m away moving slowly towards me. With both release/focus priority the hit rate is worse than what I want. I realize the depth of field is going to very narrow. Do you use flash? I've asked this before... I'm just curious if it's simply a depth of field issue (in which case I'm forced to use flash) or I have a legitimate camera problem

How about you indulge us by stating your AF mode? And yes shooting anything moving inside that far away will miss a lot. You need a flash with a AF assist beam, that's why they are there, even on top of the line models.

Are you using AF-C?

Yes, I am.

AF-C, single point.

Try AF-C 9-point. 9-point worked best best for me when tracking in low light (Though that was with a D700 and Nikon 70-200mm f/2.8 VR II for a single assignment). Not sure how low you're talking, however.

Try AF-C 3D tracking.

I thought the 85/1.8 focuses very fast. Ming Thein surely feels so (in his long report). Here is a synopsis.
Nikon AFS 85/1.8 G** (June 2012, 9.5/10)
An impressively sharp optic on every body I’ve tested it on, including the D800 and D800E; no CA, edge to edge perfection even at f1.8. Sharp and contrasty all over, probably thanks to the small number of internal elements. I have no idea how they do it – the optical design is incredibly simple, and has no aspherical elements, no ED glass, no IF/ RF groups and no fancy coatings. The T stop is about half a stop down on the 85/1.4 G for the same aperture, though – must be the effect of the Nano Crystal Coating on the 85/1.4 G’s elements. I think this may be the best general purpose/ fast 85mm out there at the moment, and I don’t say that lightly. Beats the 85/1.4G and Zeiss ZF.2 85/1.4 handily for sharpness. Highly recommended if you don’t already have the 85/1.4 G, and recommended for D800 users even if you do.
© 2012, Ming Thein

Who is Ming thein?

But isn't increasing the number of points going to decrease the accuracy?

I will try, but I was under the impression you don't want to use many points when you are shooting at large apretures / dealing with small DOF

It'll still focus on a single selected point for main focus accuracy, but uses the other 9 points to help track movement. In general, I suggest using 9 or 21 for movement depending on how erratic it is, and single only for static shots. I used 9 for the majority of covering an arangetram (indian dance recital the size and importance of a wedding) and had an over 95% keeper rate. I lost only a handful of shots in over two thousand to focusing errors.

Though I must admit, I wasn't aiming for eyelashes on a moving target at f/1.8 within a few meters, I was a medium distance away, and the DOF was bigger, too.

Still underwhelming results... I know I might be asking too much, but the lens is focusing behind the subject in a few shots.

This is a 100 % crop. The camera is focusing on the hair strands, not the eyes.

My understanding of focus tracking on the D800 is that it tracks the feature that's under the selected focus point when you start tracking.

Using more focus points allows it to follow that feature as it moves around the field of view, while using fewer focus points means that you're going to lose your track when the feature it's tracking moves outside the focus points that are active. The downside of using more focus points, I believe, is that using fewer points with a subject with predictable movement that you can keep within the fewer focus points gives the camera less opportunity to lose track by locking on to other spurious features in the scene.

The manual implies that the 3D tracking setting uses actual image color information for tracking while the other modes use more conventional contrast and distance tracking.

Based on the manual's recommendations, it looks like you might be best off trying 9 point dynamic area AF with a walking subject, because it should be easy to keep those 9 points aimed at your subject but you'll have enough points active to be able to maintain track with some subject motion from side to side as they walk.

Have you tried Auto Area AF in AF-C mode?

That particular mode includes face detection, which might give you better results in a case like this. I believe it's somewhat biased toward focusing at the nearest point on the subject, in which case you might be focusing on the tip of the nose rather than the eyes, but for a walking subject it's worth a shot.

mrwilkins said:

mrwilkins wrote:

Have you tried Auto Area AF in AF-C mode?

That particular mode includes face detection, which might give you better results in a case like this. I believe it's somewhat biased toward focusing at the nearest point on the subject, in which case you might be focusing on the tip of the nose rather than the eyes, but for a walking subject it's worth a shot.

Click to expand...

No, that is one I've not tried. I'll give it a shot. My results have been poor... but I'm not sure what the issue is.

Your initial results were probably poor because you were trying to use single-point AF. The problem with this is that the moment you hit the button, the camera relentlessly tries to focus on whatever is under that individual single point, but if your subject is walking, the distance to subject under that one AF point is going to be all over the place. The lens won't be able to focus fast enough to follow the variation even if that result were desirable.

Using more points or switching to 3D focus tracking will cause the camera's AF system to try to follow the feature you were initially pointed at with your selected focus point as it moves around a larger area of the viewfinder. This is probably just what you want -- the trick is making sure that your selected focus point is focused on a feature you want to follow when you first press the button down.

Finally, Auto Area AF uses face detection along with some heuristics about the scene to guess what it thinks you want your subject to be, and chooses what to track on its own. This is the mode in which you have the least control, but it might produce good results for this case.

Edit: This is a camera with pretty much the full range of autofocus features available in a Nikon body. Don't be discouraged by having to spend some time experimenting and thinking carefully about what the system is doing to get your head around what works. Their audience of professionals is reasonably demanding in terms of AF performance, so it's likely that you can achieve good results with the right technique and choice of setting.

mrwilkins said:

mrwilkins wrote:

Your initial results were probably poor because you were trying to use single-point AF. The problem with this is that the moment you hit the button, the camera relentlessly tries to focus on whatever is under that individual single point, but if your subject is walking, the distance to subject under that one AF point is going to be all over the place. The lens won't be able to focus fast enough to follow the variation even if that result were desirable.

Using more points or switching to 3D focus tracking will cause the camera's AF system to try to follow the feature you were initially pointed at with your selected focus point as it moves around a larger area of the viewfinder. This is probably just what you want -- the trick is making sure that your selected focus point is focused on a feature you want to follow when you first press the button down.

Finally, Auto Area AF uses face detection along with some heuristics about the scene to guess what it thinks you want your subject to be, and chooses what to track on its own. This is the mode in which you have the least control, but it might produce good results for this case.

Click to expand...

OK, the above was shot with using 9 points actually... I will give it a try a few more times.