Nikon D7000 Focus Problem?

[cichlid]

Hi All,

I unfortunately am posting about a potential problem with my camera, Nikon D7000.
I remember a few months ago some others posting problems with their same camera.

Attached (as a PDF) are two photos, identical subject and camera settings. The only thing changed was a gel on one of the lights. Though nothing was moved.

Nikon ViewNX2 confirms the focus point, which are identical. Though, you will note that there are two different areas of focus?

Can anyone shed some light on what happened here. This scares me as how can I be sure that the camera focus where I expected it to? I should not have to sharpen every photo due to the failings of the camera.

Thanks

[jprzybyla]

Hello Erik, I use the Nikon D7000. What you are showing happened to me once when photographing a flower, same focus point one sharp the next the camera lost focus. It has not happened to me again so I do not think it is chronic. I overcame it by removing the lens and reinstalling it, not sure what caused it unless communication between the camera and lens was interupted. Hope this helps. Also I had a slight backfocus with certain lenses, that was solved by using the D7000 fine tune autofocus tool in the menu.

[cichlid]

Thanks Joe.
I'll need to test on different subjects and lighting. Basically to see if I can re-create the "issue".

Erik

[Manfred M]

I don't shoot the D7000, but rather the D90, so let me make some observations based on my experience.

Shooting metal and glass seems to be problematic as the camera does have issues picking up what to focus on. Contrasty subject matter works much better. Moving the focus point around does not guarantee you that the centre of the square is where the camera ends up focusing. It just restricts the area it focuses to elements in that red square, as opposed to the normal zone used to focus . In both your pictures this seems to have worked properly, so the camera has done what it is supposed to do. In the one shot the camera has focused on the knurled surface and in the other on the piece in the back. You have certainly moved the camera between shots, so that is more than enough change the autofocus point.

In situations like this, I will use a tripod and switch to manual focus. It's really the only way I can guarantee getting the exactly what I want in focus. If you have trouble focusing while looking through the viewfinder, try going into live view and hit the magnifier button to zoom in. You can get a very tight focus that way.

No autofocus is perfect and every camera I have ever used misses occasionally.

[Andrew1]

I read an article about the placement of the focusing sensors in relation to where the camera identifies they are. You can't be 100% certain the measurement is being done in the exact center of the indicator. What lens you mount and fstop you use can also cause very small variances in the way the light hits it. It looks to me the sensor may be a bit higher than center and in the second shot the point of focus is actually on the black wire moving the focusing plane a touch to the rear. Sorry, I could not relocate the article to attach.

[Black Pearl]

Isn't the AF systems in modern Nikon bodies colour sensitive?

Wonder if the colours gel threw the system out.

[Inkanyezi]

In all autofocusing systems, for reliable focusing it is essential, that the object to focus upon is:
A) larger than the focusing point (unless anything behind completely lacks objects to focus upon, i.e. sky or similar)
B) not broken up with spaces that show objects behind
There is also a third requisite for phase detection systems:
C) the object must not have a fine grained repeating pattern

As all these three rules are broken herre, it's no wonder if it will miss focus.

Phase detection may miss when there is a pattern that repeats as in the knurled surface. Also when the point is larger than the area chosen, the AF system may find something else within the area to focus on; the same when there are spaces in which the background shows. There is no way to tell exactly what happened, but one plausible reason for shifted focus is the object to the left behind the knurled surface. AF systems often are a bit whimsical, and it is unwise to use them contrary to their purpose like this. When taking a series of shots of the same object it does not make sense to have the AF system set focus again and again; it's best done switching to manual, either after setting focus with AF or for focusing manually.

I have encountered the problem more than once with contrast detection AF, when there was something in the background with better contrast than the main subject. There are two possible solutions. Either focusing at something else at the same distance as the subject or doing it manually.