Help with Nikon dSLR auto focus

I have two Nikon dSLR cameras, a D40X and D3100. Both of these have the same problem with auto focus and I can’t seem to figure it out and I’d like to be able to why it will not focus on certain shots.

The majority of the time both cameras work great, the auto focus gets what I want if focus no problem. However, there are two or three types of shots that it never focuses on and will not take the shot. The first I kind of understand, if foggy/smoky type situations it will not focus, but there’s nothing for the focus to get a good read on. Those types of shots are rare and I don’t worry about it.

The ones that really get me though are ones of planes or other sky shots. I’ve had a bunch of shots with a plane in front of the moon that the camera will not take. What it does is focuses in, then out, in then out and just keeps trying to focus, or it will get in focus but just not take the photo at all. Push the button, nothing, just sits there, doesn’t even keep trying to focus.

I’ve had this problem on most, if not all of the settings, Manual, Aperture, Shutter Speed, Auto, all of them have given me this problem. The only way to avoid it, when I know it will happen, is to switch to manual focus, but my eyes are ever so slightly not perfect so I don’t always get an in focus shot.

The strange thing is I can take a shot of a waterfall from half mile away and it works no problem, but a plane that’s a few hundred feet above the ground, can’t to it. I’d really like to figure this out today as tomorrow they are doing a massive flyover of DC with WWII planes.

Does anyone know what’s going on and how I can get my cameras to work?

These are not the modes that adjust focus. You need to experiment with spot or multi area focus setting.

What lens are you using that a subject a few hundred feet away isn’t already at infinity?

I’ve had the same problem with my D100. Without sufficient contrast, the auto focus ‘hunts’. I can just switch to manual for most of those problems, but I’ve missed some wildlife shots because of it. Switching focus mode doesn’t seem to help much.

I’m sorry, I don’t understand what you’re saying. I know that in both manual and aperture modes it will auto focus, and will do so using multiple points. I do it all the time when shooting waterfalls, rivers and things like that.

I’m using the lens that came with both cameras, an 18-55 VR for the 3100, can’t remember the other lens as it’s 8 years old so I’d have to look.

This really only happens in a handful of situations, and taking photos of planes is one of them. I see in the above link that someone else says they have problems taking photos of flying birds, I’ve had the same problem. It will either focus but not take the shot, or just continue to try and focus.

One issue with manual focus is that with the advent of autofocus, many manufacturers quit putting stops on the lens focus at a calibrated infinity value, since the autofocus would adjust it correctly anyway. You can’t just manually twist the lens to the stop anymore, and can overshoot “infinity” when manually focusing.

http://www.kenrockwell.com/tech/how-to-focus-at-infinity.htm

You may note that allowing the lens to focus past infinity lets either you or the autofocus correct for variations due to temperature and such things.

Autofocus is based on contrast. When you’re shooting something up in the sky, you need to have spot focused and reading on just that one spot (bird, plane. etc.) if it’s got sensors ‘looking’ for something just in the open sky, it will continue to scan as there is insufficient contrast for it to lock on to.

A plane is really small and the focus sensors cover small parts of the image. So the chance that a plane is exactly where one of the focus sensors is looking is pretty small. I believe the D3100 has dots that light up in the viewfinder that indicate where the focus sensors are and which ones are active. Try setting the autofocus to single-point AF (not to be confused with AF-S mode!) using the center AF sensor, which is the most sensitive one. Then, make sure you put the plane square in the middle of the viewfinder. If a red dot lights up when you half-press the shutter, then make sure that red dot is on top of the plane.

What also works is autofocus on something sufficiently far away and then turn off autofocus and be sure not to change the focus manually.

Also, adjust the diopter setting on your viewfinder if you haven’t already. This will help in manual focus situations.

“Finally a diopter adjustment dial juts out from the upper right of the optical viewfinder’s rubber eyecup, just as it does on the D90 and D300S, but not on the D3000 or D5000. Diopter adjustment range remains the same as the D3000’s, at -1.7 to +0.5m-1.”