It won't focus properly and i don't know why. I don't change anything on the camera when i switch lenses and this one won't focus at all and if it does it take a long time. Not sure what i'm doing wrong. Any insight would be great!

That is correct. None of Nikon's 70-300mm lenses are designed as macro lenses. They won't focus closer than about 1.5 meters or 5 feet.

Try cleaning the electronic contacts on the lens and camera.

That's the backfocus problem. Take your camera to a Sony service.
If the lenses are ok, they will do an adjustment. If the lenses are defective, they will
replace the lenses, but that costs much.

if other auto-focus lenses work, then it is likely that this particular lens has a defect I suggest you send it in to Nikon for servicing

Make sure both the lens and the camera are switched to af (auto focus). If that doesn't work remove the lens and replace it to make sure it is mounted properly. You can also check the camera with a different lens to make sure it isn't a problem with the settings on the camera. If all of that fails take it to a camera or electronics store that does service work to make sure it hasn't been damaged.

Diopter is for your eye only and does not effect the focus of the camera. By using the 1/2 down method, by pressing the photo taking button half way down, you can set the cameras focus. on some cameras this is shown where it will focus by a red square. If you change the diopter this is like adding by focal lenses for reading a book. and setting the distance your particular eye needs for eye relief. On binoculars this is done with screwing the eyepiece closer or farther away from the other lens. check out online http://en.wikipedia.org/wiki/Eyeglass_prescription this will give you more details of what a diopter is or does. Always know what setting your shooting at as well. for instance if you are on a flower setting or Macro then this is for up close work. But up close doesn't mean too close. Most cameras on Macro need to be 6 inches to 12 inches away. But if you are say 24 inches away it won't focus right so don't try to zoom in, use your optical instead of digital and simply move closer or change the settings on the camera to a different setting or it may be blurry.

The 55-200mm lens does not focus closer than 1.1 meters (3.6 feet). If the subject is too close, then the autofocus tries and fails to focus or keeps trying (and squeaking). Either way, you won't be able to take a picture. In manual focus, you can take a picture but it will be out of focus and blurry.

Every lens has a minimum focus distance, closer than which it will not focus. Some lenses focus closer than others. It's a matter of design; making lenses focus closer makes them larger, heavier, and more expensive.

If you want to take close-ups, you have several options. You can buy close-up adapters that fit onto the front of a lens. You can buy a special close-up lens ("macro" or "micro" lens). You can buy extension tubes to fit between the camera and a lens.

You don't say anything about light conditions, target color, definition, contrast, and the many other things that can cause this.

Is your camera at fault? Maybe.

Take a look in these sites and many others like.
Your camera won't focus?

Digital camera focus tips

The Nikon D40 in Auto Focus mode will only allow the shutter to operate when the image is in focus. When looking through the viewfinder, the bottom edge provides information relative to what you are looking at. See the image below:



Item 2 is the focus indicator. It must be illuminated to allow the shutter to release. If it is not illuminated, the lens has not been able to determine the correct focus. This can happen when attempting to take a picture in low light conditions. Make sure that the front facing "focus assist lamp" on the camera (between the shutter release and lens) has not been blocked or disabled (Settings Menu # 09) and try again. Many times, you will see and or hear the lens "hunt" for proper focus - the lens will travel from one end of its focus range to the other trying to find focus. When it can't, it stops and prevents the shutter from releasing. You can get around this "unable to focus" issue by changing the position of the switch on the lens to "Manual Focus" mode, detailed below:

If a lens has an "A - M" switch, move it to "M" to manually focus. If the switch says "M/A - M " then leave on M/A (unless you're going to be taking several or more in manual mode), as these lenses may be focused manually at any time in either position. As soon as you tap the shutter button in M/A you return to autofocus.

More advanced lenses, like the 18-200mm, 70-300mm VR and 18-135mm, and any lens with an M/A position, may be focused manually at any time without moving any switches. Leave them in their A or A/M positions.

I hope this helps and good luck!

If you have a lens collimator and other equipment required to ensure the proper alignment of the lens elements when you reassemble the lens, then go for it. If you don't have the gear needed to reassemble the lens properly, then I strongly suggest taking the lens to a good camera shop and have a trained professional do a clean-lube-adjust. It won't do you much good to have a focusing ring that turns if the lens elements are out of alignment so it doesn't focus properly.