What is the best light to use for a Nikon d40 to take pictures at night,a flash or led lighting

What type of pictures are you planning to take at night? That is what will determine the flash needed. If it is still life or your shooting sporting events they require different types of flash.

It's for ghost hunting.. the problem is with my ISO set at 1600 or 800 it don't take the pic it just goes in and out of focus.. could it be the lens?

Many use filters for such a project. Most common is an IR filter. Then you may need an IR floodlight. There are also full spectrum filters but you still use a IR source for light.

Pushing the ISO doesn't always work and if your using a conventional flash then you need to use the respective ISO that matches the flash.

Here is one of many IR flash units.

IR Camera Flashes

You also may want to ask more questions about the subject your shooting and we have excellent forums here in this area.

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Ok thanks

I have the same trouble shooting at night with my dSLR...

My suggestion would be to set the camera or lens to manual focus, then find a good focus point. It takes practice, but it's an all right alternative until you get a good light source. After you take one picture review it on your camera's lcd screen and zoom in as far as you can to see if it looks sharp enough for you... Adjust accordingly...

I took some night ghost hunt shots a few months ago, I did the manual focusing and used my regular external flash for extra lighting. Turned the flash off a few times for some long exposures that gave some creepy results IMO... :)

I hope this helps...

OH! Also do try to keep your iso as low as you can to keep noise/interference at a minimum... :) (Try it between 600-800) Not sure which ISOs your camera has, but I used around 800 with about 1/60 shutter speed with a flash, many many seconds without a flash... :)