What kind of lenses can be put on a Nikon D40 camera. Can you put a f1.2 to f1.8 lens on it.?

I am looking to take pictures of fireflies at night and was advised to use F1.2 to F1.8 lens. Will that camera facilitate that lens for detailing those detailed objects or do I need to go to a D70 or D80 camera?

Yes, you can use any Nikon-mount compatible lens on the D40 (I have a 50mm f1.4 myself that works great). The big catch is that the D40 doesn't have an internal auto-focus motor... so while any Nikon-mount lens will FIT on a D40, the only lenses that it'll be able to auto-focus are AF-S and AF-I series lenses.

Otherwise you'll have to manually focus... not a big deal.

400mm f/2.8 AF-I work perfectly!!!!!!!!!!!!!!!!!!
( I wouldn't try anything else)!!!!!!!!

If your camera has interchangeable lens, then you can probably get one with that f-stop range. I would not use a 400mm lens because at night you will need a slower shutter speed and you will just get blurred pictures. I would go with a 28-70 zoom if it has that low of a f-stop range. You'll be able to get a wider field of vision with it and more fireflies in the pic, unless you're just trying to capture a single firefly in the pic. You will probably also have to use ASA400 or 800 film to get a shutter speed above 1/60 to avoid blur. Any higher than those speeds and your picture will start to look too grainy. If you're trying to capture a single one flying, then using 400 or 800 ASA film and a tripod, you might be able to use a 300 or 400mm lens to try and capture it. You'll need the lens to have the smallest f-stop(largest opening) possible to let in the most light and allowing for a faster shutter speed. Hope this helps.

I don't think there are any nikon lenses with 1.2 that will fit on your D40, unless going 3rd party....
There are no 1.2 or 1.8 lenses that will autofocus on your D40 since it focuses only with AF-S lenses (which currently go to 2.8 at most). I'm not a macro expert but a 105mm 2.8 AF-S might do the job...
I don't think the D40's multicam 530 will focus well at night, making it more difficult to get the right focus.

Consider getting the Nikon 105 f/2.8 VR macro lens. The VR is supposed to add up to 3 stops of light-gathering ability, which would mean you are effectively down to the 1.2 range anyhow. The depth of field would still be the f/2,8 depth of field, but you can use the lens in low light. It's an AF-S lens, too, so it would autofocus for you. I'm also suggesting this lens because it's a macro lens and you want to take pictures of fireflies. This will be hard to do with the 50 mm lenses that only focus as close as 1.5 feet and has a magnification 1:6.8 or 1:6.6, which is not really even close to macro. The 105 goes down to 1 foot minimum focus and gives a true macro 1:1 reproduction at that distance. If $800 is too much for you, as it was for me, you might consider my Nikon 60 mm f/2.8 macro lens. Okay the VR of the 105 is the main reason to buy it, but the 60 mm lens also has a 1:1 magnification.

Since you mentioned fireflies, see this photo of a bee to get an idea about the range of macro photography. http://www.flickr.com/photos/7189769@N04... While you are there, click on my name (samfeinstein) then "Tags" and then "Macro" for a few more examples with this lens.

Most certainly. You may find that images are soft at the wide apertures. You can try stopping down to F2.8 and it should help.