Nikon Reviews in a Nutshell

Nikon D40 with manual focus, AI 50 mm. f/1.4 lens

Virtually all Nikkor lenses produced since the introduction of AI in 1977 can be mounted and used on virtually all Nikon cameras produced since 1977, because the Nikon F-Mount has remained fundamentally unchanged since 1959.
In other words: Nikon DSLRs are almost 100% backwards compatible with Nikkor lenses back to 1977...
That this is no small feat, is particularly well demonstrated by the fact that Canon have had to change their mount 5 times in the same period (R, FL, FD, new FD y EOS), while these mounts are not always backwards compatible.

Mounting a lens on a camera is one thing, however, can you actually shoot with it? Yes you can!
The following manual focus (non-CPU) lenses and accessories are compatible with the D40, and if they are compatible with an entry-level camera, they ought to be compatible with most everything else.
• AI, AI-S, E-series
• AI-P (post-chipped AI)
• Medical 120 mm. f/4
• Reflex Nikkor
• Nikkor PC (perspective control)
• AI tele-converters
• PB-6 bellows
• PK-11A, 12, 13 and PN-11 extension rings.

Follow up:

Note 1: Do not mount Pre-AI, F-type (C, F, K, H & H.C Auto, N & N.C Auto, PD Auto, QD Auto, S & S.C Auto, etc.) lenses on your Nikon DSLR or you will be sorry.

Note 2: on Nikon cameras without a built-in focusing motor - D40(X), D60, D80, D3000 and D5000 - Nikkor AF (motor-less, non-AF-S) lenses work only in MF mode.

To find out exactly what usability a particular lens has on your camera, I'd strongly suggest you consult your camera’s manual, because quite a few lenses and accessories have their specific limitations.
Still, as a rule of thumb, the following:
To function properly on a DSLR, AI and AI-S manual focus lenses ought to have an f/max. of f/5.6 or less (f/4, f/2.8, f1.8, etc.).
When used in combination with accessories (extension rings, bellows, etc.), the combined f/max. ought not to exceed f/5.6, either.

Compatibility and functionality.
- Nikon D1, D1H, D1X:
AI & AI-S MF Nikkors can be mounted and used with either aperture priority or manual, center weighted, auto exposure metering.
Aperture is not indicated in the viewfinder and/or control panels (F- -), lens data (lens description, max. aperture, selected f-stop, focal length) is not included in Exif.

Non-CPU lens data. Shooting Menu, D200

- Nikon D2 and D3 series, D200, D300 series and D700:
AI & AI-S MF Nikkors can be mounted and used with either aperture priority or manual, center weighted, auto exposure metering.
Once the user includes lens data (max. aperture, (min.) focal length) in the camera’s lens database, the selected f-stop will be indicated in the viewfinder, control panel(s) and aperture priority auto exposure with matrix metering becomes available.
Lens data (lens description, max. aperture, selected f-stop, focal length, etc.) is included in Exif.
The camera's lens database can be accessed through the Shooting Menu > Non-CPU lens data, or - alternatively - assigning the FN button to this function (D200: CSM f4: assign func. button > lens data), in which case the focal length and f/number are set with the main and secondary control dials.

Nikon D40-D100. Mode dial must be set to M.

- Nikon D40-D100:
AI & AI-S MF Nikkors can be mounted and used in Manual mode only (you must set the Mode Dial to M), the exposure meter does not work.
If the camera is not set to M-mode, it will not fire and show a blinking F- - icon in the viewfinder and/or control panel(s)
Aperture is not indicated in the viewfinder and/or control panel(s), and lens data is not included in Exif.

However, it is quite easy to shoot without metering on a DSLR.
In normal daylight, start out with f/8 @ 1/125 s., review your histogram and/or blown highlights overlay in image review and adjust your shutter speed and/or aperture until achieving correct exposure. Remember that manual lenses may be set between full stops (clicks).
After some practice you will get better at “guessing” quickly.
In all cases the electronic range finder DOES work, just make sure the green dot (4) at the extreme left hand bottom side of the viewfinder goes steady to assure proper focusing.

How to identify a manual focus Nikkor lens?
By focal length f/number and serial, here.

AI and AI-S Nikkors. On the AI-S the pre-AI coupling prong is removed.

Visually:
- AI:
Screwed-on, semi-circular, pre-AI aperture coupling prong with cutouts.
Aperture ring with Aperture Direct Readout (ADR) scale (an additional series of small f-numbers engraved on the aperture ring).  This is by far the easiest way to identify (in)compatible lenses, because pre-AI lack the ADR scale.
Thus, if you have a MF lens without ADR scale, and with thick, triangular, uncut, fixed or screwed-on coupling prongs, it is pre-AI or F-type and should NOT be mounted, or it will most likely destroy your mirror (amongst others)!

- AI-S. Identical to AI, plus:
Lens type identification gouge - small semicircular indentation on the F-Mount.
Smallest aperture number marked in orange on the ADR and aperture scale.
Note: AF and AF-S type D lenses are AI-S compatible and have a smallest aperture lock (L) button (see picture).
On cameras with control dial(s), these lenses must be set to their smallest aperture - marked in orange - and locked.
If the lens is not locked at its f/min. (typically f/22), the camera shows a blinking fEE symbol on its control panel(s) and will not fire.

- E-series:
A series of more economical lenses, marked “Nikon lens Series E” not “Nikkor”. Compatible with AI, can be mounted.

- IX-series:
APS type, can not be mounted because the mount is considerably smaller than the standard F-Mount.
Marked “IX Nikkor”, they are mostly silver and black plastic (20-60, 24-70, 30-60 and 60-180 mm.).

Disclaimer:
I have not done exhaustive research on each and every Nikon DSLR or Nikkor MF-lens available.
Yours may be the exception: do not risk mounting unidentified lenses on your camera and always consult your user’s manual: section Technical Notes > Compatible Lenses > Non-CPU lenses, to find out more about compatibility and usability.
To facilitate search, you may want to download your user’s manual in PDF from a Nikon web site, open the file in Acrobat or Reader and do a search for any of these terms.
Finally, and I cannot emphasize this enough: don't even think about mounting pre-AI, F-type (C, F, K, H & H.C Auto, N & N.C Auto, PD Auto, QD Auto, S & S.C Auto, etc.) lenses on your Nikon DSLR or you WILL be sorry.