exposure - Why will low-end Nikons not meter with old lenses?

To quote the source, the following sites claim that the Nikons: D90, D5200, D3200 and so on will not meter with AI lenses.

http://www.aiconversions.com/compatibilitytable.htm

http://www.kenrockwell.com/nikon/compatibility-lens.htm

From my understanding there must be some kind of sensor inside of the camera that measures the amount of light that comes in (kind of photoresistor). If that were true, all Nikons will meter with basically all lenses, so what is different that it doesn't work?

And why the AI lenses work with high-end models: D7100, D200,... etc. ?

Answer

You can divide Nikon lenses in basically three categories (as far as metering goes):

• pre-AI: no metering functionality, mounting them on a modern camera might break the metering system for old lenses. Interestingly they can usually be mounted without risk of damage on low end cameras because they lack the metering system for old lenses in the first place (bear in mind that officially Nikon denies this, however).


• AI, AI-S: they communicate the selected aperture, maximum available aperture, etc. using mechanical prongs. They require mechanical prongs on the camera side to get these informations. High end models have these prongs.


• AI-P, AF: they communicate the selected aperture, maximum available aperture and eventually more data using electronic contacts. This is the only interface supported on low end models.