In most cases, buy for the camera and shooting you have now. On a D90, DX lenses are often the more practical choice because they’re typically smaller, lighter, and offer focal lengths that make more sense on APS-C.

FX lenses do work well on DX, and the smaller sensor uses the center of the lens, which can mean less visible vignetting and better edge performance. But the trade-off is usually more size, weight, and sometimes less suitable focal lengths on your current body.

The key question is how likely a full-frame upgrade really is, and why you want it. If you have a clear, specific reason to move to FX soon, buying selected FX lenses can make sense. If not, it’s usually better to choose the lens that best fits your current needs.

Also consider the lens type: some DX options may be limited compared with Nikon’s FX lineup, especially if you want faster primes or more advanced glass. In those cases, an FX lens may be the better choice even on DX.

Finally, lenses often hold value reasonably well, so buying the right lens now and selling later is often smarter than compromising for a hypothetical future body.