Can one use old lenses with a D90?

[JimC]

That's what I'd suggest (see what they have to say about it). Due to registration distance, some lenses may require optical elements when using an adapter in order fo allow focus to infinity (which can degrade optical quality, depending on the adapter). So, you'll want to make sure the adapter you use is well liked if that's the case.

But, if you're on a tight budget and want to use older lenses, you may be better off going that route (since you'd need to move up to a higher end body in the Nikon lineup to use Manual Focus lenses if you want metering with them).

[Squalo Bianco]

And how do you know whether a particular lens needs those optical elements or not?

But, if you're on a tight budget and want to use older lenses, you may be better off going that route (since you'd need to move up to a higher end body in the Nikon lineup to use Manual Focus lenses if you want metering with them).

Since I am, and also given that my original idea was to get a white Kx, what you say makes lots and lots of sense to me. That way, I suppose I will also be able to use new, fast prime lenses (50mm / f1.8 would do) made by Nikon, which are a tad more affordable than the -no doubt delightful nonetheless- Pentax ones.

[JimC]

I'd ask the Pentax users in our Pentax dSLR forums.

As a general rule, the same thing would apply to an entire class of lenses. IOW, Nikkor manual focus lenses may or may not require optics in the adapter to allow focus to infinity. You wouldn't need to worry about it on a lens by lens basis.

Judging from the adapter I see a photo for (the link I posted to one earlier), it looks like it probably does have optics in the adapter. But, I'd check with Pentax users to find out for sure if that's the way most adapters are designed to work with older Nikkor Manual Focus lenses.

IOW, the adapter probably acts as a small teleconverter (adding magnfication so that the lens can still focus to infinity).