singleviking wrote:
Sure. Putting a DX lens on a D800 will automatically change the sensor area to the DX format. Instant in-camera cropping, but why not frame or crop in the PP so you can determine the best framing? Granted, when ever you crop, you reduce the pixel resolution by the same ratio, so there is some limit to how much any photo can be cropped though before pixelization begins to occur.
I shoot Canon so I wasn't considering the Nikon DX/FX capability, but it is not variable as you mentioned in your OP and the only reason I can think of to use that feature is to be able to make use of a type of lens that, with Canon anyway, would be useless on a FF. As you mentioned, cropping in PP is a better option then using a DX on a full frame.
BTW, cropping has no effect on pixel density. The only time pixelization becomes a factor is when the photo is enlarged. Print it in the reduced size that cropping creates and the resolution is unchanged.