Thats exactly what I want to do. ThanksLocation I posted will forcing it to DX which will cause it to shoot in DX even with an |FX lens.
Thanks . I knew this was possible. I was unable to search the manual in my pdf reader. This all makes sense.Welcome.
Check page 74 in the online manual. It looks like you can either manually select, or put it in auto, and it will automatically go into the correct mode depending on the lens that is attached.
D810 online manual
The D810 has a 36M sensor... so cropping will reduce the sensor are but still leave more than the ubiquitous 20M available...This begs the question of the reason behind this. Yes, you will gain the 1.5 crop factor, effectively turning your 500mm lens into a 750mm. However, since you are not using a large portion of your sensor, your resolution will suffer commensurately. You can accomplish the same thing by shooting your FX lens in FX mode and then cropping in post processing. This will give you more flexibility, and you won't run the risk of forgetting to return to FX mode later down the road.
I agree with you in principle Clovis, but I also would have to believe the original poster has some kind of motivation here. Maybe his post processing workflow is automated and he does not want to crop at that time? Many possibilities I suppose. Maybe he attended the Jared Polin school of editing and refuses to crop in post ever. LOL.That's true, but my point is that shooting full frame and cropping is equivalent to shooting in DX mode.
If the OP wants to shoot in DX mode for framing or other reasons, that is fine, just pointing out that there is no resolution, or magnification gain by shooting in DX mode with an FX lens versus shooting FX and cropping.