Recently-purchased Nikon D-750, which I love. but many photos turn out a 'ovals'. Might have something to do with crop-frame vs. full-frame sensor? Mainly use a Nikkor 55-300mm lens, but it is a DX lens. Do I need to upgrade to FX lenses? (Sorry if I am a dunce on this.)
I love my D750, too. And yes. The coverage of DX lenses on a full frame sensor isn't complete. You can use it, but as you see, you may lose some of your image. I've had all full-frame lenses even with my D200. I'd upgrade if I were you. It's worth it. You're going to love the quality of the image you get from full frame. If you still have your DX, use it when you need more reach from your telephoto lenses.