Nikon D810 vs D750
I own both because I bought the D810 when it first came out in July 2014, and then the D750 came three months later in September 2014. If both came out at the same time, I would never have bought my D810. The D810 is more of a special purpose camera for people who need a quiet DSLR or for nutjobs who actually think they need 36 megapixels, while the D750 is what most people, including myself, really want.
There is no visible image quality difference between them. While I can use my laboratory to create special conditions under which I can show slight differences when splitting pixels, either camera has more than enough resolution for stunning prints at least 6 feet (2 meters) wide. How often do you print even three feet (1 meter) wide? Anyone trying to tell you that one or the other is visibly sharper or has less noise is making it up in an attempt to sell you something. We can see slight differences in a lab, but not in actual photography. They have the same size sensor and they each work as well as the other in actual use. Nikon's salemen don't want you to know that it takes some very careful technique and subjects to get enough lens resolution at the sensor to make any difference, even if you are printing 10 feet (3 meters) wide.
Autofocus is the same between them. The D600 and D610 are similar to the D750, but the D600's and D610's autofocus go blind and stop working in very dim places, like outdoors at night — while the D810 and D750 continue to focus like it's daylight. In enough light to read or brighter, all are the same.
They all take the same battery.
Neither is a professional camera; only the D4S is. Of course pros use all these cameras, especially landscape and portrait pros who use the D810. The D810 does feel tougher than the D750, but not by much.
The real difference is size and weight. My D810 is bigger and heavier than my D750, which is by comparison a lightweight. This alone may make up your mind.
The D810 is much quieter. The D750's shutter makes much more noise. Quiet mode makes the D810 even smoother and quieter and it's still almost as fast as regular mode, while the D750's quiet mode isn't quiet and makes the D750 much slower.
The D750 is smaller, lighter, less expensive and the best choice for most reasonable people.
The D810 is for dedicated full-time landscape shooters, or for people who need a quieter, smoother camera.
The D750 is faster and faster to use. The D810 is smoother and quieter and is intended more for dedicated landscape, studio and portrait pros.
There is no clear winner, even at the same price. All depends on what's important to you.
If price matters, by all means the D750 is the better camera; heck, if you're not shooting in pitch black, the D610 is the same thing, and the D7100 is almost the same as the D610, just with a smaller sensor.
The D810 has unique controls.
The D750, D610, D600, D7100 (DX) and D7000 (DX) all have the same controls. I've owned all of them, and honestly, the only way I usually can tell which one I'm holding is to look at the label. They are that similar; the biggest difference is that some have the viewfinder data lit in green or in white.
Smaller sensor DX cameras are so good today that the biggest real difference is that FX full-frame cameras have larger viewfinders and shallower depths of field. For sports where you need long lenses or where you want more depth of field, the D7000 or D7100 may even be a better camera than full-frame.
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