On an earlier thread there is a long running discussion/debate about FX lenses on DX cameras. I have come to think the whole thing is based on a false premise that started innocently enough. Before there were digital cameras there were many different types of camera, a SLR was the most popular others included range finders, twin lens and etc. They all used the same size “sensor”. The 35mm 24x36 negative. During this period certain lenses became standard depending on the situation. Pretty much the 50mm was considered normal, a 35mm modest wide angle, 85mm and 105mm were portrait lenses.
Then the digital camera was introduced with the first common sensor size being smaller in size than the 24X36. I believe that the camera manufacturers in an attempt to help potential users understand which lens to use came up with a crutch ‘the crop factor’. IE; the 1.5X rule of thumb for Nikon. This confused as many people as it helped. For those who had been using 35mm film it helped to understand when which of the above lenses should be used with the new smaller sensor. If instead they had said something simple like, “the standard lens for these new cameras is the 35mm and a moderate wide angle is a 24mm etc.” there would be less confusion. By coming out with the “crop factor” they provided us old geezers with a crutch to quickly figure out what to use in a given situation, but it didn’t help the beginner one bit.
I think the crutch muddied the waters far more than it helped, particularly when you consider that most entry level DSLR purchases are comprised of a camera and kit (read ‘zoom’) lens. Wide, normal, portrait are meaningless at that point. The beginner should learn with that lens, figure out what setting they use the most and from that when they want to go to prime lenses they’ll know what is standard for them.
In the past we didn’t fool around with ‘factors’ when putting down the 35 and picking up the medium, we knew what lens to use when. Today for most users it doesn’t matter, as the zoom has become most users ‘prime’ lens.
I would like to see us stop using crop factor and instead say things like ‘for the D7200 a 35mm lens is the normal lens.

This topic tends to end up making tempers flair for some reason.

My answer is who cares? If you are used to the APS-C, then you don't really wander about thinking in terms of "crop sensor"
I started out with Medium Format film, so actually, 35mm is a crop of that, so does that mean "Full Frame" is a crutch?

I was a fairly early adopter of digital, and we didn't lament having a "crop sensor" I'm so used to seeing what I see through my lenses (mostly an 18-55 2.8, and 50-150 2.8 (sigma...favorite lens) that I don't have to do math in my head. I needed a new body this past year, and instead of going with a used/refurbished D-810, I went with the D-500. The focus in extremely low light is amazing (same focus system as the D-5.

I've yet to ever have a client ask me what size sensor I use. I just sold a 20x80 panorama to a medical center last week, and not once, did anyone get worked up about the model of camera, and if it was a full frame.

It's a tool. Learn your tool, know it inside and out, and push it to its limits.

I too disagree. All that matters is that the person using a DX (or any other format camera) knows what to expect when they put a particular lens on their camera. The lenses I bought for my 35mm camera never had the medium format equivalent stamped on them.

While there are people who are confused by the equivalent nonsense, there are also people confused with large numbers representing small apertures. If they choose not to learn by whatever means necessary they will go through life confused.

I've heard it said that ignorance is bliss. Maybe that's why folks shooting with cell phones look so happy. They don't need to know an aperture from a hole in the wall.

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The whole topic shows a widespread lack of understanding of optics and photographic history.

A 50mm lens is a 50mm lens. The only thing that makes it "normal" is history. The focal length of the lens is not important in itself. It's the Field Of View (FOV) that is important here. The FOV depends on both the focal length and the sensor size. You can't characterize the FOV purely by the lens's focal length. It's the tendency to boil things down to minimal information that is what confuses people. "Everything should be made as simple as possible, but no simpler" (Attributed to A. Einstein).

So why is a 50mm lens sometimes called "normal"?

There are a number of explanations, some of them are probably guesses. Most of them are subjective. When 35mm cameras became widely used the common lens that was supplied was about 50mm focal length. So photographers got used to that field of view (defined by the 50mm focal length and the 24mm x 36mm sensor [film]). Since people were used to it, it became "normal".

A given lens will only be "normal" for a given system. "Normal" is not inherent in the lens.