photography when it's cold outside?

Today there was beautiful frost on the kale.  It would make a great photo.

But it's below freezing and I worry about the camera being grumpy at me.

What do I  need to know about taking photos in sub-zero temperatures and what else do I need to know about bringing it in from the cold into the house?

The only thing that doesn't like cold is the battery, so it may be wise to keep the battery in warm pocket. (I think it should still work up to -10°C.)
The image sensor will actually have less noise at lower temperature.

Bringing the camera from a warm room into the cold: I think the biggest worry is the humidity of the air in the camera condensing in the surfaces. That only happened to me with my old camera after using it in the rain or submerging it in water (don't ask).
You can keep your camera in a camera bag with desiccant for a few days… or just hope it doesn't condense on the glasses. The sensor will heat up during use, so this isn't an issue.
About bringing it from cold to warm: The humidity of the room air may condense on the camera. Mine is supposedly rain proof, so I don't care about that. The lens will be fogged up until it warms up.

Over all I don't expect any damage.

I do almost all of my geological exploring in the winter, and I have never had a problem with the camera in the winter. It gets pretty cold here, so this includes temperatures down to 0 degree (f). I am not sure I would have problems colder than that with the camera, it is just too cold for me, and I cannot get motivated to get outside to explore when it gets below zero.

I have to wrap my camera up so that it is padded in my backpack, just because I have a lot of tools for taking ore samples. Cameras banging onto hammers and chisels would not be a good thing, so I pad it. So all that padding, in my backpack, next to my back just means when I get to a spot where I am going to take my pictures, I let the camera acclimate a bit so that condensation does not get on the lens as I take photos. I also carry spare batteries with me just in case. Cold takes a toll of battery life, but it would be a silly photographer that does not take spare ones along anyway.

As for winter photos, I really like them. In fact my most miserable time is right now. I am caught between seasons so everything is dead and brown, and all our snow melted. Until we get more tomorrow, it is pretty dismal looking.

 When talking photos in the cold, I keep extra batteries in a pocket inside my jacket, and even zip my camera inside my jacket sometimes.

 It is tempting to buy less expensive spare batteries, but in my experience they don’t do well in the cold, buy the same brand as your camera. So I buy the $40 canon battery instead of the $10 one from Batteries+.

Also, try to avoid bringing your camera directly from cold to hot, this can cause condensation inside your camera. Try to warm it up gradually. This applies in the summer as well. Instead of taking you camera from an air conditioned house or car into the hot sun, simply placing it inside a bag or backpack before you go out will be more gradual.