The best equipment to have for christmas shooting is a tripod
Fast lens can work of course but you probably need to bump up the iso a little
And you might find that 50mm or 85mm might not be wide enough IMHO
Eh why do you want to use 85mm lens to shoot xmas lights? It's too narrow unless u want to zoom in on 1 person in particular. Stick to the kit lens, it's usually wide enough 17mm-xx or around there. Don't have to set it to f/22 even onwards f/11-13 etc maybe enough depending on where you are or the effect you want to achieve. Just play around with it, we cannot spoonfeed you or tell you specifically, it's better to practice yourself and once familar, you'll more or less can judge with your eyes.
For night shooting, your foundation is the most important part of setup. A tripod is almost compulsory. Taking in the festivities around you, I would try not to use anything like 50 or 85mm or longer unless I am trying to intentionally isolate a certain scene for a reason. Shooting a festive scene is usually about capturing as much of the mood, ambients ..etc as much as possible...
I would go with super wide to wide lens for most of my shooting. But yes at times I will use even a telephoto length (anything from 50 to 200 or more) if there is a good reason to do so but at those kind of length, you need the help of a tripod to stabalise your camera for shot as you more then likely be shooting at much lower speed.
Fast lens as in lens that have maximum apeture of 1.8mm as compared with f4.5 or f5.6..eg might offer you abit more higher speed but you do so at the risk of less dept of field in your overall shot. Another thing to consider when you are shooting with a tripod.
Take note that using slwoer speed is more accessible being on tripod but if you are shooting outdoor scene, the wind is blowing so all those lighting fixture on the tree will not be sharp but you will see streaks of lighting due to their movement. What I do sometime is just wait there till the wind die down if I want to capture shapr not blur or streaking shots. Of use the swaying of the wind effect all the lighting to your advantage for something difference.
Also note...VR is not going to help you in that kind of scenarios. VR minimise your hand shaking but it can not prevent the rest of the world from shaking to let you shoot a sharp stationary shot heheheh....just thought I bring that up because there are still some people who still does not understand what a VR can and can't do.
So you will still have to make some decision before you shoot. In a busying scene where too many people are walking about infront of your scene...well you can use longer exposure to turn walking people into "ghost" or make them transparent BUT at the same time look at all the lighting around you to see how bright they are shining. Long exposure play havok with strong light which can burn in the picture cause you to have bright spots in your final photo. These are just some stuff you need to look into apart choice of lens.
Sometime with lens...if it is too long, you move back as far as you can. If too short, you walk near to your subject! heh. But in between all that...don;t forget there are other things to consider too before you press that shutter button.
oh and ah....try not to use f22. It is true that F8 is sharper or have more depth of field then say f4.5 and that F16 has even more then f8 ...and so on. But this also depend on which lens you bought. As you now go further to like f22..quality starts to drop as it has past the lens optimum quality. It happens lah. Lens are all not built equally. Generally lens are made to be at its best in the f8 - f11- f16 region. Some would not agree with that but in general that has always worked for me from way back till today. For me all my night scene are shot in the f8 to f16 region. As I shot wide angles most of the time, I want depth and sharpness.
oh..if you have a remote use that instead of your finger if possible to lessen vibration especially if you have what I call "heavy" trigger fingers. Every bit helps.
Perhaps prime lenses should be better in this case as compared to zoom lenses? The only shortfall is you have to do some walking and hunt for the better spot to take a shot, while in some situation there might be no way for you to get to the best spot to take the shot anyway.
Still lighting plays a deciding role so bobian better get a tripod so that you can get the correct shutter spedd and better overall picture quality pushing other parameters to the best maximum without getting too much noise.