I'm using a Nikon D800 with a prime Sigma 50mm f1.4 lens to photograph a selection of artwork for producing an art book. I will shoot in raw format. My questions are:
Personally I would not choose a 50mm lens for this but a higher focal distance. 100-200, but depends on the art size. The more focal leinght you have less distortion you get. But as it is a prime lens, it is a good option. If you get noticeble distortion you can correct it on postproduction.
The lenses have an optimum performance in the mid range, normally on F8-11. If the FStop is too wide you can have some zones out of focus, and if you force it too much you could have diffraction.
Try to see if there is a review on your specific lens here: http://www.dpreview.com/lensreviews/?utm_campaign=internal-link&utm_source=mainmenu&utm_medium=text&ref=mainmenu
I am not sure, but you will probably want to reduce the reflection on oil paintings, but on the other hand you probably want to show them if the paint has a lot of texture. If you want to reduce it use a polarizer filter. The same case with the glass.
Outdoors will be my last choice... I would not consider it at all for a painting.
I would consider natural light (next to a window) only if the context is important, where the painting is, the painter's studio. Only in thoose cases.
So the option is artificial light.
If the painting is old you should not use a strong flash becouse the photons can afect the painting in the long term. If it is relative new I would use 2 flashes with a large softbox on 45° of the painting on each side as far as you can go, to have the most uniform light you can get.
If the painting has a lot of texture, I would move a main light closer to the plane of the painting to enhance the texture. But far enough to have little diference of light on the side next to the light and the far side.
To avoid almost any reflection your studio should be dark with the lights only pointing t the paint. So you would not use white transparent umbrellas, but a softbox with a grid on it.
But if you can, remove the paintings from the glass frame. Pastel painting also has texture.