I'm into night sky photography and I recently picked up a Rokinon 16mm f2.0 lens for my Canon 70D. It appears that it's one of the best night sky lenses for APS-C cameras and I'm really excited to use it. It's a manual lens and I understand the need to adjust focus and aperture via the lens itself and not the camera.
What I'm confused about right now is that my camera doesn't seem to tell me the appropriate exposure at all prior to taking a picture. Basically, if I set my shutter speed to what my camera says is zero/in the middle/ perfectly exposed - the photo image when I view it is significantly underexposed. In order for me to expose properly where the histogram is in the middle - I actually need to take the picture at somewhere around +3 on my meter. This doesn't seem right to me, but then again I'm still very much a novice.
I bought the lens used, so most importantly I want to make sure that I didn't get hosed by a faulty lens (or maybe there is even something wrong with my camera? But I don't seem to have this problem with my kit lens).
This problem happens regardless of which metering mode I'm in. When I try aperture priority it doesn't seem to be an issue, but I don't plan on using aperture priority mode for anything with this lens.
If I'm not wrong it's a totally manual lens, so what the camera meter it's exactly what you'll shoot. And in this sense no, you shouldn't have this problem at all.
But given that, again, is totally manual and there is no electronic contact at all, there is nothing the lens can do to confuse the camera meter. So the only explanation that comes to my mind is that your metering is set in a way that prevent the camera to correctly measure the light, maybe like having the metering set to central spot and pointing the camera to a star...but you already excluded that.
Apart from that...it happened to me to have taken just a few shoots of the sky at night (not in sky photography at all), and they obviously have the histogram all to the left...I can't think a way to shoot at something so dark like the sky at night and have the istogram perfectly in the center. Are your shoots underexposed, or is you who are expecting something unrealistic?
Finally: no point in using Av mode, you have to shoot in M.