The grey-tinted double paned glass gives a color cast and cuts down on light entering the lens. I just tested at ISO 640 (with my 40d) and got f/5 at 1/500 through the glass and f/5 at 1/1000 with the door open. At this time of day there are strong reflections on the three panels of glass at the right (facing west) from the windows on the left. If I get all the way up to the slding door area without scaring a bird away I can eliminate a good deal of the reflection, but not all of it. I try to make up for lack of contrast in the post processing. Best bird photography through glass is on overcast days, but then, of course, the light is even worse and ISO has to go up. Update: I took this picture and edited it before I had cataract surgery. I now see that I oversharpened it!