Deer eysight is I think very different to ours, poor binocular vision, therefore hard to detect object distance in anything other than good light. They detect movement exceptionally well, especially fallow, who seem to have radar! As has been said, walking in a straight line to the animals, (roe especially) will as long as they are not observing you directely, allow you to close a long way. I was in daylight a few weeks back in on roe that had been heavily disturbed by constant activity. There were 5 animals in the group, I carefully closed from about 220m to 135 by walking straight at them and watching heads and stopping as soon as one looked my way, I needed to be certain as to exactely what was being culled, hence the distance. As for night, once light starts to drop, a deers ability to discern you against backgrounds seems to drop markedly and as others have eluded too, you can get in a lot closer even when its still light enough for you to to see them without aids. I have closed in on red in the morning pre-sunrise light, where no cover is available, but against a hedge, from 500 M to 150 ish relitively easily on a good number of occasions. In evenings, when getting debateable light to shoot, but still legal, I have found as long as you are downwind, the wind is stable and you dont make noise you can almost walk up to animals, many times to within 20 m of an animal, even fallow, but by then its usually too dark with optics alone, which is all I use on the rifle.