The night vision devices are different from the telescopes. Between the objective lens and eyepiece there is an image amplifier, therefore focus needs to be adjusted at both sides (both at the objective lenses and at the eyepieces). Somewhat similar to camcorders where the image is produced on a small screen and then an eyepiece is used to magnify the image. The performance of a night vision devide depends on the objective lens size, focal ratio and the properties of the image amplifier. In Hungary beside the first generation devices the 2nd and 3rd generation devices are also allowed to use.
Using two objective lenses it is not needed to split the incoming light beam into two. We aren't Cyclopses after all meaning that it is quite important to use both eyes especially during low light contiditions. The two separately processed light beams makes it easy to identify objects even at 250 meters.
The digital night vision devices represent a completely new generation, they do not belong to nor I neither II generation. Depending on the sensor they can perform similarly to generation II night vision devices (can be used freely), but one needs to get used to the different usage. The image is rectangular and sometimes is too bright to the human eye. They provide additional functions as well, for example the image can be recorded.
The infrared illuminators can be used to extend the detection distance of the night vision devices to several hundreds of meters. Using these flash units the image gets much brighter and more details can be observed. With a laser IR flash unit the image quality can be enhanced almost to the level of 2nd generation devices.