The secret lens is made to obtain high-quality images, while the pictured subject is not aware of having someone taking pictures of them, therefore allowing for a perfect level of realism and candidness.
Perfect to take pictures of people in natural poses or for covert photos (for example when paparazzi want to take sneak pictures of an important subject in compromising situations), the spy lens it works thanks to conveniently placed mirrors, which can allow you to shoot photographs in any direction and at any angle.
Thanks to the hole on the side of the lens, while you give the impression of pointing your camera at some target in front of you, you will actually be able to catch a subject located on your right side, on your left, above or below you. Of course, if the subject being photographed is alert and afraid of paparazzi (probably either because he has something to hide or he is simply very privacy-oriented), and he is close enough, he will be able to notice the unusual shape of your lens and become suspicious, but you will just need to be quick enough to save yourself from suspicions.
This kind of spy lens can work with lenses of 50mm at least, and is compatible with most reflex cameras manufactured by brands such as canon, Nikon, Olympus, Pentax and Sony, on which it can be comfortably mounted.