In optics, two positions that image each other reciprocally — focus on one, the other stays sharp. Critical for macro work and mirror rigs where lens distance matters.
Two positions within an optical system that image each other reciprocally — if you focus the camera on position A, position B will automatically appear sharp, and vice versa. This sounds theoretical but is concretely relevant on set, especially when working with macro lenses or mirror systems, and the lens distance is no longer negligible.
The practical significance lies in depth of field and focus calculation. With long focal lengths and short working distances — for example, when macro filming products or insects — you cannot simply use the standard focus formula. The distance from the lens's optical center to the sensor is not identical to the distance to the subject plane. Conjugate points precisely describe this mutual dependency: if your macro lens is sharp at a distance of 10 cm from the subject, then a second conjugate point exists in the light path that is also in focus — usually behind the lens, within the optical system itself. You don't need to see this, but it explains why the calculation differs.
This becomes most important with mirror camera systems (especially reflex designs) and with teleconverters or other extension optics. A 2x teleconverter shifts the conjugate points — the focal plane moves closer, the depth of field becomes shallower. You notice this because your focus markings no longer align. The same happens when you attach a macro lens to a bellows system: the distance between the lens and the sensor changes, and with it, both conjugate points shift.
Practically, this means: do not rely on the distance scale on the lens for macro work. Instead, measure the actual working distance from the subject to the front lens element, or use Live View and focus visually. With mirror systems, it is also important to understand that a mirror telephoto lens cannot simply move closer to the sensor — the mirror path is fixed geometry, conjugate points are fixed. This explains why these lenses are so compact, but also why you cannot work at arbitrary close distances with them.
For depth of field calculation: forget the simple formula at short working distances. The magnification ratio becomes relevant — the closer you get, the more your aperture affects the image, and the differently the depth of field is distributed in front of and behind the focus point. Conjugate points are the geometric explanation for this.