Photo & Cine Lenses
First Quantum of Lens Design
In the past Photo lenses have been designed individually, showing specific image characteristics in dependence on object point position and lens aperture, and having dedicated names like Planar, Tessar, or Hektor.
Specific to Cine lenses is their development in families, thus groups of lenses with different focal lengths, showing very similar and specific optical performance.
Actually, major optical companies have introduced the concept of lens families also for photo applications, having dedicated names, and matched optical performance.
The trend to fusion of Photo and Cine lenses seems to be irresistible.
It certainly follows the trend of photo cameras showing increasing features for video capture.
Fixed focal length lenses have a two-dimensional performance variation: with focus position and f#. That means that for a given focus position and the f#, the image of any 3D object is fully determined.
Power distribution is key in optical design
The distribution of positive (P) and negative (N) optical power inside a lens is determined by two factors: the ratio of the focal length to image diagonal and the main method used to flatten the field.
While for wide-angle lenses the correct structure is N - P, for tele-lenses the P - N distribution is the proper one. A good way to prove this principle is to observe the position of the main positive power group inside a family of cine lenses. The positive power moves from rear to front, while the focal length increases.
We will see that this principle is also the motor of zoom lenses.
Here is a modern 25mm lens for cine cameras, a member of a family of lenses.
The P - N power distribution is obvious, the red surfaces are aspheric and the grayed lenses are special glass material.
Special glasses are key for good color correction
Low dispersion and short flints are glasses of choice.
The use of aspheric lens elements is key for compact high-performance lenses
High-performance lenses must have high precision mechanics, especially for all moving sub-assemblies. Also high precision radial and axial adjustment means are mandatory. The mechanical structure has increased stability in order to avoid backlash and any kind of hysteresis. Another characteristic of cine lenses is that the focus scales are very precise, with a defocus error of only a few microns.
Photo and Cine Lenses have to transfer emotions from object to image. This function is called Character of the lens and describes the way the lens is influencing this transfer. When creating the character of the lens, a holistic image assessment is necessary, that means that the image formation of the whole object space has to be considered.
Design is not only science,
it is art too
