JTX Lens & Focusing

If you find that you are unable to achieve perfect focus while using your JTX Dual Prism Anamorphic Lens, do not be alarmed. This is common in any two prism setup. The lack of ability to focus stems from two primary factors: astigmatism and chromatic aberration.

Astigmatism
It should be understood that any two prism anamorphic lens will produce some amount of astigmatism in the projected image. The results is that perfect focus is achieved in only one single plane at a time. If you display a vertical/horizontal cross hatch test pattern you will see that you can focus the horizontal lines, but the vertical lines will be slightly “fuzzy”. In the same way, if you focus the vertical lines, the horizontal lines are “fuzzy”. The best focus is achieved when both the vertical and horizontal lines share an equal amount of “fuzzieness”. This is why more expensive anamorphic lenses use circular optics rather than flat or wedge prisms. Circular lenses can focus in more than one plane. Unfortunately, these circular anamorphic lenses can cost more than $4k+. Still, some lenses incorporate a $300+ corrector element that corrects for astigmatism, but fails to address the chromatic aberraion issue (viewed by many as the greater offender). At these costs, it is easy to understand why many have decided to integrate a two prism lens and accept the focusing issues.

Chromatic Aberration
Chromatic aberration (or CA) is a type of distortion in which a lens is unable to perfectly focus all colors of light to the same convergence point. It occurs because lenses have a different refractive index for different wavelengths of light. In other word, a lens will bend different wave lengths (or colors) of light in differing amounts. This results in colors separating or “fringing”.

The good news is that the effects of astigmatism and CA are minimized toward the center of the screen. In most movies this is where the attention of the viewing audience is kept. Even at the edges of the projected image, where these effects are greatest, focusing issues often go undetected for those immersed in a movie.