Optical correction of anamorphic stretch — restoring 1:1 aspect from anamorphic glass. Done in post (desqueeze) or on-set with corrective lenses.
Anyone shooting with anamorphic lenses must eventually restore the horizontally stretched image format to its natural proportions — this is the core of de-anamorphosing. Anamorphic optics compress horizontally during recording, creating that characteristic oval bokeh and wide flares. During editing, or already on set, this stretching is reversed by compressing the image horizontally until it has 1:1 pixel ratios again. Without this step, your talent would look like they were filmed through a funhouse mirror — faces oval, movements strange.
In practice, this almost always happens in post-production. Your DIT or colorist loads the RAW files into DaVinci Resolve or another system and applies a desqueeze node — a horizontal compression by a factor of 2.0 (for standard anamorphic lenses) or 1.3 (depending on the lens and manufacturer). The math is simple: if you record with 4:1 anamorphic glass on a 4K sensor, you are effectively storing 8K image information but only need to output 4K. The desqueeze compresses the horizontal axis, and you get your correct, cinema-ready widescreen format.
However, there are also physical solutions for the set workflow: so-called desqueeze lenses or correction attachments that you stack in front of the monitor or camera. This allows you to see the image already desqueezed in the viewfinder — practical for adjusting focus pulls without having to mentally calculate. Digital monitors like Atomos Ninja or SmallHD can also calculate this in real-time by simply selecting the anamorphic type in the menu.
The critical point: quality loss is minimal if you shoot with sufficient resolution and use clean interpolation. Desqueezing 6K anamorphic footage to 4K is completely standard. But if you are already compressing in the codec (ProRes, etc.), you must desqueeze beforehand — otherwise, error accumulation is pre-programmed. Some crews also consciously desqueeze only during grading to have maximum control over sharpness and color adjustment. However, this requires a 32-bit or float project and excellent rendering.