https://kipon.com/0-7x-pl-pl-focal-reducer/

Focal reducer adapter: The
primary function

Creative work like photography and video graphy requires using a
variety of tools and accessories to present the finest creations. The
right tools can significantly influence the quality and versatility of your
work. One such tool is focal reducer adapter.
What is a focal reducer adapter?
A focal reducer adapter, an optical device, is placed between a
camera body and a lens. Its primary purpose is to “reduce” the focal
length of the lens being used, effectively broadening the field of view
and increasing the light transmission to the camera sensor. This device
is particularly useful when adapting full-frame lenses to smaller sensor
cameras
Key functions of a focal reducer adapter
Shortening the effective focal length
This is the primary function. By reducing the focal length, the adapter
creates a wider field of view, allowing you to capture wider angle.
Increasing light transmission (aperture boost)
The focal reducer concentrates the light entering through the lens onto
the smaller sensor. This results in an increase in effective aperture,
allowing more light to reach the sensor. For instance, an f/2.8 lens
might behave like an f/2 lens when paired with a focal reducer. This
enhanced light-gathering capability is particularly beneficial for low-
light photography and achieving shallow depth of field effects.
Improving image quality
High-quality focal reducers often enhance the sharpness and contrast
of images by optimizing the optical path between the lens and the
sensor. They help mitigate issues like vignetting and edge softness,
common when using lenses on sensors smaller than they were
designed for.

https://kipon.com/0-7x-pl-pl-focal-reducer/

Lens adaptation
Many focal reducers are designed to adapt lenses from one mount
system to another. This opens up a world of possibilities for
photographers, enabling them to use lenses from different brands or
vintage lenses that might otherwise be incompatible with their
cameras.
Benefits of using a focal reducer adapter
Versatility
Allows photographers to use a broader range of lenses on their
cameras.
Cost efficiency
Provides full-frame-like performance on smaller sensor cameras
without investing in a full-frame system.
Enhanced creative control
Wider field of view and better low-light performance offer more
creative flexibility.
Considerations when using a focal reducer adapter
While focal reducers offer numerous benefits, they are not without
limitations. Some lower-quality adapters may introduce optical
distortions, such as chromatic aberration or reduced edge sharpness.
Additionally, compatibility with autofocus and electronic
communication between the lens and camera may vary depending on
the adapter’s design and the camera-lens combination.