High Image Quality: Typically offer excellent sharpness, contrast, and color rendition due to their prime (fixed focal length) design.
Large Apertures: Many 300mm primes boast wide maximum apertures (e.g., f/2.8 or f/4), allowing for shallow depth of field, excellent low-light performance, and faster shutter speeds.
Telephoto Reach: The 300mm focal length provides significant magnification, ideal for sports, wildlife, and portrait photography where distance to the subject is often necessary.
Professional-Grade Construction: Canon’s L-series 300mm lenses are known for their robust build quality, weather sealing, and durability.
Image Stabilization (IS): Many modern Canon 300mm lenses include Image Stabilization technology to reduce camera shake, allowing for sharper handheld shots at slower shutter speeds.
Fast Autofocus (USM): Often equipped with Ultrasonic Motors (USM) for quick, quiet, and accurate autofocus performance, crucial for capturing fast-moving subjects.
Optex Modifications (General Benefits):

Cine-Style Enhancements: Optex often rehouses broadcast lenses to make them suitable for cinema cameras, adding features like geared focus and iris rings for use with follow focus systems, and standardized front diameters for matte boxes.
Mount Conversion: They may convert the lens mount to different professional standards (e.g., PL mount) for compatibility with a wider range of cinema cameras.
Improved Mechanics: Rehousing can sometimes improve the overall mechanical feel and durability of the lens for demanding filmmaking environments.