ProRes video recording remains limited to 1080p quality at 30 frames per second on the 128GB model of the iPhone 15 Pro, unless the device is recording directly to a connected external storage drive, according to Apple. On the 256GB and higher iPhone 15 Pro and iPhone 15 Pro Max, ProRes video recording is supported in up to 4K quality at 60 frames per second to both internal storage and external storage.
Apple does not mention this information on the iPhone 15 Pro's tech specs page on its website, but the limitation is listed when comparing the iPhone 15 Pro to another iPhone model in the Apple Store app, as seen in the screenshot below.
The same limitation applied to iPhone 13 Pro and iPhone 14 Pro models with 128GB of storage, but those devices cannot record ProRes video to external storage, so at least iPhone 15 Pro users have that option this time around. The limitation does not apply to the iPhone 15 Pro Max, as that model starts with 256GB of storage.
ProRes video files are very large. Apple says a one-minute 10-bit ProRes video is approximately 1.7GB in 1080p and 6GB in 4K. Apple likely feels that offering 4K ProRes video recording to internal storage on the 128GB model would be a subpar user experience. With recording to an external storage now an option, it is less of an issue.
Introduced on the iPhone 13 Pro models, ProRes refers to a family of codecs from Apple that can compress videos into smaller file sizes with "impressive image quality preservation." Apple says ProRes is designed as a final delivery format for high-quality videos like commercials, feature films, and broadcasts.
ProRes video recording is still not supported at all on the standard iPhone 15 and iPhone 15 Plus.