English Translation
I’ve never seen as many people as this photograph the same rainbow.
The 撮影した「ものを」part sounds very unnatural. The word order is also awkward.
The original Japanese sentence is closer to “I have never seen such a compilation of photos in which so many people took a photo of the same rainbow.” The English sentence means that the speaker was surprised at so many people rather than at a compilation of photos.
撮影する is also vague. It means either “to take a photo” or “to videotape”.
My alternative translation is:
これほど多くの人が同じ虹を写真に収めたのを見たことがありません。（これほどおおくのひとがおなじにじをしゃしんにおさめたのをみたことがありません）
Verb + のを + 見る = I see someone doing. Verb + のを (in this case, 写真に収めたのを) is a gerund.
写真を撮る (instead of 撮影する) is okay but not so good as 写真に収める in this context. The connotation of 写真に収める is “to take a photo in order to memorize the precious moment”. One of the meanings of 収める is “to store something in a box/shelf/storage”. Therefore, the object of 写真に収める is usually a memorable scene.
FYI: The original English-Japanese pair is sourced from Tanaka Corpus (田中コーパス), which spreads so many unnatural translations. The Japanese sentence is currently adopted by a non-native speaker. That means no native speaker has reviewed it yet.