[color=red]a. He had not been informed that I had seen some of the photos.
b. He had not been informed that I had seen some of the photos.
Which means
He was not aware that I had seen any of the photos
and which means
There were certain specific photos that I had seen and he had not been informed of that. Maybe he had been informed that I had seen certain other photos.
?
a. He had not been informed that I had seen some of the photos.
b. He had not been informed that I had seen certain of the photos.
?
The question is: which could be used if he had been informed that I had seen some of them? He knew I had seen some of them, but he did not know that I had seen certain others.
a. He had not been informed that I had seen some of the photos.
b. He had not been informed that I had seen certain of the photos.
Neither could be used to definitely carry that meaning.
He had not been informed that I had seen a certain selection of the photos.
comers closer, as it does somewhat imply that you had seen others, but it is not 100% definite. It can still mean you haven’t seen any.
You would have to preface the sentence with something like
Although he knew I had seen a number of them, he had not been informed that I had seen certain photos.