The author could very well agree that “resolution” captures something important about the quality of photographic instruments and materials, such as camera lens and film. The point, however, is that the differences in resolution of camera lens aren’t practically relevant. “Resolution” is still an important feature, but the differences in camera lens resolution don’t have to be.

(B) is necessary because if it were not true — if differences in lens resolution DO compound (in other words, make worse) the deficiencies of film, then this is a reason lens resolution might make a practical difference. So the author must assume that differences in lens resolution do NOT make worse the deficiencies of film in order to conclude that there’s no practical difference between different lens resolutions.

(C) doesn’t say anything about camera lens resolution. The author doesn’t have to assume anything about film resolution except as it relates to camera lens resolution. If variations in processing film do have large effects on film’s resolution, that doesn’t undermine the author’s point, which is that camera lens project more detail than any film can capture in an image, and that this is why lens resolution differences don’t matter.

The author asserts that modern lenses project far more detail onto film than any film can reproduce. But there’s no indication the author believes we “need” to use “flawless” technique achieve this level of detail or any other level of detail.

(E) isn’t necessary, because even if there are other factors important to the level of detail in the final print (such as the quality of a printer, perhaps), the author’s reasoning involves only the relationship between lens resolution and the detail reproducible on a developed image. After we get the image, there might be further factors that affect the detail of a printed image, but the author’s argument doesn’t relate to this stage.