That is a great question! Yes, theoretically the FOV and magnification have a relationship to the dioptric power such that a high diopter implies higher FOV. However, the size and design of the lens also plays a role in performance. While the 90D theoretically should have a wider field of view, due to the 90D being smaller in size than the 78D, the field is essentially "cropped" in the 90D to allow for a small size. As a result, the 78D has both wider field and higher magnification than the 90D. The smaller size of the 90D allows for easier manipulation within the orbit which coupled with its undilated exam ability makes it a popular choice and a classic industry standard lens.