We take for granted that the mid-sized models will have larger fields of view than their full-sized siblings, but is there a physical reason for this based on the binocular or objective lens size or do manufacturers design them that way arbitrarily?
Another way to ask would be, is it easier to get a wider field with a medium sized objective or what?
As I understand it FOV is determined by the design of the eyepiece lens. A basic formula found in the TeleVue website (www.televue.com., then go to Telescopic Formulas) is: Field size (in degrees) is equal to the eyepiece field stop diameter divided by the telescopes focal length then multiplied by 57.3 degrees. The only way an objective lens affects the field of view is if it lengthens or shortens the focal length of the binocular.
Bob
There seem to be very few 50mm models in roofs with a wide field. There must be some reason. On the other hadn people want brightness in these models, so that is the priority.