black crow
Well-known member
I'm sure this has been answered many times but I must have been away those days. Why is it that in a binocular line the 42mm will often have a smaller FOV than the 32mm?
I'm sure this has been answered many times but I must have been away those days. Why is it that in a binocular line the 42mm will often have a smaller FOV than the 32mm?
Field of view is not based on aperture; it’s based on magnification and field stop. Those things are based on what the manufacturer has determined to be his or her market for the particular instrument. Bigger is not always better. Unless, of course, you are talking about ... ice cream. :cat:
Bill
Would they really use the same size prisms in 32 and 42mm glasses?
Maybe the principle will be clear if I describe the simplest case: an 8x42 and an 8x32 using the same prisms, with the same size eyepiece field stops, but different focal length objective lenses and eyepieces.
The short focal length objective of the 8x32 focuses a small image of the FOV at the plane of the eyepiece field stop. The longer focal length objective of the 8x42 focuses a larger image at the plane of the same sized eyepiece stop. Because the 8x42 image is larger the eyepiece field stop in the 8x42 can't include as wide a FOV as the same sized field stop in the 8x32, so the FOV is smaller.
The last wrinkle is that, because the objective lenses have different focal lengths, the eyepieces must also have different focal lengths for the two binoculars to both be 8x. The 8x42 requires a longer focal length eyepiece which magnifies its eyepiece field stop less than short focal length eyepiece in the 8v32, so the eye seems to see a smaller field stop and therefore a smaller apparent field in the 8x42.
Best I can do just before going to bed.
How about the FOVs of f4 42mm binoculars? Most 7x42s have 24mm eyepieces and a real FOV of 8º while most 8x42s use a 21mm eyepiece and have real FOVs of around 7.7º.
If you want wider FOVs do you manipulate the focal lengths of the objectives or fiddle with the ocular lenses or do both? For instance, the Zeiss Victory 7x42 FL has a real FOV of about 8.5º and the Zeiss 8x42 SF has a real FOV in that neighborhood.
Bob
Would they really use the same size prisms in 32 and 42mm glasses?
Wide FOV is one reason I've remained happy with a 32mm for years. I always assumed that 42s had a narrower view because this is the most popular size among birders who carry them all day, and want them kept small and lightweight. Some are barely taller than 32s. One can only make so many compromises.