John Russell
Well-known member
I wear glasses so adequate eye relief is critical for me, particularly as the converging lenses for my far-sightedness effectively reduce the eye relief, whereas diverging lenses for the near-sighted would effectively lengthen it.
I was looking at 8x32s recently at my dealer's and he insisted on showing me a Leica Ultravid but, before looking through it, a look at it was sufficient to tell me that the eye relief would be impossibly short. The eye lenses of the oculars were very small. Large eye lenses will allow a large AFOV and/or long eye relief.
AFOV is 2 arctan 0,5(eye lens dia. - exit pupil dia)/eye relief.
Exit pupil is objective dia./magnification and as AFOV is defined by FOV (in m @ 1000 m or yds @ 1000 yds) and magnification, it ends up with a simple calculation:-
Eye relief = 1000(eye lens dia. - exit pupil dia)/FOV. magnification
This value for eye relief, of course, applies to the plane of the eye lens.
Here are some values calculated for bins to which I had access:-
Swarovski SLC 7x42, 16,3 mm
Meopta B1 7x42, 18,2 mm
Swarovski EL 10x42 SV, 19,8 mm
Kowa Genesis 10x33, 15,7 mm
These values tie up rather well with my subjective impressions.
John
I was looking at 8x32s recently at my dealer's and he insisted on showing me a Leica Ultravid but, before looking through it, a look at it was sufficient to tell me that the eye relief would be impossibly short. The eye lenses of the oculars were very small. Large eye lenses will allow a large AFOV and/or long eye relief.
AFOV is 2 arctan 0,5(eye lens dia. - exit pupil dia)/eye relief.
Exit pupil is objective dia./magnification and as AFOV is defined by FOV (in m @ 1000 m or yds @ 1000 yds) and magnification, it ends up with a simple calculation:-
Eye relief = 1000(eye lens dia. - exit pupil dia)/FOV. magnification
This value for eye relief, of course, applies to the plane of the eye lens.
Here are some values calculated for bins to which I had access:-
Swarovski SLC 7x42, 16,3 mm
Meopta B1 7x42, 18,2 mm
Swarovski EL 10x42 SV, 19,8 mm
Kowa Genesis 10x33, 15,7 mm
These values tie up rather well with my subjective impressions.
John