Apparent Brightness Comparisons

In the previous thread, (Low power - Zeiss Victory / Fujinon impression), the discussion relative to apparent brightness struck my attention.

Recent studies have shown that binoculars do not deliver all the light expected based on the simple calculations of aperture area and exit pupil. The brightness of the exit pupil is considerably affected by vignetting in the binocular system. Even for some of the best binoculars, vignetting is present. The amount present can cause considerable differences between models of the same sizes. Brightness cannot simply be assumed based on light gathering area and exit pupil.

Without getting into how this is measured (you can read all of that if you wish in the CN binocular forum), I will give a few examples of light loss in some very good binoculars. This is a factor that must be taken into consideration when comparing the image brightness from one model to another.

Fujinon FMT-SX 16x70
Light entering at 70% out on the diameter of the objective lens is providing 100% illumination to only the central 50% of the diameter of the exit pupil, 25% of the exit pupil area.

The light entering the objective at 90% out from the center, produces an exit pupil only 50% the width of full size. This is the furthest point out on the objective that is fully illuminating any portion of the exit pupil. The effective aperture of full illumination is 90% x 70mm = 63mm.

At the outer edge of the objective, light entering produces an exit pupil only 40% the width of a full size exit pupil. If you rotate that 40% width exit pupil about the central axis, you would find that the central 20% of the exit pupil receives no light at all from the outer edge of the objective.

This is one of the better examples. Most others have proven less. With the exception of a binocular telescope, all binoculars I tested ranged between 70% and 90% effective aperture of full illumination.

Fujinon FMT-SX 10x70
Light at 70% out on objective diameter illuminates only the central 25% dia of the exit pupil.

The furthest point out on the objective that is fully illuminating any portion of the exit pupil is 85%. The effective aperture of full illumination is 85% x 70mm = 60mm.

From the outer edge of the objective, light entering produces an exit pupil only 35% full width. The central 30% of the exit pupil receives no light at all from the outer edge of the objective.

A Minolta Activa 7x35 has a 70% effective aperture of full illumination, only 24mm. Light from the outer edge does not reach the exit pupil at all.

The brightness of the image in the exit pupil is entirely dependant on the light delivered from the objective. The % illumination has an effect on this outcome.

For a more complete post see

http://www.cloudynights.com/ubbthreads/showthreaded.php?Cat=1,2,3,4,5,8,9,10&Number=102901&page=0&view=collapsed&sb=5&o=&vc=1

edz

Hi Ed,

I re-read your original CN post on vignetting in binoculars and am curious about something. When you viewed into the edges of the objectives, did you have some kind of sight tube perpendicular to the binocular tube end? This seems important to verify the extent to which on-axis rays are being vignetted. It would not be as damning to have a dropoff in limiting magnitude at the outer edges of the field as long as the center is 100% illuminated. This would be caused by vignetting of rays that are both toward the outer edge of the objective and off-axis by some percentage of the 1/2 FOV. In binoculars where you are talking about several degrees, this might be difficult to judge. In a Newtonian reflector, the central part of the field is (almost) always 100% illuminated, but off axis illumination is commonly reduced by the secondary or focuser.

Thanks,
Milt