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Apparent Brightness comparisons

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#1 EdZ

EdZ

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Posted 05 May 2004 - 03:31 PM

Often we read of a user comparing the apparent brightness of the image in the exit pupil from one model binocular to another. Assuming equal size exit pupils, sometimes it seems difficult to understand how or why it might be that a smaller objective binocular might appear to have a brighter image, or assuming equal size objectives, why one with a smaller exit pupil may appear brighter.

Take the example of two 100mm binoculars. Why might the one with a 4mm exit pupil appear to give a brighter image than the one with a 4.5mm exit pupil. Or compare 12x60 vs a 10x50. Both have a 5mm exit pupil. One would think the 60mm binocular might provide more light to the image just based on the light gathering area of the aperture. But that is the simple calculation, and all is not always what it may simply appear to be.

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 vignette in the binocular system. Even for some of the best binoculars, vignette is present. The amount present can cause considerable differences between models of the same sizes and between varying sizes of objectives producing the same size exit pupil. 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 under the topic Vignette), 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 radius 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. To see the affect this has, as the light source is moved completely around the circumference of the objective, the image of the light forming the 40% exit pupil would follow opposite. As the 40% width exit pupil rotates about the central axis, you will find that the central 20% of the exit pupil receives no light at all from the outer edge of the objective. Any exit pupil less than 50% the width of the full exit pupil indicates light entering at that point on the objective is not providing full illumination.

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

Fujinon FMT-SX 10x70
Light entering at 70% out on the objective radius fully illuminates only the central 25% diameter 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. Rotated about the central axis, you can see the central 30% of the exit pupil receives no light at all from the outer edge of the objective.

Celestron Skymaster 25x100
Light entering at 70% out on objective radius fully illuminates only the central 40% diameter of the exit pupil.
The Celestron Skymaster 25x100 has a 90% effective aperture of full illumination, 90mm.
The central 20% of the exit pupil receives no light at all from the outer edge of the objective.

Oberwerk Giant 22x100
Light is off-center in both objectives. Light from the center of the objective does not produce the maximum exit pupil. The results given are an average of four readings taken across the binocular objective, one line thru the maximum exit pupil and one line perpendicular to that. Light at 70% out on the objective radius, on one side of center thru the maximum point, illuminates 87% of the exit pupil. Quite the opposite on the other side of center. No point in the exit pupil is fully illuminated and the central 25% diameter of the exit pupil gets no light at all.

Average light at 70% out on objective radius illuminates only the central 56% dia of the exit pupil.
Average across the objective, Oberwerk 22x100 has a 75-80% effective aperture of full illumination, = 75-80mm.
On average, the central 40% of the exit pupil receives no light at all from the outer edge of the objective.

Minolta Activa 7x35 has a 70% effective aperture of full illumination, only 24mm and not 35mm. Light from the outer edge of the obbjective does not reach any portion of 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. Light delivered by the objective cannot be assumed at full area of the aperture or at full illumination from every point on the aperture. Take this into consideration when comparing binoculars.

edz


#2 KennyJ

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Posted 05 May 2004 - 03:44 PM

Ed,

I've just responded to this over on Astromart.

Not that I wrote anything YOU don't already know !

It just seemed a good opportunity , particularly on THAT site , to comment on how these revelations COULD make something of a mockery of the "optical theorists" who were so swift to contest my practical experiences with very small exit -pupil binoculars such as 10 x 25s, which "in theory" may have provided "sufficient brightness" to make no difference for daytime viewing alongside say a
10 x 40 ,but which I insisted all along did NOT.

Yes , amongst many other things Ed., your tireless work on this vignetting issue has certainly provided SOME kind of "technical back -up" to what I was trying to say all along.

There were times when I felt similar to how General Custer
must have felt regarding this issue , but I'm even more pleased now that I stuck to my gun regardless of the crossfire and ceaseless bombardment.

Regards , Kenny.


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