Here is my dilemma, sorry if this is long winded. From all the researching I have done on binoculars it seems to me that it boils down to a few important factors; exit pupil diameter, relative geometric brightness, and twilight factor. The exit pupil diameter is easily determined by dividing the objective lens diameter by the magnification, example 8 divide by 42= 5.25, so your exit pupil is 5.25mm. This is very important for several reasons; to reduce eye fatigue, the amount of light that is allowable to enter the eye, and ultimately the brightness of the image. The human eye when young and healthy can open and accept 5.2mm of light entering the retina. As we age this amount is reduces. So for binoculars to have maximum benefit the exit pupil has to be close to this number (5.2mm). If your exit pupil is equal or greater than 5.2mm the less fatigue your eyes experience, so the longer you can glass. For example 8x56?s the exit pupil is 7mm, so it's easy for your eye to compensate in low light conditions. Therefore, the 8.5x42 have an eye relief of 4.9mm and the 10x42?s have an eye relief of 4.2mm. So the 8.5x42 create less fatigue during low light conditions. Relative geometric brightness determines brightness capabilities of the binocular, not to be confused with the quality of glass, just how bright they can be by there dimensions, the exit pupil and objective lens combine. So the calculation to determine this is; the objective diameter divided by exit pupil and then squared. Examples; 42 divide by 8.5= 4.94 squared=24.4 is the relative brightness, for the 10x42?s the relative brightness is 17.64 and say for 8x56?s,the relative brightness is 49. The higher the factor the brighter the glass, but you have to remember your eye can only allow in so much, so there can be over kill. Twilight factor can not be confused with brightness and is a direct correlated function of magnification.
So I find it difficult not to go with the 8.5x42?s, I'm not sure if I want to sacrifice brightness, field of view, and eye fatigue for 1.5 extra magnification? I am trying to consider size or I might of even considered the 8x56?s. I am going to purchase top quality bin?s because I feel that your binoculars are one of your most important tools for hunting sheep. With long hours of glassing and most animal activities in low light conditions, dawn and dusk. Well that's my dilemma, what are your thoughts?
Cheers, Ramchaser
