binocular/monocular magnification

[HenryC]

This is a side issue to our other discussions on this topic, but I will give you my opinions on magnification anyway.

For the novice, magnification in a telescope or binocular/monocular is everythng.  After all, isn't making distant things look closer the whole point? Well yes and no. High magnification does make distant objects appear closer, and small objects appear bigger, but it also has many drawbacks.

High power makes the glass harder to hold steady and find your target, especially on a moving platform like a boat. You can't find what you want to see, and when you do find it you can't keep it in view, and when it is in view you can't hold it steady enough to get any detail.  And if the magnification is too high, the image is too dim. This is very important.  One of the major advantages of the popular 7x50 is that most people can learn to hold it steady enough to aquire a target, keep it in the field of view, and keep it still enough to inspect it in detail.  I can hold up to a 10x glass steady enough, but that's about my limit.  The 7x is much easier. With a 7x I can easily follow a football from kickoff to catch, or follow a bird in flight.  With a 10x I still can, but not as easily.  Above 10x and I really need a tripod to keep the glass on target.

However, I now am starting to prefer the 10x50 for general use  (I still prefer the 7x50 on the water).
Here's why.  For any given magnification, the size of the front lens determines how much light you scoop up. In my opinion, the minimum for any kind of low light conditions, at night or in a canopied forest, is 50mm.  A 7x35 is great for a well-lit ball park or rock concert, but is inadequate for birding in deep woods, for night use on a boat, or for looking at the Milky Way.  Images look dim if you're not collecting enough light, or if you're at too high a power, so balancing the two for your eyes is critical.  The way to do this is to determine the "exit pupil", the size of the cone where the image is projected behind the eyepiece.  The bigger the exit pupil, the more light goes into your eye and the brighter the image.  With a 7x50, you can go out at night and see if there's a prowler sneaking in the shadows around your neighbor's house a block away.  With a 7x35, you can't.  This is why the 7x50 is called a "night glass", and the 7x35 a "day glass".

You determine the exit pupil size by dividing the lens size by the magnification, so a 7x50 has an exit pupil of roughly 7mm, a 7x35 an exit pupil of 5 mm. At night, the bigger the lens diameter the better (it doesn't matter in bright sunlight).  But if the exit pupil is bigger than the pupil of your eye, the extra light can't get into the eyeball and it is wasted.  For a young man, the dark-adapted pupil of the human eye is about 7mm in diameter.  But as you get older, the ability of the eye to open up decreases.  I'm almost 65, and I reckon my exit pupil opens up to only 5 or 6 mm at most, so the optimum exit pupil for me is around that size.  Since a larger lens would be too heavy to carry around, and I still want as much magnification as I can hold steady, the 10x50 works for me.

I own a pair of jumbo-sized 11x80 binoculars I use for astronomy, I really need a tripod to use them because they are so heavy.  I would gladly trade them in for an 11x60, which would give me an identical view.  All that extra light gathering power is wasted on my ancient eyes. I also own a 100mm (4 inch)refracting telescope I use for astronomy.  I specialize in large, faint objects, like galaxies, clusters and nebulae, so magnification is not as important to me as low light performance.  My telescope isn't designed to make small things look bigger (planets or lunar craters),  its for making dim things look brigther.  Even though I can push it to its maximum theoretical magnification of 200x with a high-power eyepiece, the eyepiece I use the most is 20x.  100/20 = 5mm exit pupil.

Incidentally, here's a useful rule of thumb for people shopping for telescopes of any kind.  The lowest power a telescope can be operated at (for observing faint, extended objects)  is roughly 5x per inch of lens aperture, because of the exit pupil discussion above. The highest power any telescope can be pushed to before the image deteriorates is about 50x per inch. (1 inch = 25.4mm).  Regardless of what's printed on the box it came in, optical physics tells us any telescope's theoretical magnification range is limited to between roughly 5 and 50 times the aperture in inches.