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Binocular compared to Spotting scope.. (1 Viewer)


Well-known member
Everyone one obsesses over magnifcation, but in truth what we seek is resolution, which is solely a function of aperture. (assuming quality optics)

Magnification, beyond a certain point for a given aperture is so-called “empty magnification”.

All you do is turn a smaller blob into a bigger blob, with no additional detail revealed.

Yust my opinion, based on absolutely no qualifications or credentials.


Well-known member
No standard binocular gets near the resolution limit as regards aperture.

Unless one has 20/6 vision.

Empty magnification occurs at much higher magnifications than a standard binocular, unless the binocular is truly awful optically.

Empty magnification usually occurs in telescopes.

But for me a very well aligned 25x-135x80 binocular had empty magnification above 80x.

My 12.5 inch Dall Kirkham had empty magnification above 700x.



Well-known member
A pair of binoculars or a Spotting scope works at its maximum resolution power when it reaches the resolving magnification. Resolving magnification tells us how many times we need to magnify an image at an angle so that the eye reaches the separation power of the lens. The resolving magnification is equal to 1/2 of objectives diameter.
For example, a 80mm binoculars or spotting scope reaches the limit resolution for our eyes at 40x. Any power greater than 40x will not give to our eye more information. A 42mm binoculars reaches his maximum resolution for our eyes at 21x


Well-known member
Although a theoretical magnification of 40x may near the resolution limit of a 80mm scope. in practice this isn't the case.

With 20/10 vision the best observers use about 190x on a 300mm scope.
However, where conditions allow, say on Mars, these same observers use 300x and if the Seeing allows 600x.

Rev. Dawes, who had very fine vision, used 65x per inch of aperture with his 6.3 inch aperture refractor.
This is on Mars, Jupiter and Jupiter's moons. About 420x. The 420x was used to extract the finest detail.

This 65x per inch of aperture is quite normal with many of the best planetary observers.

Double star observers go even higher.

What is surprising to me is the detail Dawes saw and drew on Jupiter's moons, which I have not seen with a fine 12.5 inch telescope.



Well-known member
Empty magnification is more related to the quality of the optics than any 'theoretical' aspect.

The Pentax 100mm f/12 refractor was used on planets at 300x and tested at 400x on stars.

400x was certainly not empty magnification.

With a good spotting scope of 100mm aperture, I suppose empty magnification might occur at 200x.

Empty magnification is also not a function of Seeing.

With binoculars that have objectives of about f.3.6 the optical quality means empty magnification occurs at lower magnifications than spotting scopes or fine astro scopes.

As to the 190x used on 300mm telescopes by observers with excellent vision, this is used to see or glimpse fine contrast detail on Jupiter and Saturn.
To see small higher contrast detail one switches to a higher magnification.

Because the surface brightness on Mars is higher than on Jupiter and Saturn higher magnifications are usually used.
Also the detail on Mars is often higher contrast, although cloud storms are lower contrast.

With binoculars, which are wide field instruments of poorer quality than telescopes, one uses whatever magnification and aperture is most useful, depending on conditions.



Well-known member
Well, I can still look for wildlife or watch animal behaviour for hours with my 10x56 binocular (on a tripod if I want), which has 5.6mm exit pupil, which is significant difference compared to, for example, exit pupil of a 3.5mm. If you use 18x binocular you also have smaller fov, so you won't see as wide area as with 10x (or 8x) but still won't be able to ID the distant birds.

I also doubt your statement that most people can only use scope for brief periods before getting a headache...

It may work fine with large mammals or other large targets, or at relatively close distances with birds, but just take my words; in general you will loose many bird ID, if not using higher magnification (=scope). I still think that is the main reason why just relatively low magnification binocular on a tripod is not going to be popular among bird watchers.

Regards, Juhani
10x can be limiting, especially handheld plus 18x can have 4.5 degrees FOV with an even greater AFOV

Eye fatigue is one of the top reasons why people switch from scopes to high powered binoculars in other hobbies.

tines are not large targets

and it is popular with bird watchers that try them, my binoculars are rarely unattended with meetup groups…the scope is though


Well-known member
Once again missing out on looking for wildlife or watching animal behavior for hours on end , since most people can only glass through a scope for brief periods of time before getting a headache
"Most people can only glass through a scope for brief periods of time before getting a headache"?

You must be joking.



Well-known member
Yes you can achieve greater magnification on scopes

but let’s talk real world many astronomers prefer to be around 2mm exit pupil. With you being on the cloudy night forum, I’m sure your aware of this.

Plus many can’t be bothered with those super high magnifications since they are a hassle to deal with

in addition many birders are carrying small 50mm scopes which starts to dim beyond 25x. So if 20x gives a better view why bother with the scope when you can have 18x binoculars
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