Usable aperture in larger binoculars

[Buzzard]

I need new binoculars and am considering something like 15x70s. However, having read some threads on Stargazers' Lounge, it seems that quite a few of the more budget options (eg, Celestron ) are fitted with prisms that don't capture all the light from the lenses, effectively stopping a pair of 15x70s to 15x60s, or whatever. 

This seems bizarre to me, not to mention a possible breach of marketing and sales regulations! Do the manufacturers do this to save costs on expensive prisms, or to stop light from the outer rim of lenses in order to reduce optical distortions from what might not be top quality lenses? Is there an easy way to tell if binoculars do this?

Does anyone have any info. on this, or tips about reasonably priced binoculars that don't do this?

Many thanks

[Stu]

Hi.

I believe the reasons for stopping down are as you describe ie saving money on the prism and preventing the distortions apparent with using the whole objective, which I guess is to do with faster focal ratios etc.

Check out www.binocularsky.com for some great advice on binos

[Alien 13]

The one important thing about optics is that each part of the objective produces the whole image, this is why maks and newts work or why you can use a scope with just the small hole in the cap open. The small prisms will loose some of the collected light but the resolution from the objectives should be the same, a proper field stop is usually at the eyepiece end but that does tend to remove imperfections of the eyepiece itself.

Alan

[andrew s]

8 minutes ago, Alien 13 said:

The small prisms will loose some of the collected light but the resolution from the objectives should be the same, a proper field stop is usually at the eyepiece end but that does tend to remove imperfections of the eyepiece itself.

The resolution is defined by the aperture stop if the prism stops down the objective then the resolution will be reduces along with the light grasp.

Regards Andrew

[BinocularSky]

2 hours ago, Buzzard said:

I Do the manufacturers do this to save costs on expensive prisms, or to stop light from the outer rim of lenses in order to reduce optical distortions from what might not be top quality lenses? Is there an easy way to tell if binoculars do this?

Yes, yes & yes.

For "why?", see the bottom part of http://binocularsky.com/binoc_minefield.php

For "how to tell" see http://binocularsky.com/binoc_aperture.php

11 minutes ago, Alien 13 said:

a proper field stop is usually at the eyepiece end

In stopped down binoculars it is usually at the entrance pupil of the prism assembly.

[trailogy]

Get the celestron spymaster pro 15x70. I got a pair and measured the aperture, it is as marketed 70mm aperture. Make sure the pros though. The other ones are really 15x60.

Cheers,

-trail

[Stu]

4 hours ago, trailogy said:

Get the celestron spymaster pro 15x70. I got a pair and measured the aperture, it is as marketed 70mm aperture. Make sure the pros though. The other ones are really 15x60.

Cheers,

-trail

Interesting. How did you measure them, using the exit pupil method?

[John]

On 29/08/2016 at 12:39, Alien 13 said:

The one important thing about optics is that each part of the objective produces the whole image, this is why maks and newts work or why you can use a scope with just the small hole in the cap open. The small prisms will loose some of the collected light but the resolution from the objectives should be the same, a proper field stop is usually at the eyepiece end but that does tend to remove imperfections of the eyepiece itself....

Just a couple of minor points:

- As far as I know you don't get the benefit of the whole objective or mirror if only the small hole in the cap is open in terms of light gathering and resolution. The performance is restricted to the aperture of the small hole.

- The field stop of an eyepiece does not generally cut down the light coming from the objective or primary mirror because I think the light cone has narrowed to a point where it's almost a point source when it reaches and passes through the field stop of the eyepiece - ie: much narrower than the aperture that it's passing though.

Happy to be corrected if I have hold of the "wrong end of the stick" with the above

More on topic, I have a pair of 11x70 Opticron binoculars which are Japanese rather than the current chinese made ones. I have the feeling that these do make use of the full 70mm aperture, or at least the majority of it but I need to test them to find out for sure. I'm intending to use the "flashlight test" as described here:

http://garyseronik.com/getting-the-most-from-astro-binoculars/

Are there any potential flaws with that approach that I ought to be aware of ?

[BinocularSky]

2 minutes ago, John said:

http://garyseronik.com/getting-the-most-from-astro-binoculars/

Are there any potential flaws with that approach that I ought to be aware of ?

None at all, as long as the torch is sufficiently far from the eyepiece: As I wrote in the link I gave above, " I was unable to detect any difference in diameter of the emergent beam when the light source (the camera-flash LED on my mobile phone) was more than 200mm behind the eyepiece, and it was within a millimeter when the light was 150mm from the eyepiece. "

 

[trailogy]

6 hours ago, Stu said:

Interesting. How did you measure them, using the exit pupil method?

Not sure what the method is called but I measured them by putting a flashlight up to one of the oculars. I then measured the light that was coming out of the objective lens by having it shine on a wall and then measuring with slide calipers. It does seem to be 70mm if not maybe half a mm less. 

Looks like someone is also planning to do what I did. My advise is to put a head lamp about 7 inches back so you get the full objective lens and not pieces. 

 

-trail