Designing binoculars of different magnification (1 Viewer)

[tenex]

Contemplating my lovely new E2s prompted me to wonder about changes in the optical design of binoculars with different magnification. The 20th-century approach was mainly to add longer tubes with larger objectives onto the same prism/ocular housing to increase magnification, as the Nikon E/SEs illustrate with 8x30, 10x35, 12x40 or similar (with a curious exception for any 7x models, perhaps for marine use?), and likewise early roof-prism models like Dialyt (8x30, 10x40). But then sometime in the 1990s the idea arose to have more consistent lines of 32, 42, 50mm models each in (7x), 8x, 10x, (12x), achieved instead by using higher magnification oculars on the same prism/objective housing.

Which approach is optically more advantageous and why? (And was Trinovid BA "Ultra" the first such complete line to appear, or was it SLCs? Interesting given how little Leica is thought to innovate today...)

 

[BKoh]

In one case you keep the eyepiece but change the objective, in the other you keep the objective but change the eyepiece.

This is like using one eyepiece in different telescopes, versus using different eyepieces in the same telescope. Some combinations work better than others, there is no "best" strategy.

I would guess there is probably a "reference design" which is then modified, and that reference design is likely the "best" performer.

 

[jring]

Hi,

I would expect the change from the old same eyepiece different objectives approach to keep the size the same and change eyepieces to have happened when optical design software became readily available.

Designing a doublet objective (most binoculars use cemented doublets) is really straightforward when compared to a complex wide angle EP... and doing the latter without the help of a computer is quite a lot of number crunching...

Joachim

 

[Tringa45]

In one case you keep the eyepiece but change the objective, in the other you keep the objective but change the eyepiece.

The latter approach seems to be more common today but in the 2000s Zeiss made four Conquests with two objectives and two eyepieces.
The 8x30 and 10x30 had different eyepieces and they used 45 mm objectives with 1,5x the focal length to make a 12x45 and a 15x45.

The Zeiss 7x42FL had a shorter body than the 8x42 and 10x42 FLs, so it probably used a shorter focal length objective with the same eyepiece as the 8x42 FL.

As Joachim pointed out, eyepiece design is a complex task, particularly if you wish to maintain the same eye relief for different focal lengths. The designers have gone the easy way with some binocular families and just scaled the design up or down for the different magnifications.
An example is the Meopta Meostar where the eye lens diameter and eye relief gets progressively smaller from 7x42 to 8x42 to 10x42.
This is also the case with the 56 mm Swarovski SLCs. Eye relief for the 8x56, 10x56 and 15x56 is 23 mm, 19,5 mm and 16 mm respectively.

A puzzle to me are the Swarovski SLC 7x42B and 10x42WB (not the subsequent HD version) from the turn of the century. The 7x42B is 17 mm longer and 80 g heavier than the 10x42WB and has 19 mm eye relief vs 14 mm for the 10x42WB. They appear to be completely different optical designs without any commonality.

John

 

[tenex]

Is it generally true then that better optical results are more easily achieved by increasing the focal length of the objective than decreasing that of the eyepiece, especially given its complexity... but computers help, and the difference may not matter that much at the relatively low magnification of binoculars?

 

[Swedpat]

Interesting topic. As mentioned above there are two main strategies for binocular series. For example 7, 8, 10x42 models in same line usually share the same objective but use different eyepiece focallengths.
And a line of 7x35, 8x40 and 10x50 usually share the same eyepieces but different objectives. In the latter case it's obvious when as well AFOV and ER can be the same for all models. Which is optically more advantageous because you don't sacrifice either brightness or ER with the higher magnifications. At the expense of size and weight, though.