Light throughput equation (1 Viewer)

[tenex]

Given enough money, there is no reason why the increase in AFOV can't continue.

And yet it declines, usually even worse at lower magnifications. I've often wondered why. Carry more, get less FOV?

 

[kimmik]

Might be aspherics - cost of aspheric element exponentially rises with size, so larger models fall back to spherical lenses which then limits focal ratio and fov.

Slc 56 costs the same as NL 32 after all, despite being way larger.

 

[kimmik]

Now that the SFL official specs are out, here is the completed Zeiss premium line up, showing well developed patterns:

40, 42 and 54 perform similarly in terms of light throughput per weight
25, 30 and 32 perform similarly

 

[has530]

And yet it declines, usually even worse at lower magnifications. I've often wondered why. Carry more, get less FOV?

It is the nature of the larger objective lenses. As the objective lens diameter grows you typically also get a longer focal length to keep a reasonable f/ and image quality. Then in turn you need a longer focal length eyepiece in order to keep the focal ratio (i.e. magnification) low. As you increase eyepiece focal length you need to increase field stop diameter to maintain the same AFOV. This causes two problems: enormous eyepieces which are unpractical in size and weight for for handheld binoculars (like nikon WX), you also need larger and larger prisms to fully illuminate the full field stop of the eyepiece which again... large, heavy (scales roughly as r^3 compared to ~r^2 for lenses) and expensive. Therefore the usual compromise that is made is sacrificing FOV to have reasonable sized eyepieces and prisms. This also manifests itself in why 7x50 typically has lower AFOV than 10x50. The TFOV is set by the focal length of the objective and field stop diameter of the eyepiece so when the field stop diameter (and prism size) stay roughly the same you get a similar TFOV and the lower focal ratio then gives a reduced AFOV.

 

[tenex]

Therefore the usual compromise that is made is sacrificing FOV to have reasonable sized eyepieces and prisms.

Exactly... but what I wish is that not every manufacturer made the usual compromise! We would have much more interesting choices.

 

[has530]

Exactly... but what I wish is that not every manufacturer made the usual compromise! We would have much more interesting choices.

Not every manufacturer, the Nikon WX did not make such a compromise (although even that somewhat did as the 7x50 has a markedly smaller FOV than the 10x50) and could be yours for a mere 4-5k over in the classifieds! Or a handful of older Japanese UWA 7x50 but some have pretty poor image quality because they get around the problems I discussed above by placing a focal reducer in front of the prism cluster.

 

[kimmik]

"The amount of flux which can be transmitted through a prism or other optical element depends on both its angular aperture and its cross-sectional area. The greater the amount of flux which can be transmitted, the better the throughput or light-gathering power (sometimes called
étendue or luminosity) of the system. The maximum throughput of a prism is thus proportional to the product of the prism’s solid angle of accep- tance and its cross-sectional area perpendicular to the prism axis. Hence, a large Glan-Taylor prism having an 8° field angle may, if suitable magnification is used, have a throughput comparable to a small Glan-Thompson prism with a 26° field angle. In general, to maximize prism throughput in an optical system, both the angular aperture and clear aperture (diameter of the largest circle perpendicular to the prism axis which can be included by the prism) should be as large as possible."

Handbook of optics, Bass 2009

The name of the concept this thread has been discussing, is étendue (french) or luminosity, light-gathering power, light throughput (english).

 

[John A Roberts]

Hi kimmik,

It’s a bit hard to evaluate the general application of a single technical paragraph, in relation to two types of polarising prisms,
where the significant difference is whether there is an air gap.


But what it seems to be indicating more generally is that (unsurprisingly), for a given type of prism:
one that has a larger entry face, and a correspondingly larger body, has a larger diameter light column.
So a greater amount of flux in terms of the area of the light column in the prism.


John

 

[kimmik]

Hi John (apologies!), the first sentence is the key concept, and was stated in a generally applicable fashion. These two polariser prisms offer one example of practical application of the light throughput concept. I have in effect adapted the concept to binoculars as a whole, which are at their core a transmissive optic with an angular aperture and a cross sectional aperture.

The amount of flux which can be transmitted through a prism or other optical element depends on both its angular aperture and its cross-sectional area.