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Why does a smaller objective give a wider field of view? (1 Viewer)

Winterdune

Well-known member
I've been using binoculars all my life and have literally only this week realised that x32 bins frequently have a wider FOV than their x42 equivalent models (eg Swaro ELs). This seems really counter-intuitive to me. Can anyone explain the reasons for it?
Thanks
Sean
 

Binastro

Well-known member
Hi Sean,

The FOV is determined by the eyepiece, not the objective.

However an 8x32 may have objectives, say 120mm focal length.
The eyepiece 15mm focal length.

An 8x42, say 160mm objectives focal length.
Eyepiece 20mm focal length.

The field stops may be similar sizes, also the prisms.
So the 20mm focal length eyepieces have a smaller field than the 15mm eyepiece, but probably longer eye relief.

Regards,
B.
 

tenex

reality-based
Antarctica
Since I often wondered this myself, can we formulate an answer in terms of why components are chosen for 42 vs 32mm models as Binastro notes they tend to be? For example, is it primarily the desire to keep the 42mm model relatively compact?
 

Binastro

Well-known member
If the 42mm binocular was just a scaled up 32mm it would be 31% bigger in each dimension.

I suppose the weight of the 42mm would be perhaps 70% or more than the 32mm depending on the thickness of metal.
But the glass elements would I think be cubed rather than squared.
There would be more light loss with the thicker glass.

So normally they are chosen to be more compact.

B.

P.S.
The binocular cannot just be scaled up 31% as our head would need to be scaled up 31% also.

The IPD has to remain at say 54mm to 74mm.

Just scaling up would give a 42mm binocular a 226% bigger volume than the 32mm.

Our head would also on average need to be 226% bigger :)
 
Last edited:

Chosun Juan

Given to Fly
Australia - Aboriginal
I've been using binoculars all my life and have literally only this week realised that x32 bins frequently have a wider FOV than their x42 equivalent models (eg Swaro ELs). This seems really counter-intuitive to me. Can anyone explain the reasons for it?
Thanks
Sean

The other question to ask is - why do binoculars with objectives smaller than 30mm nearly always have narrower Fov's ...... ? ;)







Chosun :gh:
 

Binastro

Well-known member
Hi Chosun,

Presumably because they are sold as pocket binoculars.
Large prisms etc. may make them too large for a pocket.

However, the 6x24 Amplivid has a 12.2 degree field and goes in my pocket.
6x25 Karlstein micro binocular 11.5 degrees, pocket yes.
7x25 micro binocular 10 degrees, pocket no.

5x25 VisionKing 15.8 degrees, pocket no.
6x24 Russian 12.2 degrees, pocket no.
4x22 Dowling and Rowe 16.5 degrees, pocket maybe.

There is a Russian 8x30 with a 13 degree field. Unfortunately I have never seen one.

Regards,
B.
 

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