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Challenge - shortest, clearest explanation of why 8x FOV tends to be greater than 7x and 10x (1 Viewer)

Dr. K

Bad Weather Birder
United States
Posts can get super in the technical weeds here, and frequently that is what I'm here for, so thanks for that. Not this time thought... The challenge here is to explain to me, as clearly but briefly as possible, 1) why magnification seems to relate to FOV, and, 2) why does it seem to be the case that greater magnification (10x+) have narrower FOV, but at the same time, few 7x have wide fov?

Ready set go - and thanks!
 
(1) an instrument provides a certain apparent angle of view, and magnification determines (inversely) what real field that corresponds to
(2) 7x bins are built today as lazy modifications of an 8x model, instead of being designed for the purpose

Related to #2 there is the further fact that even 8x bins usually don't have quite the FOV you'd expect by calculating from the 10x model, so presumably compromises in size (eyepiece and/or prism) are being made even there that limit FOV at lower magnifications.
 
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7x50s are usually either marine binoculars or simple binoculars with fields typically of 7.1 degrees.

There are wide field 7x35s, such as the Minolta Standard MK with 11.05 degrees measured field and some up to about 12.5 degrees, but the 7x50s usually have smaller fields.
The Minolta Standard MK 8x40 has a 9.5 degree field and the 10x50 7.8 degrees.
So all similar AFOV with the same body but different length tubes.

Same with Kronos. 6x30 12.5 degrees, 7x35 11 degrees, 8x40 9.5 degrees, 10x50 7.5 degrees.

Old Trinovids had wide angle and normal angle versions.

!0x binoculars usually have wider AFOV but slightly less real FOV.
The 10x eyepieces may have extra lens elements.
Also the prism size affects the eyepiece field lens size, which determines the field size.

There is a Soviet 8x30 with a 13 degree field.

The Bresser Superwide is 7x32 13 degrees plus, 8.5x42? 11 degrees and 10x50 9 degrees plus.

The reason for the typical fields of 7x, 8x and 10x binoculars is historic.

With good makes the fields and magnifications are accurate.

However, the Celestron 8x30 is 6.7x 27.

The Bushnell Xtrawide 4x21 is 3.5x21. 5x25 is 4.4x25, so although the fields are wide the magnifications are inflated by about 15%, which is a bit naughty, as I think 10% is the limit and 2% to 5% acceptable.

Regards,
B.
 
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1. The objective forms an image
2. The ocular magnifies that image.

In order to magnify the image more, you have to get closer to it, which means you can see less of it.
 
Thanks for playing. I'm afraid I can't identify a winner because we all got pretty off track real quick, as one does on a forum... Tenex is closest though.

(1) an instrument provides a certain apparent angle of view, and magnification determines (inversely) what real field that corresponds to
(2) 7x bins are built today as lazy modifications of an 8x model, instead of being designed for the purpose

Related to #2 there is the further fact that even 8x bins usually don't have quite the FOV you'd expect by calculating from the 10x model, so presumably compromises in size (eyepiece and/or prism) are being made even there that limit FOV at lower magnifications.
This is interesting... I recall reading a comment somewhere, saying that the Zeiss FL 7x42 were made by shortening the length of the housing but otherwise using the same parts. At the time that appealed to me, as it means smaller/shorter, maybe a hair lighter, and while I'm still in love with my FL7x42, I do now wonder if there were any 7x roof bins that were designed around that magnification rather than modified from another. ALSO, is this true also of 8x? what's the first configuration, the one the original design for a model is made?

1. The objective forms an image
2. The ocular magnifies that image.

In order to magnify the image more, you have to get closer to it, which means you can see less of it.
This would be my expectation too, however, the important part of my question is the exceptions - fov, in recent binoculars, seems to not fit that pattern, i.e., 7<8>10, when it should logically be 7>8>10.

Have a look here. Especially #4.
Thanks for the reference... There's some good stuff in there that I will have to work through. My reply to #4 would be the same as post #6 in that same thread.

Thanks again!
 
I do now wonder if there were any 7x roof bins that were designed around that magnification rather than modified from another.
Apart from WX 7x50? SLC 7x42 would fit that description, as at the time there were only a 7 and 10x and their optical design was very different. But despite being quite heavy, it still had only the usual 420ft/140m. Dialyt 7x42 may have been another example, and achieved 450ft/150m... and was the FL really not derived from it? But that's still less than I'd want in 7x.
 

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