Field of view / eye relief relationship (1 Viewer)

The relationship between eye relief and field of view is still exercizing me and I feel there should be a simple relationship but then I doubt my assumptions.

In the case of the new Monarch HG 10x42 the FOV is wider than the original HG or the subsequent HG-L which were 6 degrees but the eye relief was 18.5 mm: the Monarch HG is 17mm.

If a 6 degree fov 10x42 had an eye relief of, say, 18mm and that is assumed to be on the limit to a user needing spectacles as I do, how much of that FOV will be unseen if the eye relief were only 17mm.

Would someone please do the math for me, so that I may apply it to other hypotheses?

It mainly depends on the eyepiece design.
With older eyepieces there was a fairly strict relationship.
Nowadays long eye relief eyepieces can provide a lot of eye relief for very short focal lengths, even down to 2.5mm focal length eyepieces.
In general very wide angle eyepieces have shorter focal lengths and maybe less eye relief.
Some are complex and use exotic glass.
I think you are on to a loser if you try to make predictions for unknown eyepieces.
It also depends on the focal ratio of the objectives.

P.S.
As to 18mm and 17mm eye relief, to nearest approximation probably 5% loss, but there are so many variables, only trying it will work.

Generally, if you know the accurate eye relief and the diameter of the eye lens, simple triangulation will often give a good appromoximation for the AFOV. I tend to forget that stuff so calculators like this help.
http://www.1728.org/angsize.htm

Likewise if you know the ER is 17mm and the AFOV was 60° then the eye lens diameter would need to be 19.6mm. If viewed from 18mm and using the 19.6mm value then the AFOV should be 57°. That's the theory. With a quick look-see, it seems about right when extending the eyecup about 1mm past optimum, but the view seems to be disproportionally narrower than it should be at 5mm too long. so I suppose other factors must come into play that I'm unaware of.

David

Thank you, Binastro and Typo, for your responses.

The query had a general nature but was targetted following the introduction of the Nikon Monarch HG, which do not yet have a selection of reviews, but lots of opinions. There is a choice between buying these, the original HGs and the more recent HG-Ls.

I don't have access to any of these (Park Cameras is its usual unhelpful self) and so I cannot measure the ocular lens diameter. This is 24 mm on my Endeavor IIs.

Given my past experience with Nikon, I may just only consider makers who make quality bins of adequate, or nearly so, eye relief.

Thank you Ceasar,

I have yet to see a 10x42 edg for sale used, and new they cost more than I would be willing to pay.

The-Wanderer said:

Thank you Ceasar,

I have yet to see a 10x42 edg for sale used, and new they cost more than I would be willing to pay.

Click to expand...

I deleted my original post because I thought it was not needed but I thought I this comment about the Nikon 10x42 Premier would be useful to you.

The Nikon 10x42 Premier is still listed on the USA Nikon site under "Specialty Binoculars." It is also listed as currently out of stock. Its ER is 18.5mm which may be long enough for your use with spectacles if you can find one.

It does have a reputation for "Rolling Ball" however.

Bob

PS: My error. They are not "Out of stock," they are "Back-ordered." There is a difference I guess. Perhaps we can assume they aren't discontinued?:smoke: