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ZEISS. Discover the fascinating world of birds, and win a birding trip to Columbia

Can you interpret this statement for me re Nikon eyepiece (1 Viewer)

rdfish1

Well-known member
what kind of fov is this vs a wide angle fixed 30 or maybe a 25-75 zoom ep?

The lowest power eyepieces take up the entire opening in the scope. There is no way to get more. Therefore the apparent field is fixed max , in this case I believe about 45-50 degrees. But you are getting the maximum real field possible along with brightest image.
 

wllmspd

Well-known member
Different eyepiece designs can give different apparent fields of view (50degrees is pretty narrow, beyond 80degrees is possible). Spotting scopes normally give real field, the apparent field is (approximately) the real field width multiplied by the magnification. Most zooms have a narrow apparent field of view at the lowest power and thisnwidens out at higher powers. Depending on the focal length you need (magnification is scope focal length divided by eyepiece focal length). Long focal length and wide apparent field of view can be hard as sometimes the e pieces need to be larger than the normal hole in the back of the scope (why astro scopes often have a 2” eyepieces as well as 1.25” diameter ones).
If you mainly use your scope at one power then a wide field eyepiece will show more, making finding and tracking things easier.
Did you have any specific models you wanted feedback or comment on?

Peter
 

jring

Well-known member
Hi,

yes, there is a relationship between the field stop diameter (often the diameter of the field lens, the one closest to the objective) and the apparent field of view of an eyepiece. The apparent field of view in degrees can be calculated as follows:

afov in deg = 57.3 deg * field stop diameter in mm / EP focal length in mm

Since the field stop can not be larger than the eyepiece opening of the scope body, that is a hard limit.

Often manufacturers offer one EP with the widest possible true field - in case of the Nikon fieldscopes that would be the 20/25x MC with 55 deg afov.

As can be seen, the 13-40x/20-60x/25-75x MC II zoom also offers 25x in the 82mm body at the low end but the afov is quite a bit narrower at 38 deg... like looking through a straw...
That is typical for zoom EPs, they tend to be narrow at the low mag end and widen up a bit at higher mags...


Joachim
 

Alexis Powell

Natural history enthusiast
United States
Often manufacturers offer one EP with the widest possible true field - in case of the Nikon fieldscopes that would be the 20/25x MC with 55 deg afov.
The 20/25x is not a wide-field design. Its angular FOV is the same as the 24/30x WF and MC and the 16/24/30x DS eyepieces. Those all have an angular view of 3.0 or 2.4 degrees (on 60 vs 78 or 82 mm scope) and an apparent FOV of 64 degrees. But how about the often forgotten 15/19x sibling eyepiece to the 20/25x? Does anyone know its angular and apparent FOV? And what is the eye-relief of that one? I don't have access at present to my old catalogs.

--AP
 

jring

Well-known member
Hi Alexis,

thanks for double checking, when I wrote this (not quite up to standard) post, I had the MC EP specs page open but obviously didn't scroll down far enough to the 24x/30x wide but only to the 27x/40x/50x and that is quite a bit narrower than the 20x/25x.

Both the 20x/25x and the 24x/30x wide do indeed result into a true field of 3 deg on 60mm bodies and 2.4 deg on 78/82mm bodies.


Same for the 16x/24x/30x DS - and a whopping 4.5 deg at 16x in the 50mm...


As for the 15x/19x (Type 7772) I did only find an old BH Photo Video page - but alas, no field of view data listed.


As for whether the 15x/19x can offer a wider true field than the 20x/25x or 24x/30x wide, we can make an educated guess... If the field lens diameter of any of the latter two EPs plus a mm or two for the barrel is close to the inner diameter of the eyepiece receptacle of the scope body, the true field is as wide as we can get... the formula given above is (while correct) not ideal for what we want to do...

afov in deg = tfov in deg * magnification = tfov in deg * telescope focal length in mm / EP focal length in mm = 57.3 deg * field stop diameter in mm / EP focal length in mm
Multiplying both sides by EP focal length in mm and solving for tfov in deg yields:

tfov on deg = 57.3 deg * field stop diameter in mm / telescope focal length in mm

which is much more useful in our case. It is easy to see that with any given scope body and thus a fixed scope focal length, field stop diameter and true fov are directly proportional and thus if the field stop diameter for an eyepiece is at the maximum dictated by the diameter of the eyepiece receptacle, the true field for another eyepiece can not get any larger.

Sorry for the maths...

Joachim
 
ZEISS. Discover the fascinating world of birds, and win a birding trip to Columbia
ZEISS. Discover the fascinating world of birds, and win a birding trip to Colombia
ZEISS. Discover the fascinating world of birds, and win a birding trip to Colombia

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