Focal length of a Nikon ED 78?

[william j clive]

Would anyone on the forum happen to know the focal length of this scope please?

Clive

[tlb]

Clive,

Looks as if it is 1000mm. I took this from specs for Nikon's 35mm photo adapter as used with Nikon's 60-, 78- and 82mm scopes.

Here is the link to B&H Photo's website from which I took the info:

http://www.bhphotovideo.com/bnh/controller/home?O=NavBar&A=getItemDetail&Q=&sku=219286&is=REG&si=spec#goto_itemInfo

Hope this helps,

Tony

[Leif]

Clive,

Looks as if it is 1000mm. I took this from specs for Nikon's 35mm photo adapter as used with Nikon's 60-, 78- and 82mm scopes.

Here is the link to B&H Photo's website from which I took the info:

http://www.bhphotovideo.com/bnh/controller/home?O=NavBar&A=getItemDetail&Q=&sku=219286&is=REG&si=spec#goto_itemInfo

Hope this helps,

Tony

It won't be as high as 1000mm. Most of these scopes are about F6, which gives a focal length of just under 500mm. The Leica APO 77 has a focal length of 440mm, and the focal lengths of the Zeiss 85 and Swaro ATS 80 are similar. What the camera adaptor does is multiply the focal length using some optics i.e. bascially a Barlow tube. I think this is to make the focal plane of the scope accessible to the film chamber of a camera.

The focal length of the Nikon 20-60x zoom ranges from 21mm to 7mm and I believe that on the 78 ED the zoom goes becomes a 25x to 75x.

Therefore 25 = FL / 21 => FL = 525mm. And this implies that the F ratio is 525/78 = 6.73.

[andytyle]

It won't be as high as 1000mm. Most of these scopes are about F6, which gives a focal length of just under 500mm. The Leica APO 77 has a focal length of 440mm, and the focal lengths of the Zeiss 85 and Swaro ATS 80 are similar. What the camera adaptor does is multiply the focal length using some optics i.e. bascially a Barlow tube. I think this is to make the focal plane of the scope accessible to the film chamber of a camera.

The focal length of the Nikon 20-60x zoom ranges from 21mm to 7mm and I believe that on the 78 ED the zoom goes becomes a 25x to 75x.

Therefore 25 = FL / 21 => FL = 525mm. And this implies that the F ratio is 525/78 = 6.73.

Leif's estimate is probably about right. In my experience, 60mm spotting scopes tend to be between f/6 to f/8 and 80mm (ish) spotting scopes usually between f/5 to f/7, (we are talking about refracting spotting scopes here).
If memory serves me correctly, the Nikon Field scopes have the barrel type focus with an internal focussing lens (or at least they used to). This means that - a.) The true focal ratio of the objective lens is probably no more than about f/3. b.) The focussing lens then acts not only as a focusser, but also as a tele-extender (like a Barlow lens), giving an 'effective' focal ratio of f/6, f/7 - (whatever it is). If this is the case, then the magnification changes a little depending on how far from infinity you are focussing. I can't remember which way it goes, but essentially this means that a bird on the extreme horizon (near infinity) may be 30X with the 30X eyepiece, but could be 33X or 27X at close focus. A moving lens which acts as a tele-extender will be changing the 'effective' focal ratio/focal length, so this means that magnification is constantly changing during focus. In other words, it is acting like a very weak zoom. Life is never easy is it.
The only time this won't occur is when 1. The focussing lens is a relay lens (changes the focal position without altering F or f/), 2. Moving prism focussing (e.g. Optolyth TBG/TBS and the older Kowa TSNs, and probably the new 82mm Kowas as well). These have objective lenses with 'true' focal lengths of f/5 to f/7.

Best regards
Chris

[william j clive]

Phew, thanks guys

Clive