Spotting scope focusing mechanism

[vlaiv]

Here is an interesting question I can't find answer to online (I guess I'm either bad at searching of people simply don't write on this subject).

Let's for example use cheap Chinese spotting scope - like one from Svbony. Let the scope be 80mm with range of zoom of x20-x60.

On the label it says - close focus 5.5meters, so I'll guess focus range from infinity down to 5.5 meters.

A bit of simple math tells us that focal length of such scope is, if we assume 7-21mm zoom eyepiece to be FL of 420mm, or F/5.25 scope. It might even be F/6 instrument if we take 8-24mm eyepiece.

In any case we have a range of focal lengths that we can work with and let's go with 420mm or 0.42m

1/focus_distance = 1/focal_length - 1/object_distance

1/focus_distance = 1/0.42 - 1/5.5

(for close focus or)

1/focus_distance = 1/0.42 - 1/infinity

(for infinity focus)

Second is easy - focus_distance = 420mm (as we would expect for lens with 420mm focal length) and first one is, If I'm not mistaken:

0.4547m or 454.7mm or about 34.7mm over original position.

As far as I can tell - neither eyepiece nor objective lens move - yet spotter is able to accommodate at least 35mm of change of light path.

How does it do that? Does it have a set of mirrors that change the distance?

[andrew s]

Moves internal lenes. Think like moving a Barlow in a normal scope. Some camera lenes do the same.

Regards Andrew 

Edited August 24, 2023 by andrew s

[vlaiv]

9 minutes ago, andrew s said:

Moves internal lenes. Think like moving a Barlow in a normal scope. Some camera lenes do the same.

Regards Andrew 

I was not aware that spotting scopes have internal lens.

Would that not change focal length and hence magnification as well?
 

I have 100mm Svbony spotting scope "on loan" from a friend. It's not so much of a loan as trying to get it in working order as it's missing eyepiece and some bits, so I'm employing 3d printing to sort out those things.

I've made adapter to 17mm plossl - which works very good. I was hoping to adapt a zoom eyepiece to it, but most zooms seem to have Smyth lens or something and their focus position is not compatible as they can't be pushed in further than protective clear window (which probably holds nitrogen filling as well) stands in the way.

In any case - I did not notice any magnification shift when refocusing.

Edited August 24, 2023 by vlaiv
I did not write what I actually started to write :D

[vlaiv]

I think I figured it out - it uses two 90 degree prisms to do the focusing.

Top prism is moving back and forth thus changing the focus position by twice it's movement

[andrew s]

I have been looking for a reference without success.  I think it depends on the scope. The simplest is to move the prisms back and forth but I don't think that is possible with a roof prism design.

Regards Andrew 

PS beat me to it 😊

Edited August 24, 2023 by andrew s

[andrew s]

Here is an example of lens focus

Regards Andrew