Minimum focus distance. (1 Viewer)

[John goshawk]

Hi,

Does anyone know what effects the minimum focus distance in a spotting scope.
I was looking at two spotting scopes with same specs. However the straight through spotting scope had a greater minimum focus distance than the angled one.
Why is that?
Thanks John

 

[Binastro]

Probably the length of the optical path differs because of different prisms.

Regards,
B.

 

[henry link]

Are you referring to straight and angled versions of a particular
scope?

 

[John goshawk]

Yes. But they both have same magnification for all opticron eyepieces?

 

[henry link]

Which particular Opticron scopes?

 

[Binastro]

The objectives have the same focal length, but the prisms are different.

Regards,
B.

 

[Tringa45]

Most modern birding scopes are waterproof, which dictates that there is a fixed distance between objective and eyepiece.
Focussing is achieved by shifting a focussing lens (usually negative) inside the scope body instead of extending the eyepiece.
To achieve close focus the negative lens would be shifted away from the objective towards the prisms thus shortening the focal length of the objective and placing the image plane at or near the focal plane of the eyepiece, depending on the observers eyesight.
Perhaps there are assembly tolerances, or as Binastro indicated, different glass paths for straight and angled prisms.
Better close focus usually goes hand in hand with reduced focus overtravel at infinity for near-sighted observers.

John

 

[jring]

Hi,

another way of internal focus drive would be to change the distance between porro prisms... obviously only if those are being used...

But in any way, for a closer minimum focus, you need to either change the position of a prism or a negative element by a mechanical drive, which seems to have either a different travel length in angled and straight examples of the same model...
Or it's just sample variation of the focal lengths of different lenses which results into infinity focus being a a slightly different point and thus close focus is different. Or did you try enough of those pairs to do a proper statistic?

Joachim

 

[John goshawk]

These are great answers. I understand better now the mechanics of what's going on. Many thanks.
I guess an angled scope may have more room for prism position to allow for closer focus.
It was just something I noticed on the opticron archive website when looking at the specs of an old scope I have.
Much appreciate all your answers.
John

 

[billb9430]

If you are looking to focus closer, spotting scopes that accept standard 1.25" eyepieces can provide that. Closer minimum focus distance can be achieved by pulling the eyepiece barrel partially out from its fully seated position, by using the eyepiece in a Barlow tube with lenses removed, or by increasing eyepiece distance from the scope using commercial extension tubes.

For example, my Pentax PF-65EDII with a Pentax XW20 eyepiece fully seated will focus at a minimum distance of about 15 feet. With the eyepiece barrel partially pulled out before clamping the eyepiece-holding collet , minimum focus distance decreases to a little over 10 feet. Using that eyepiece in a "Shorty" Barlow with lens elements unscrewed and removed gives close focus of about 6 feet. Longer 1.25" extension tubes could be used to make the minimum focus distance even less, but then your subject targets would likely be grasshoppers or other insects!

This method for decreasing minimum focus distance should work equally for straight or angled scopes, so long as they accept separate standard 1.25 inch barrel diameter eyepieces. - Bill

 

[John goshawk]

Thanks Bill, that's an excellent tip I never thought of.
John