Reduction in the diameter lens in spotting scopes. (1 Viewer)

Hello,
I'm a big fan of Birding, also for astronomical observation, I read many times in astronomical forums, which in some binoculars (15x70 Celestron SkyMaster, for example) the diameter of the front lens, the lens is not really that he says advertising instrument, in this case, is not 70mm but rather close to the 60-62mm,
Here I would like to ask if anyone knows anything about it in relation to the spotting scope as prisms also used to straighten the image ..
Anyone know of any proven case in particular?
In astronomical forums are very knowledgeable fans who test, test star., .to See if really a binocular is working below its lens diameter.
This problem would occur in the case of spotting scopes?
Thanks for your information
Regards
Wachi

Wachi,

I always test for this.

I don't recall finding any significant reduction in clear aperture among spotting scopes that use internal focusing lenses, but the few I've tested that use a moving prism for focusing have all suffered a gradual loss of clear aperture as the prism is moved out of its optimum position toward close focus. The worst I recall was a Pentax 80ED which was more or less 80mm at infinity focus, but I think had only around 67mm of clear aperture at 10m. A Zen-Ray 82ED I tested was 80mm at infinity focus, down to 75mm at 10m and about 70mm at the closest focus of 5m. I haven't tested every scope that uses this type of focus but I imagine they all have the same problem to some degree.

Henry

Thanks Henry for his valuable information.
So how can we know if a spotting scope has internal focusing lenses or moving prism?
This depends on the model? such as the Pentax 80 you said?
Advertising on the characteristics of each model tends to reflect this information?
The use of internal focusing lens is usually associated with models, more expensive and better quality?

Thanks again
wachi

Well, pretty much all modern birding scopes have an internal focussing mechanism and thus might have this problem. Exceptions are drawtube scopes or the few ones which are derived from astro scopes and have a classic focuser moving the eyepiece (Televue Pronto/Ranger).

Joachim

wachipilotes said:

Thanks Henry for his valuable information.
So how can we know if a spotting scope has internal focusing lenses or moving prism?
This depends on the model? such as the Pentax 80 you said?
Advertising on the characteristics of each model tends to reflect this information?
The use of internal focusing lens is usually associated with models, more expensive and better quality?

Thanks again
wachi

Click to expand...

Wachi,

I'm afraid the method of focusing is seldom specified. To be certain you have to examine the interior of the scope.

However, you can be fairly certain that any inexpensive spotting scope will probably use a moving prism. At the high end all the current scopes from Swarovski, Leica, Zeiss, Nikon and Kowa use focusing lenses. Meopta appears to be the last alpha brand to still use a moving prism.

I forgot to mention one other case were I strongly suspect, but have not actually measured, reduced aperture. That is the 100mm scope models from Celestron and others. My suspicion comes from the lower focal ratio of the 100mm versions in scope lines that use the same prism/eyepiece assemblies for 65mm, 80mm and 100mm models. If the prism apertures are "just large enough" for an 80mm f/6 objective, they would be about 10% too small for a 100mm f/5.4 objective, even at infinity focus. I don't know this for sure, but I would not be surprised to see the clear aperture of the 100mm Celestron Regal for instance come in around 90-92mm at infinity focus and reduce to 80mm or less at close focus. If true, that's probably not all bad since the aberrations of the full aperture of an inexpensive 100mm f/5.4 doublet are not likely to be pretty.

Henry

Size of the prism is also important. The bigger the prism the brighter the image but i dont know if its a relative measure depend of the objetive size or an absolue term. All the models comes with the same prism module no matter if they are 60 80 or 100. Its obvious that the bigger lens brings always the brighter view but maybe somehow could suffer a little bottle neck effect. I read somewhere the vortex razor comes with cemented prism and tjat degrades the view, but maybe cemented wasnt the cause or i dont understand the problem, what something "wrong" happen with the razor when compared with the alphas (thats what the subject was about)

Cosme,

The one Vortex Razor I tested was just a poorly made unit, not necessarily an inherently bad design. The prism arrangement was the conventional Porro used by all scopes with moving prism focusers, uncemented since one of the prisms needs to move independently of the other for focusing. In the angled version I tested that was followed by the usual semi-pentaprism for changing the viewing angle, also of course uncemented since it's a monoblock.

henry

Hi henry.

It wasnt this forum where i read about the vortex, thats for sure, but theres something about that vortex inherent desing that some expert dont like it.

However thats not the matter of this thread .... what can you tell us about the same size prims housing in a same model on their differents size
scopes?

Hi Cosme,

All I can tell you is what I see when I look at the pictures on the Vortex website. The 65mm and 85mm sure look like they use the same eyepiece and prism housing. Since I found the 85mm to have an unobstructed clear aperture of 85mm at infinity focus I would expect the 65mm to also have full aperture at infinity focus. Judging from its magnification range the 65mm appears to have a slightly higher focal ratio than the 85mm (f/5.66 vs f/5.41), so its front prism really ought have a little extra room outside the diameter of the objective light cone compared to the 85mm.

Henry

henry link said:

so its front prism really ought have a little extra room outside the diameter of the objective light cone compared to the 85mm.

Henry

Click to expand...

That mean the lens size advantage of lens size in brightness, even still very noticeable, of course, could be smaller that numbers say?