Extenders (doublers, triplers, etc) (5 Viewers)

[Ignatius]

If one uses an extender, such as a Swarovski ME 1.7x or a Zeiss Mono 3x12 on an optical device, what happens to the fov of that device?
The Zeiss SFL 8x40 has a fov of 140 m. Does this become 140/3 = 46.67 m when using the Zeiss tripler?
The Swaro BTX85 has a fov of 38 m. With the ME1.7x that would become 22.35 m - if my assumption is correct. Correct?

 

[PeterPS]

For a good extender the AFOV of the combination bino+extender is identical to that of the bino, thus the FOV of the combination can be approximated as in your post (the difference from the results obtained using the tangent formula is small, and even that formula is an approximation---the actual FOV must be measured).

 

[DRodrigues]

If one uses an extender, such as a Swarovski ME 1.7x or a Zeiss Mono 3x12 on an optical device, what happens to the fov of that device?
The Zeiss SFL 8x40 has a fov of 140 m. Does this become 140/3 = 46.67 m when using the Zeiss tripler?
The Swaro BTX85 has a fov of 38 m. With the ME1.7x that would become 22.35 m - if my assumption is correct. Correct?

You are right for the Swaro extender but not for the Zeiss monocular, since this has a reduced AFOV when compared to the AFOV of the binocular or spotting scope eps. So the resulting reduction of the TFOV will be higher than 3x. This would happen also on the use of other extenders that are applied after the ep - Swaro had one for binoculars (2x), and Opticron had a 2.5x version and presently produces a 2x, that can be used on binoculars and spotting scopes.

 

[Ignatius]

Thanks guys, but now I have two somewhat competing answers.
I can understand that there is a difference where in the optical train an extender is placed, and that the Swaro BTX/ME example is good.
But where does that leave the Zeiss tripler?
The Mono 3x12 has a fov of 180 m / 12.41° or ISO AFOV of 36.16°.
The SFL 8x40 has a fov of 140 m / 7.97° or ISO AFOV of 58.26°.
My calculation (dividing the SFL fov by 3) is roughly correct according to PeterPS, but not according to DRodrigues.

I do understand that for an accurate result it must be measured. However, I cannot imagine Zeiss engineers running around with measuring tapes, averaging out what three of them can see side to side when looking through a bino. There must be ways to do this mathematically. As a non-scientist these ways might be difficult to understand, but I'd nonetheless like to know.

 

[Tringa45]

There is a principal difference between the extenders offered for scopes such as those for Kowa, Leica and Swarovski and doublers or triplers for binoculars.
The former are negative (barlow) lenses placed between objective and eyepiece, which effectively increase the focal lengths of the objectives and should preserve the AFoV of the eyepieces with a corresponding reduction in TFoV.
Doublers and triplers are small telescopes in their own right and contain an erecting prism. With such small magnifications it is impossible to incorporate an adequately dimensioned prism, so they all have a very limited AFoV. I measured about 37° AFoV on my Zeiss 3x12 and IIRC it was very similar on a Vortex doubler I once owned. The doublers offered for Meopta B1s and old Swarovski SLCs would have had similar values.
The Zeiss 3x12 is the only one amongst these with independent focussing as it was designed primarily as a monocular.
Any monocular or binocular can be used as a booster, although magnifications much above 3x do not make a lot of sense.

John

 

[John A Roberts]

Swarovski used to offer a 2x booster (introduced in 1999 and discontinued around 2009).
It fitted the original EL series introduced in 1999, along with the original SLC series (excluding the 7x30 and 8x30 models).

Here's parts of two pages from a 2005 catalogue showing the effects on the various aspects of FOV (?):




For details of the booster in use on the original SLC 15x56, see Roger’s review: Swaro Booster Review

And in relation to being able to adapt it to other models, see Jan’s comments: How to make 2x Booster compatible with current SLC 15x56


John

 

[DRodrigues]

...
The Mono 3x12 has a fov of 180 m / 12.41° or ISO AFOV of 36.16°.
The SFL 8x40 has a fov of 140 m / 7.97° or ISO AFOV of 58.26°.
...

If you use a trigonometry calculator you will obtain tan 36.16º=0.73 and tan 58.26º=1.616, this means that a 58.26º AFOV ep has TFOV 1.616/0.73=2.21x wider than a 36.16º AFOV ep, with the same focal length. So the SFL 8x40 with the Zeiss mono 3x will have a FOV=140/3/2.21=21.12m

 

[Ignatius]

Excellent @DRodrigues . Now I know how to calculate that myself in future. Thank you very much for the leg-up.

 

[Tringa45]

If you use a trigonometry calculator you will obtain tan 36.16º=0.73 and tan 58.26º=1.616, this means that a 58.26º AFOV ep has TFOV 1.616/0.73=2.21x wider than a 36.16º AFOV ep, with the same focal length. So the SFL 8x40 with the Zeiss mono 3x will have a FOV=140/3/2.21=21.12m

The tangent error increases with increasing angles, so the ISO calculation for AFOV is based on twice the tangent of half the angle. It assumes that you would be closer to the object by a factor of the magnification of the binocular.
In the case of the 8x40 SFL you would effectively be viewing a 140 m FoV at 125 m, so the AFoV would be 2 arctan 70/125, i.e. 58,5°.
The TFoV would be 2 arctan 70/1000, i.e. 8° which would give 64° AFoV by the simple calculation.
The ISO AFoV of the Zeiss 3x12 Mono is 36,5°.
How, btw, did you arrive at figures such as 36,16° and 58,26°?
AFoV can be measured to about 0,5° but any calculations involve errors due to distortion or magnification tolerances.

John

 

[Ignatius]

He got those figures from me. And I got the FOV in metres/degrees from the manufacturer's literature. The rest I got from this site, where mere mortals like me who cannot do simple calculations go to get such figures. Anyway, according to my calculator your 'better' figures give the same result: 21.12 m.

 

[Tringa45]

Quoting AFoV values to two decimal places is "Erbsenzählen" considering that in the case of the 8x40 SFL there is a 5,5° difference between the ISO and simple methods and that the real value lies somewhere in between.

 

[Ignatius]

Jawohl oh Meister.

 

[Tringa45]

Your thread requesting explanations, so if you don't understand, sarcasm and sour grapes are out of place.

 

[Ignatius]

Your thread requesting explanations, so if you don't understand, sarcasm and sour grapes are out of place.

Several people bothered to give answers, some better than others. Along comes you, evidently without following the thread or you'd have read where DRodrigues' figures come from, then you proceed to split hairs, give your own, of course much better or more correct figures, and guess what: those lead to the same result. To top it off you arrogantly point out, that giving a fov to two decimal places is a beancounting accuracy that is unwarranted.

When this haughty attitude is made a little fun of you go into a huff.
In future I'd prefer to not receive your kind of arrogant lecturing to my questions. In fact ... where is that button again?

 

[Tringa45]

Who' splitting hairs and who's in a huff?
I did not reach the same result. Perhaps you didn't notice, but I pointed out that taking the tangent of a complete AFoV (as per DRodruiges) makes absolutely no sense at all. Simple trigonometry is not rocket science

 

[Hermann]

As an aside: Henry's thread about the Zeiss 3x12 is IMO essential reading for anyone interested in that nice little device: Fun with a Zeiss Tripler

Hermann