Impact of TSN-EX16 on FOV? (1 Viewer)

[Gzoladz74]

Morning!

I am trying to find out of there is any impact in the FOV with and without the extender at equal magnification...for example if I set the Prominar 88 at 40x without the extender and I compare the FOV to that with the same magnification but with the extender installed...is there a reduction? If so, how much?

I have read that light loss is estimated at 5%, but I found no reference to FOV impact.

Thanks
G

 

[Grando]

Morning!

I am trying to find out of there is any impact in the FOV with and without the extender at equal magnification...for example if I set the Prominar 88 at 40x without the extender and I compare the FOV to that with the same magnification but with the extender installed...is there a reduction? If so, how much?

I have read that light loss is estimated at 5%, but I found no reference to FOV impact.

Thanks
G

I think it will just be the associated loss of FOV that comes with greater magnification. So it will be proportional, I would think. So, if the FOV with the Kowa zoom eyepiece at 40x is roughly 33.87m then at 64x (40x + the 1.6 extender) I'd expect it to be 33.87/1.6 = 21.17m.

(I have the TSN EX-16S converter for the smaller Kowa scopes and I don't think I'm seeing a smaller FOV when I use it; it's very good, I'd say, so would expect the converter on the 883 to work even better given the scope's larger objective.)

 

[Gzoladz74]

Thanks! I hope that's the case.

 

[dogbreath]

I wonder what the FOV would be with the eyepiece at 25x plus the 1.6 extender, so the magnification of the combined setup is 40x?

 

[Gzoladz74]

I wonder what the FOV would be with the eyepiece at 25x plus the 1.6 extender, so the magnification of the combined setup is 40x?

That was actually my question, but from the previous poster I understand it should be the same (40x with and without extender).

 

[henry link]

Morning!

I am trying to find out of there is any impact in the FOV with and without the extender at equal magnification...for example if I set the Prominar 88 at 40x without the extender and I compare the FOV to that with the same magnification but with the extender installed...is there a reduction? If so, how much?

I have read that light loss is estimated at 5%, but I found no reference to FOV impact.

Thanks
G

The extender is basically a Barlow lens placed in front of the eyepiece field stop, so it has no effect on the apparent size of the field stop as viewed from the eye behind the field stop. The apparent FOV follows the same curve from about 60º at the lowest magnification to about 80º at the highest magnification with or without the extender. This means that there is a loss of real FOV at 40x when the zoom eyepiece is set to 25x and combined with the 1.6x extender compared to setting the eyepiece directly to 40x without the extender because the AFOV with the eyepiece set to 25x is about 60º while the AFOV is about 70º when set directly to 40x without the extender. So, the real FOVs at 40x are about 1.75º without the extender and about 1.50º with it.

 

[Gzoladz74]

The extender is basically a Barlow lens placed in front of the eyepiece field stop, so it has no effect on the apparent size of the field stop as viewed from the eye behind the field stop. The apparent FOV follows the same curve from about 60º at the lowest magnification to about 80º at the highest magnification with or without the extender. This means that there is a loss of real FOV at 40x when the zoom eyepiece is set to 25x and combined with the 1.6x extender compared to setting the eyepiece directly to 40x without the extender because the AFOV with the eyepiece set to 25x is about 60º while the AFOV is about 70º when set directly to 40x without the extender. So, the real FOVs at 40x are about 1.75º without the extender and about 1.50º with it.

Thanks Henry, I had to read it 2 or 3 times but I think I got it?

-Real FOV gets wider as magnification increases, hence
-Real FOV at 40x without the extender > Real FOV at 25x without the extender = Real FOV at 40x (25x x 1.6) with the extender.

 

[Binastro]

AFOV gets wider as magnification increases.

Regards,
B.

 

[henry link]

Thanks Henry, I had to read it 2 or 3 times but I think I got it?

-Real FOV gets wider as magnification increases, hence
-Real FOV at 40x without the extender > Real FOV at 25x without the extender = Real FOV at 40x (25x x 1.6) with the extender.

Looks like my explanation didn't quite succeed. Real FOV does not get wider as magnification increases. In the Kowa zoom and in most other zoom eyepieces it's the apparent FOV that increases as magnification increases, but that increase is much too small to make up for the loss of real FOV as the magnification increases.

To find the real FOV in degrees at 40x without the extender simply divide the apparent FOV at 40x by the magnification (70º/40 = 1.75º.) To find the real FOV at 40x with the extender (25x x 1.6x =40x) divide the apparent FOV at 25x by the final 40x magnification (60º/40 = 1.50º.)

 

[Gzoladz74]

Noted....

I think the answer to my opening question is: FOV @ 40x (without extender) > FOV @25x1.6 = 40x (with extender).

 

[Gzoladz74]

I had the opportunity to visually test this...and to confirm, the FOV @ 1000m is about 25%/30% narrower with the extender vs without the extender at the same magnification.

Views with the extender are still impressive and immersive.