It was asked on the thread Zeiss 6x30 IF whether a small 3x prismatic binocular was better than a 3x opera glass.
I don't have a small 3x prismatic binocular, although I do have 3.5x, 4x, 4.4x and 5x larger modern prismatic binoculars.
These larger binoculars are too large to consider here.
The 6x15 Teletur has a field of about 8 degrees.
The well-made French AFSA 2.2x-3x26 opera glass is about the same size as the Teletur but lighter.
The AFSA is made of aluminium or a similar alloy. I think the Teletur is brass.
The AFSA focuses, which is why the magnification for me varies from 2.2x to 3x.
Because the opera glass is so short the focus range is large compared to the rest length of the opera glass.
There is no dioptre correction, but I unscrewed one front tube and wrapped a long rubber band around the open joint.
When unscrewing the front tube collimation may be lost as the tube wiggles, but the tight rubber band restores alignment.
This gave me my dioptre correction.
There is no hinge, just a fixed curved bridge.
The field of the AFSA is about 8 degrees and if glasses are worn only about 5 degrees.
This is the problem with Galilean optics. Tiny fields.
But the image is bright.
I suppose the transmission of the Teletur when new was about 60%, the AFSA 80%.
Both may be 1920s, although I am not sure.
In poor light the AFSA has a much brighter image than the Teletur.
But a 3x15 small prismatic would have a 5mm exit pupil and should be quite bright.
The field might be 15 degrees.
There is an interesting problem with the AFSA.
Because the magnification changes a lot as the separation of objective and eyepiece changes, providing a 1 dioptre correction changes the view by apparently a 2 dioptre correction.
This makes it difficult for the brain to make a good image without strain.
So, a 3x prismatic probably has advantages over a similar size opera glass, at greater complexity and cost.
A modern 2.5x or 3x coated opera glass with about 90% transmission can be very useful for its intended purpose.
Opera or theatre.
Regards,
B.
I don't have a small 3x prismatic binocular, although I do have 3.5x, 4x, 4.4x and 5x larger modern prismatic binoculars.
These larger binoculars are too large to consider here.
The 6x15 Teletur has a field of about 8 degrees.
The well-made French AFSA 2.2x-3x26 opera glass is about the same size as the Teletur but lighter.
The AFSA is made of aluminium or a similar alloy. I think the Teletur is brass.
The AFSA focuses, which is why the magnification for me varies from 2.2x to 3x.
Because the opera glass is so short the focus range is large compared to the rest length of the opera glass.
There is no dioptre correction, but I unscrewed one front tube and wrapped a long rubber band around the open joint.
When unscrewing the front tube collimation may be lost as the tube wiggles, but the tight rubber band restores alignment.
This gave me my dioptre correction.
There is no hinge, just a fixed curved bridge.
The field of the AFSA is about 8 degrees and if glasses are worn only about 5 degrees.
This is the problem with Galilean optics. Tiny fields.
But the image is bright.
I suppose the transmission of the Teletur when new was about 60%, the AFSA 80%.
Both may be 1920s, although I am not sure.
In poor light the AFSA has a much brighter image than the Teletur.
But a 3x15 small prismatic would have a 5mm exit pupil and should be quite bright.
The field might be 15 degrees.
There is an interesting problem with the AFSA.
Because the magnification changes a lot as the separation of objective and eyepiece changes, providing a 1 dioptre correction changes the view by apparently a 2 dioptre correction.
This makes it difficult for the brain to make a good image without strain.
So, a 3x prismatic probably has advantages over a similar size opera glass, at greater complexity and cost.
A modern 2.5x or 3x coated opera glass with about 90% transmission can be very useful for its intended purpose.
Opera or theatre.
Regards,
B.