Telescope Reviews: True ( SHARP ) field of view

I've been using the measure of resolution of double stars at accurately measured points along the axis. Each separation of a pair gives me a reading of the limit of resolution at that point on the axis. Positioning is determined by placing some other field star at exactly the edge of the field stop. Measurements are taken from very large scale atlases to determine the position of the test double, by using the placement of the field star at the field edge to measure from.

Collecting the data is not so simple as it may seem. For any given binocular 4, 5 or 6 different doubles must be viewed at various points along the axis to determine the position at which it can still be seen clearly, where it can be detected and finally where it can no longer be detected as double. So for any given binocular there might be 10 or 20 or more readings.

Keep in mind it is not the separation of the double alone that makes the reading, it is the separation times the magnification, or the apparent separation. For instance lets say a 10x60 binocular can clearly see a 36" double at 85% out from center. That's 360 arcseconds apparent at 85% out. In order for a 12x binocular to equal that performance, it needs to clearly see a 30" double at the same position and a 16x binocular needs to resolve a 22.5" double. A 20x binocular would need to see an 18" double.

I believe this method takes out all subjectiveness from reporting these values and gives a fairly accurate comparison of one binocular to another. Over time I have built a file with hundreds of readings. This is how I compare sharpness across the field from one binocular to another.

John asked, "Why refer to Tfov rather than Afov?"

Well personally, I don't give much attention to what the Afov of a binocular is. With the exception of interchangeable ep models, no binocular is magnification independant. In the field, the most important thing to me is the Tfov. So I take all my measurements in reference to the Tfov. However, if you prefer to read sharpness in reference to Afov, this is a very simple matter of flipping the calculation to get the value you want.

A number of wide field binoculars gain nothing by employing wide field eyepieces. The Nikon Action Extreme line uses epieces with Afov 62-64°. However the field of view at 60% out falls off and by 70% out gets rather poor. By 75% out it is unusable. The Burgess LW 20x80 uses Afov = 74° eyepieces, one of the widest I know of, but not to very good advantage since this binocular looses sharp image beyond 60% out.

I suppose it may be true that most narrow Afov binoculars display better sharpness to the outer edges, for example the various Pentax models with Afov ranging from 45° to 50° have edge sharpnees nearly unrivaled. The Oberwerk Mariner 10x60 has Afov= 51° and at 85% out it beats the Fujinon and Nikon SE. But that is not always the case.

The Oberwerk 8x56 has Afov= 48°, as narrow as the Pentax, but image sharpness while still good at 65% out, falls off in the outer 20-30%. Orion Vista 8x42 and Orion Giant 16x80 have 52° and 53° Afov and both are rather poor in the outer 30%-40% of the field. Orion Minigiant 15x63 has Afov 57°, but is poor in the outer 20% of field. Celestron Skymaster 25x100 has a 60° Afov and is poor at 60% out and extremely poor at 80% out.

Fujinon 16x70 have a 65° Afov, but only degrade in the outer 20%. Likewise Oberwerk 15x70 have Afov = 64° and degrade only across the out 30%. Nikon SE 12x50 has Afov =61° and has far sharper images at the edge than the Fujinon.

edz

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