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Why 26mm - who decided?


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Why do eyepiece manufacturers make 26mm eyepieces... i mean specifically 26mm - rather than 25mm or 20mm or 27mm

Why 9.7mm instead of just 10mm?

They seem 'odd' numbers to pick.

What's the original reason for it? - Is 26mm a conversion from 1.1 inches. Is it just what we've always made? 

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50 minutes ago, Mark Foster said:

Why do eyepiece manufacturers make 26mm eyepieces... i mean specifically 26mm - rather than 25mm or 20mm or 27mm

Why 9.7mm instead of just 10mm?

They seem 'odd' numbers to pick.

What's the original reason for it? - Is 26mm a conversion from 1.1 inches. Is it just what we've always made? 

There are 25mm eyepieces, 20mm eyepieces, 27mm eyepieces (and 19mm, 21mm, 22mm, 23mm, 24mm, and 28mm eyepieces).  I've never seen a 29mm eyepiece, though.  I guess it's time to introduce one to be the first to market.

The 9.7mm may just be an honest measurement of the focal length instead of rounding it to the nearest integer.  I have a 5.2mm Pentax XL that became a 5mm when they introduced the XW line.  The progression was 5.2mm, 7mm, 10.5mm, 14mm, 21mm, 28mm, and 40mm.  Except for the 40mm, they were all progressions of the square root of two.  For the XW line, to make folks like you happy, they changed to 3.5mm, 5mm, 7mm, 10mm, 14mm, 20mm, 30mm, and 40mm.  I recall the Speers-Waler line having lots of odd-ball focal lengths as well.  No logical progression that I could discern, though.

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