Hello every one.
This is my first post here, but I read a lot on the forum.
I have a short question concerning the apm / lunt 5mm 110 eyepiece. I read several time on this forum that this eyepiece has actually a focal length of 4.7 mm (or even 4.77 mm). I searched on the internet but never find any source / specification that confirm this.
Now, I don't think that this really matters as I would probably not see any difference between 4.7 and 5mm. But I am still wondering if it is a 5 mm or a 4.7 mm eyepiece.
Do someone know which focal length the eyepieces actually has?
Unless they changed the optical configuration, it's probably 4.77mm.
For what it's worth, marketing departments will often round off to make it simpler to advertise and sell the eyepiece.
For example, the old pseudo Masuyama eyepiece sold in the early '90s as a 3.8mm eyepiece was really 3.75mm and they simply rounded off.
Today, most sellers would call a 4.77mm eyepiece a 4.8mm.
But, perhaps that was the design spec and the production came out closer to 5mm. Had it been 4.95mm or 5.05mm, I could see selling it as a 5mm.
Production specs are often a little different than design specs, but very rarely more than 0.1mm different.
So is it 4.77mm or 5mm? It's fair to consider it a 5mm eyepiece until a lab test shows something different.
I compared it to a 4.7mm TeleVue Ethos SX and could see no difference in magnification, but then, the difference would be between 383x and 365x in my scope and I couldn't reliably see a difference of less than 5% at that magnification.
And who's to say the Ethos SX is exactly 4.7mm?
I'll agree with Dick--ask Markus Ludes about it. I'd bet he knows the complete history of the eyepiece.