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Started by 1953, 07/11/2008 10:26AM
Posted 07/11/2008 10:26AM Opening Post
Ok, my questions for the day:

1.Just how accurate are the focal lengths that are stamped on the side of eyepieces? Is 25MM really 25MM?

2. How can I measure the true value of the eyepiece's focal length?



"Don't sweat the petty things and don't pet the sweaty things" 8O
Posted 07/11/2008 11:11AM #1
Accuracy varies. I know that there have been cases where different distributors of the same eyepiece sets from the same manufacturer have offered slightly different focal lengths. I suspect that sometimes manufacturers will round off the number (maybe because it fits nicer when etched on the eyepiece.) Other times, the focal length stated just seems wrong. I have a 17mm Plossl that seems to give more magnification than a 15mm Erfle that I have.

You can measure the focal length directly but it is difficult to do accurately. See this post.

Posted 07/11/2008 04:42PM #2

Dan's link shows the accurate way. I have an quick, rough and dirty way too

My experience has also been that it can vary. The ones that are often very different to advertised are those with a built in barlow. I think that in this case it's not that the design or manufacture is inaccurate, it's that how far down your optical chain it will sit at focus depends on the focal length of your scope. So what might be a 5mm in a 1000mm scope might be a 5.3mm in a 300mm scope.

I've often wondered what the experience of binoviewing people was on variable FL. The barlow issue wouldn't matter as it would be the same for both paths but presumably a 5% variation in the native FL would be a really big deal in that situation?

all the best