How Does A Barlow Increase Eye Relief?

[Refractordude]

Example

If I barlow a 10mm eyepiece with an eye relief of 8mm, how much more eye relief will I get?

Thanks to you all

[notFritzArgelander]

This is NOT an easy question to answer since it depends of the designs of the Barlow lens and the eyepiece involved.

Only a design like the TeleVue Powermates will preserve the placement of the exit pupil and therefore the eye relief is my understanding. So if you're using an eyepiece that has a native 8mm eye relief that should be what you get (approximately) with all of the Powermates.

With a classic Barlow you get a little more eye relief maybe 9-11mm. Maybe more. It depends on details.

I've not desired to go into the weeds on this. From a practical POV with a Powermate I put my eye where I usually put it. With a classic Barlow I know I'll get more eye relief. But as a practical matter for an 8mm eye relief ocular I'd just start at 8mm and back off.

On CN there was a post on this topic with formulas specific to various Barlows. FWIW it's here:
https://www.cloudynights.com/topic/4857 ... ye-relief/

I'm not going to bother since I regard this as lily gilding. I prefer to assume the eye relief is roughly the same and go from there.

[Gfamily]

I'm reading the text in the attachment as saying that you 'increase' the eye relief because...
A 20mm eyepiece will have a longer eye relief than a 10mm eyepiece (typically at least)
A 20mm eyepiece used with a barlow will have a longer eye relief than a 10mm eyepiece without one (ditto)
so using a 20mm eyepiece with a barlow will give the same magnification as a 10mm, but with more eye relief (usual caveats apply!)

I was not aware that barlowing an eyepiece would change its eye relief - so thanks nFA, for the info.

[Lady Fraktor]

I have found that using a barlow if it does increase eye relief it is a slight amount as nFA has posted above.

Some people find it an advantage to use a barlow with eyepieces under around 10mm as the eye relief starts getting shorter so using a barlowed 20mm as a 10mm with the 20mm eye relief.

[Refractordude]

I have found that using a barlow if it does increase eye relief it is a slight amount as nFA has posted above.

Some people find it an advantage to use a barlow with eyepieces under around 10mm as the eye relief starts getting shorter so using a barlowed 20mm as a 10mm with the 20mm eye relief.

Yep, my 10mm is a barlowed 20mm.