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Thread: Eye relief

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    Default Eye relief



    Happy Father's Day! I have this Zhumell 9 mm Plossl ep that came with my Z8. It does a good job; however, it has virtually no eye relief. I have to almost touch my eye to the ep to get a full view. I read online that a barlow would help extend the eye relief but my GSO 2x barlow doesn't seem to help. Is there anything else I can do to increase eye relief for this ep? Thanks.
    Zhumell Z8 Dobsonian, Pentax SP 12 x 50 WP Binos

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    Default

    Not really. With scaled eyepiece designs like plossls, everything scales down - including the eye relief - as you go to shorter focal lengths - so an 18mm plossl will have twice the eye relief of a 9mm.

    (Or to look at it another way, if you get the same magnification by using a barlowed 18mm instead of a 9mm plossl, you have twice the eye relief to start off with.)

    You may want to look at some of the premium design eyepieces - these are often non-scaled designs that manage to keep a long eye relief (and a large eye lens) even at shorter focal lengths (often by incorporating a Barlow like element in the design).

    Up to an AFOV of around 68° or so, many ranges manage a constant 20mm eye relief at all focal lengths. The ultrawide 80° plus eyepieces often tend to have less to keep the size down a bit.

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    Default Re: Eye relief

    Quote Originally Posted by jerryTheC View Post
    Not really. With scaled eyepiece designs like plossls, everything scales down - including the eye relief - as you go to shorter focal lengths - so an 18mm plossl will have twice the eye relief of a 9mm.

    (Or to look at it another way, if you get the same magnification by using a barlowed 18mm instead of a 9mm plossl, you have twice the eye relief to start off with.)

    You may want to look at some of the premium design eyepieces - these are often non-scaled designs that manage to keep a long eye relief (and a large eye lens) even at shorter focal lengths (often by incorporating a Barlow like element in the design).

    Up to an AFOV of around 68° or so, many ranges manage a constant 20mm eye relief at all focal lengths. The ultrawide 80° plus eyepieces often tend to have less to keep the size down a bit.
    Good info, thanks. I think my next ep will be a 2 in 15 mm to 12 mm with wide field. These type of eps seem to work well with my Dob.
    Zhumell Z8 Dobsonian, Pentax SP 12 x 50 WP Binos

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    Default Re: Eye relief

    The barlow won't help with the 9mm. What the barlow does is allow you to use an 18mm instead of the 9mm, with the same magnification and field of view as the 9mm, but with the eye relief of the 18mm.
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    Default Re: Eye relief

    A barlow did boost my 32 plossl's ER by about 50% making it unsuitably long without eyecup assistance .... but for your 9mm plossl even with 50% more it remains less than 10mm eye relief

 

 

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