Barlow changes to eye relief?

[Bugdozer]

Does anyone know if using a Barlow lens can change the eye relief for a given eyepiece, or affect the shape of the exit pupil plane? 

[John]

A barlow does extend eye relief. The amount depends on the focal length of the barlow lens I believe. The focal point will usually change as well. Not sure about the shape of the exit pupil plane 🤔

[Mr Spock]

If you don't want a change in eye relief you are better of with a tele-extender such as a Powermate.

[Louis D]

I've found Barlows can also create SAEP (kidney-beaning) where none existed before.

[Second Time Around]

2 hours ago, Louis D said:

I've found Barlows can also create SAEP (kidney-beaning) where none existed before.

That's interesting, Louis. You learn something new every day.  Can you give any specific examples please?

[Bugdozer]

3 hours ago, Louis D said:

I've found Barlows can also create SAEP (kidney-beaning) where none existed before.

That’s what I was wondering. I think I have experienced this using my Celestron 25mm Plossl with the Celestron 2x Barlow - weirdly enough both items being supplied with the telescope, so you would expect good compatibility, but it could be that I just didn't adjust my eye position properly to account for the increased eye relief produced by the Barlow. It's a combination I rarely use, I prefer to just switch to a higher power eyepiece. 

[Louis D]

1 hour ago, Second Time Around said:

That's interesting, Louis. You learn something new every day.  Can you give any specific examples please?

IIRC, it happens with simple positive-only eyepieces like Plossls and shorty Barlows with rapidly diverging light cones.  I think it's what is causing SAEP in the 6mm and 9mm 66 degree "UWA" eyepieces as well.  Eyepieces with a well designed Smyth lens group tend to tame down the incoming diverging light rays pretty well.

[Bugdozer]

What is a shorty Barlow? 

[Louis D]

A shorty Barlow tends to be any Barlow that seats all the way into a diagonal.  They have deeper curves in the lens group to diverge the light rays in a shorter distance to get to the same magnification factor that traditional long Barlows get to over a longer distance.

Here are some images I found illustrating the difference.

Shorty Barlows next to a PowerMate:

Long Barlows next to a PowerMate:

Edited January 3 by Louis D