Eyepiece physical sizes

[trynda1701]

Updating my eyepieces from my old Kellners of pre-1990 vintage, I've noticed something about eyepieces. When I got my Celestron C8, and used the few Kellners from my home made 150mm reflector, eyepieces were tall at long focal lengths, and got shorter as the focal length got shorter, so 40mm = tall, 9mm is shorter. My even older 0.965 eyepieces followed the same trend.

But I've noticed that some modern ranges seem to grow in physical height as you go to the shorter focal lengths, like shown here with the Baader Planetarium 'Hyperion 68' or the Celestron 'X-Cel LX' ranges shown on the FLO site...

http://www.firstlightoptics.com/baader-planetarium/baader-hyperion-68-degree-eyepiece.html

http://www.firstlightoptics.com/celestron-eyepieces/celestron-x-cel-lx-eyepiece.html

What's the technical reason for this? I know modern eyepieces like Plossls have more elements than the Kellners, but why the 'reverse' in size compared to focal length in some ranges?

Mark

[Naplesnut]

Eye relief may be a factor and also orthos, erfles, naglers etc have more elements/groups than plossls, but I am no expert so take this with a pinch of salt (mods please note) 

[ronin]

I would guess it is the requirement for wider views so what you end up with is a section to collect over a large angle then a section to perform the normal eyepiece action.

If you look at the general 3 and 4 element eyepieces they all seem to have a FoV of around 50 degrees so for 60, 70, 80, 100 I presume an additional set of lens for the wider collection. I know there are plossls then I think Meade make a plossl variant that has wider field but it is a 5 element construction.

[RikM]

Most modern eyepieces have Smythe lens elements. Like a Barlow built into the barrel. To gain magnification (reduce the focal length) you move the 'eyepiece' elements further away from the 'Barlow' hence the total eyepiece is longer at shorter focal lengths.

[John]

More and sometimes larger glass elements are required to deliver the longer eyerelief, wider field of view and to be able to handle todays faster scopes without undue distortions. Additional sets of glass elements are often used in the eyepiece barrel to give short focal lengths while retaining these characteristics. The spacing between these lower lenses and the upper set is spaced further apart to get even shorter focal lengths which is why a 5mm eyepiece, for example, is often longer than a 10mm one in the same range.