Ep's like pin holes ... Why?

[autonm]

Can anyone explain why when I love through my say 25mm or say 10mm I get quite a good field of view.

However when i look through my 5mm it's as if I am looking through a pinhole and really have to squint my eye and hold things directly on to see anything.

Im sure there is a simple answer.

Or do I just need to buy a 5mm with a better fov?

Thanks.

[knobby]

Don't know the techie answer but its how the plossl design works.

If you move away from plossls to a multi element design then the glass is wider... And generally more expensive.

BST explorers are a good well priced alternative at about £50.

Hope that makes sense, someone will explain it better I'm sure

[Zaphod360]

Could be to do with exit pupil size. Take the focal length of the eyepiece and divide it by the focal ratio of your scope. So a 25mm ep in an f5 scope would give you a 5mm exit pupil which is a nice bright comfortable size. A 5mm ep would give you a 1mm exit pupil which to be honest should be fine, it's when you get down to 0.5mm I start having issues.

I have personally found that some brands of eyepieces (at specific focal lengths) need more precise eye positioning to see, but never really understood why.

[andrew63]

I'm not sure what 5mm lens you use - but a plossl or orthoscopic will have a 'small' top lens to look through. Something like a 5mm BST Explorer as mentioned will have a much larger eye lens to view with and hence be more comfortable to use - more like looking through your 25mm and not like looking down a barrel !

andrew

[Matt Scunthorpe]

This is the reason i moved to Celestron xcels which have really nice eye relief. I have a 7.5mm SW plossi, which i only use on planets anyway, but its like smashing your eye against the glass to see anything.

[autonm]

can anyone give me a link to a 5mm MST Explorer (UK) - so I can check the price?

Did a quick search on google, but returned plenty of posts in forums no stores.

thanks

[cs1cjc]

Link: http://www.skysthelimit.org.uk/telescope%20eyepieces.html

[garethmob]

heres skys the limits website to eye peices, http://www.skysthelimit.org.uk/telescope%20eyepieces.html

and their ebay store (im directly linking the 5mm eye peice too) http://www.ebay.co.uk/itm/1-25-5mm-BST-Explorer-Dual-ED-eyepiece-Branded-Starguider-/380547862433?pt=UK_Telescope_Eyepieces&hash=item589a6b8fa1

hope it helps

[autonm]

appreciate it thanks.