Eye relief: nominal vs actual

[YKSE]

Eye relief (ER) is a recurrent topic here, not least by glass-wearers, many eyepieces do provide nominal ER, assuming the numbers are correct (not always the case), there're still other factors affecting the actual available ERs, such as recessed or concave eyelens etc, since ER is defined from eyelens center to eye ploint.

There's an interesting aspect brought up in the other site, which might affecting the ER positively

https://www.cloudynights.com/topic/561801-bargain-widefield-ep-performance-in-a-flatfield-scope/?p=7624593

Here's the illustration picture:

The point is: when observer's eye pupil is larger than the exit pupil produced, he can actual move his eye further away from exit pupil position and still see the whole FOV.

I've made a couple of graphs to see its effect, the first one assuming dark-adapted eye pupil of 5mm, and eyepieces have 3mm recessed eyelens,

It shows that an ortho (42° AFOV) with 4mm nominal ER (red line) behaves like having 10mm ER, despite 3mm recessed eyelens, that's about the same ER with a plossl (52°AFOV) of 6mm nominal ER, in another words, 5mm ortho is about as comfortable as an 8mm plossl.

Second graph is with not well-dark-adapted eye, i.e. 3mm eye pupil, everthing else the same,

The same ortho has only 7.5mm available ER now, compare to 10mm when pupil was wider,and the plossl drops to 8mm.

Whats' your experience?

 

[martinl]

Very interesting. 

Your plots pretty much match my experience. My 50 degree, 10mm ER, Nagler zoom seems to offer just a touch less usable ER than my 100 degree, 15mm ER Ethoses, for example - just as your calculations predict. Orthos are definitely easier to look through than Plössls and wider designs at the same stated ER. 

Perhaps the minimum-glass, narrow field, aficionados are on to something after all...

[jetstream]

I find that using my shorter orthos-9mm,7mm,5mm- when fully dark adapted (faint galaxies) not very difficult to use. It does seem on bright targets such as the moon or Jupiter that the eyerelief gets "closer" if this is possible.

I wonder what our eye pupil diameter really is on these bright targets?

VG thread Yong...

[25585]

https://en.m.wikipedia.org/wiki/Eye_relief

[YKSE]

2 hours ago, 25585 said:

https://en.m.wikipedia.org/wiki/Eye_relief

I'm glad you find Wiki as a good resource to get a first insight of questions, I certainly hope more should do it.

TWe can,  however, find more insight to any question deeper than Wiki can provide. I'm under impression that you have many posts about eye reliefs, what your experience of your perception of ER depending how dark-adapted your eye is?

[25585]

On 17/01/2018 at 22:26, YKSE said:

I'm glad you find Wiki as a good resource to get a first insight of questions, I certainly hope more should do it.

TWe can,  however, find more insight to any question deeper than Wiki can provide. I'm under impression that you have many posts about eye reliefs, what your experience of your perception of ER depending how dark-adapted your eye is?

Sorry, what/who is TWe?

My experience is from use. I wear spectacles, have to. My Back Vertex Distance uses up anout 10mm. An eye cup &/or recessed eye lens &/or downward curved eye lens can also use up eye relief making the Effective Eye Relief much less, which affects how much TFOV I see, how much light is lost and how happy I am with what that leaves me. 

Larger exit pupils giving more light can therefore allow more to reach my eyeball, but may be too much for those fortunate enough to have better optics, both human and manufactured.

 

[YKSE]

Sorry for the typo. it's my thick fingers, it should be "We".

I'm not sure I've expressed the message clear, let me try to put it in another way

With the same eyepiece in a scope, if you just go out from a well-lighted house/room, your eye pupil will be about 2-3mm, looking through  the eyepiece, you may feel eye relief short or just right; after you stay out and avoid looking at direct light for, say half an hour, your eye pupil will get quite wider, presumably near your max eye pupil, looking through the eyepiece again, you're likely to feel the ER is longer than earlier. This is  the content of original post.

[25585]

49 minutes ago, YKSE said:

Sorry for the typo. it's my thick fingers, it should be "We".

I'm not sure I've expressed the message clear, let me try to put it in another way

With the same eyepiece in a scope, if you just go out from a well-lighted house/room, your eye pupil will be about 2-3mm, looking through  the eyepiece, you may feel eye relief short or just right; after you stay out and avoid looking at direct light for, say half an hour, your eye pupil will get quite wider, presumably near your max eye pupil, looking through the eyepiece again, you're likely to feel the ER is longer than earlier. This is  the content of original post.

I have found what you say is true with binoculars for sure, but not certain about eye pieces. Binoculars get used most as day and night suitable, so the difference is much more noticable - moreso with roof prism than porro types.

Longer eye relief and less critical eye placement when in one-eyed viewing at night is quite different and more crucial to ease of use than in daytime telescope use, whatever the telescope type or size, as only vision and a sense of orientation can help at night.

[jetstream]

1 hour ago, 25585 said:

I have found what you say is true with binoculars for sure, but not certain about eye pieces. Binoculars get used most as day and night suitable, so the difference is much more noticable - moreso with roof prism than porro types.

Longer eye relief and less critical eye placement when in one-eyed viewing at night is quite different and more crucial to ease of use than in daytime telescope use, whatever the telescope type or size, as only vision and a sense of orientation can help at night.

Do you notice a difference in eyerelief with short fl plossls or orthos on the moon vs viewing DSO with them (dark adapted)?

[25585]

7 minutes ago, jetstream said:

Do you notice a difference in eyerelief with short fl plossls or orthos on the moon vs viewing DSO with them (dark adapted)?

I don't use those types, opting for wider FOV. Vixen LVW or Pentax XW, each have an advertised 20mm eye relief. I have them for 10mm and smaller FL.