Extra Flat 27mm or Starguider ED 25mm?

[Thalestris24]

Hi all

I don't think I've ever posted in this forum before! I was wondering about any comparisons between the above two EPs? I'm actually thinking more in terms of daytime / bright light use  but maybe astro also. I have some experience of a BST Starguider ED 25mm and it is excellent. But the EF 28mm apparently has an extra 3mm eye relief which might make it a tad more comfortable for me (with my glasses on). All the blurb on the EF 27 says it has  'excellent colour rendition' - not sure what that actually means, but I wonder if it's as good as the ED 25mm and welcome input from anyone who has used one .

Many thanks

Louise

[Don Pensack]

Bear in mind there is a difference in apparent field, the 27mm being 53° and the 25mm being 60°.

[Thalestris24]

1 hour ago, Don Pensack said:

Bear in mind there is a difference in apparent field, the 27mm being 53° and the 25mm being 60°.

Hi thanks, I do know that. What I don't know, and would like to find out, is whether there is any difference in quality - sharpness, colour correction, contrast.

Cheers

Louise

[Louis D]

Here's a review of the Smart Astronomy 27mm EF Eyepiece which I believe is the same eyepiece.  Not quite as good as the 28mm Pentax XL, but not bad at all.

[JeremyS]

3 minutes ago, Louis D said:

Here's a review of the Smart Astronomy 27mm EF Eyepiece which I believe is the same eyepiece.  Not quite as good as the 28mm Pentax XL, but not bad at all.

Interesting you mention the XL 28. Not often cited on here. Is it a favourite of yours?

[Thalestris24]

1 hour ago, Louis D said:

Here's a review of the Smart Astronomy 27mm EF Eyepiece which I believe is the same eyepiece.  Not quite as good as the 28mm Pentax XL, but not bad at all.

Thanks for the link. Shame they didn't give a little more performance detail or post some daytime phone photos (Perhaps I ask, or hope, for too much!).   Ironic there is an advert for the Paradigms/Starguiders on the same page... Anyone on here by any  chance actually have an EF 27 ?  I'm particularly interested in the actual eye relief, and any apparent colour aberrations (though it doesn't sound like there are any).  People always say things like you can't expect the performance of a 'low end' eyepiece (or whatever) to match a high end one, though they rarely describe the practical differences!  

Thanks

Louise

ps can anyone confirm if the bottom section (the 1.25" coupling) unscrews? I assume it does - but...

Edited February 21, 2021 by Thalestris24

[vlaiv]

31 minutes ago, Thalestris24 said:

People always say things like you can't expect the performance of a 'low end' eyepiece (or whatever) to match a high end one, though they rarely describe the practical differences!

That highly depends on telescope you'll be using them in.

In F/5 scope - you'll clearly see the difference between high and low end eyepiece. In F/12 - there will hardly be any difference if at all.

Outer field performance is usually compared. Most eyepieces have quite decent performance in the center, but outer part of the field starts to show aberrations like astigmatism and such - which blurs things in daytime use (and distorts stars in night time).

If you are worried about lateral color - some expensive and high end eyepieces also suffer from that. It is hard to get good lateral color performance in wide field eyepieces. That is something that might show up in daytime. Best cure - stick with eyepieces up to 60° or so.

What will be intended use for this eyepiece? In those focal lengths (25-30mm) Plossl eyepieces have enough eye relief and are excellent performers. For example Vixen NPL 30mm:

https://www.firstlightoptics.com/vixen-eyepieces/vixen-npl-eyepieces.html

has 50° AFOV and 24mm of eye relief.

If you really want wider FOV and decent eye relief, maybe this one:

https://www.firstlightoptics.com/explore-scientific-eyepieces/explore-scientific-62-series-ler-eyepieces.html

26mm focal length. It is said to be very good performer and it has max field stop for 1.25" barrel size.

 

[Thalestris24]

16 minutes ago, vlaiv said:

That highly depends on telescope you'll be using them in.

In F/5 scope - you'll clearly see the difference between high and low end eyepiece. In F/12 - there will hardly be any difference if at all.

Outer field performance is usually compared. Most eyepieces have quite decent performance in the center, but outer part of the field starts to show aberrations like astigmatism and such - which blurs things in daytime use (and distorts stars in night time).

If you are worried about lateral color - some expensive and high end eyepieces also suffer from that. It is hard to get good lateral color performance in wide field eyepieces. That is something that might show up in daytime. Best cure - stick with eyepieces up to 60° or so.

What will be intended use for this eyepiece? In those focal lengths (25-30mm) Plossl eyepieces have enough eye relief and are excellent performers. For example Vixen NPL 30mm:

https://www.firstlightoptics.com/vixen-eyepieces/vixen-npl-eyepieces.html

has 50° AFOV and 24mm of eye relief.

If you really want wider FOV and decent eye relief, maybe this one:

https://www.firstlightoptics.com/explore-scientific-eyepieces/explore-scientific-62-series-ler-eyepieces.html

26mm focal length. It is said to be very good performer and it has max field stop for 1.25" barrel size.

 

Hi Vlaiv

Um, well if I told you the prime purpose was not for an astro telescope at all, someone might kick me out!

I was looking for >20mm eye relief - which is what the BST Starguider  25 has. The latter is the only Astro EP I've had any experience of (other than SW kit EPs). The BST has some distortion towards the edge of its field but it's acceptable. Colour is also good. Eye relief is ok - as mentioned, I could do with a couple of mm extra - but only if the image quality was as good as the Starguider, so it would be pretty much a straight swap. Hence I was looking at the EF 27 which is supposed to  have the extra 3mm eye relief.

Cheers

Louise

[Zermelo]

How's your Russian?

http://astro-talks.ru/forum/viewtopic.php?f=32&t=865

[Thalestris24]

22 minutes ago, Zermelo said:

How's your Russian?

http://astro-talks.ru/forum/viewtopic.php?f=32&t=865

Thanks for that  

Only as good as Google translate, I'm afraid! It doesn't tell me much more than I already know. At a glance, doesn't do any comparisons - just the review. Does give a table of physical and optical properties which is useful  But I don't think I'm much the wiser. It's from 2012 so I don't know if the 27mm eyepiece has changed in any way since then?

Oh, I might be doing some imaging through it so the flat field is important. I see the Russians quote 22.5mm eye relief rather than 23mm but it's still at least 2mm longer than the BST  .

Cheers

Louise

[vlaiv]

34 minutes ago, Thalestris24 said:

Hi Vlaiv

Um, well if I told you the prime purpose was not for an astro telescope at all, someone might kick me out!

I was looking for >20mm eye relief - which is what the BST Starguider  25 has. The latter is the only Astro EP I've had any experience of (other than SW kit EPs). The BST has some distortion towards the edge of its field but it's acceptable. Colour is also good. Eye relief is ok - as mentioned, I could do with a couple of mm extra - but only if the image quality was as good as the Starguider, so it would be pretty much a straight swap. Hence I was looking at the EF 27 which is supposed to  have the extra 3mm eye relief.

Cheers

Louise

Actually - specs differ on different websites for some reason.

BST 25mm has 16mm of eyerelief.

TS for their version of Extra flat 27mm say it has 21mm of eye relief:

but then again, they quote field stop of 28mm and I think they are pushing it since 1.25" filter thread is 28.5mm - I think that max field stop in 1.25" format is closer to 27mm.

FLO on the other hand - lists even more eye relief:

Quote

Available in focal lengths of 16mm, 19mm & 27mm, with apparent fields of view of 60°, 65° & 53° and eye relief of 17mm, 19mm & 23mm respectively.

I did google translate of that link @Zermelo posted and it looks like FF27mm is not well suited for you.

Ernest says that effective eye relief is more like 16mm since eye lens is recessed (it is 21mm from eye lens)

Field stop is only 25mm and that gives AFOV of 50 degrees.

I think plossl would work better in that focal length than this eyepiece.

Maybe look into ES62 26mm if you have the budget for it.

Check out this link: https://www.cloudynights.com/topic/592703-es-62-26mm-vs-panoptic-24mm/

and in particular post by russell23 as well as this brief description by @Nyctimene:

 

[Thalestris24]

27 minutes ago, vlaiv said:

Actually - specs differ on different websites for some reason.

BST 25mm has 16mm of eyerelief.

TS for their version of Extra flat 27mm say it has 21mm of eye relief:

but then again, they quote field stop of 28mm and I think they are pushing it since 1.25" filter thread is 28.5mm - I think that max field stop in 1.25" format is closer to 27mm.

FLO on the other hand - lists even more eye relief:

I did google translate of that link @Zermelo posted and it looks like FF27mm is not well suited for you.

Ernest says that effective eye relief is more like 16mm since eye lens is recessed (it is 21mm from eye lens)

Field stop is only 25mm and that gives AFOV of 50 degrees.

I think plossl would work better in that focal length than this eyepiece.

Maybe look into ES62 26mm if you have the budget for it.

Check out this link: https://www.cloudynights.com/topic/592703-es-62-26mm-vs-panoptic-24mm/

and in particular post by russell23 as well as this brief description by @Nyctimene:

 

Yes, it's confusing trying to compare various parameters - which is why I posted on here! FLO quotes the ovl ef 27 eye relief as 23mm and the bst 25mm as 20mm. I suppose the max value is taken from the glass surface. But, anyway, the absolute value doesn't matter - just the difference between the two eyepieces and whether one is longer than the other. I think I'm probably getting the feeling that the ef 27 isn't really any better than the Starguider and I won't know whether the ef 27 has longer eye relief but pretty much the same quality unless I try one.  I already know the Starguider is ok and it's hard to tell if the ef 27 would be worth the extra money (£65 vs £45 for the Starguider). I don't want to spend any more than the cost of the ef 27.

Cheers

Louise

[vlaiv]

13 minutes ago, Thalestris24 said:

bst 25mm as 20mm

It lists it at 16mm eye relief.

Same usable eye relief as EF27mm.

I have ES82 11mm which has 15.6mm eye relief by specification. I feel that 12mm Plossl is more comfortable to use although it has far less eye relief - only 8mm.

Do you need 60° AFOV?

Good plossl will offer better eye relief and excellent performance - with only "drawback" being 50° AFOV.

https://www.365astronomy.com/25mm-GSO-Plossl-Eyepiece.html

In stock and won't break the bank.

[Louis D]

3 hours ago, JeremyS said:

Interesting you mention the XL 28. Not often cited on here. Is it a favourite of yours?

Never tried it, but I love my 14mm and 5.2mm XLs.  It's just that the 28mm XL is generally regarded as the best 1.25" eyepiece in that focal length range ever made.

[Thalestris24]

31 minutes ago, vlaiv said:

It lists it at 16mm eye relief.

Same usable eye relief as EF27mm.

I have ES82 11mm which has 15.6mm eye relief by specification. I feel that 12mm Plossl is more comfortable to use although it has far less eye relief - only 8mm.

Do you need 60° AFOV?

Good plossl will offer better eye relief and excellent performance - with only "drawback" being 50° AFOV.

https://www.365astronomy.com/25mm-GSO-Plossl-Eyepiece.html

In stock and won't break the bank.

Elsewhere the eye relief is quoted as 16-20mm for the bst 25. I think that's maybe because of the rubber eye cup which you can easily remove. Not sure if it's the same with the ef 27. It's the focal length I want to keep to around 25mm. The point is I want to be able to comfortable use the ep whilst wearing glasses. Having the large lens diameter that I get with the bst 25 is nice. I think life is too short and I'll get another Starguider, or two. At least I know where I am with them. Thanks for your time and advice - much appreciated, as always.

Louise

Edited February 22, 2021 by Thalestris24

[Louis D]

If the 25mm GSO Plossl is similar to my Taiwan made (GSO most likely) 26mm Orion Sirius Plossl, then it has only 11mm of measured, usable eye relief.  This is much too short to take in the entire, measured 53° AFOV with eyeglasses for me.  The problem is the highly recessed, 23mm eye lens.  Due to edge distortion, it works out to have only a 49° eAFOV.  It has a 22.3mm measured field stop.

The 25mm Starguider BST (Paradigm) has a true 60° AFOV, a 61° eAFOV, 17mm of usable eye relief, and a 26.7mm field stop.  It does fall off quite noticeably in sharpness in the outer 25% of the field at f/6.

The now discontinued 25mm Meade HD-60 actually has a 58° AFOV, a 57° eAFOV, 18mm of usable eye relief, and a 24.9mm field stop.  It remains sharper to the edge, but the FOV and FS values are less than the BST.

The real winner at this focal length for long eye relief and maximum true field of view is the 24mm APM UFF, which is now available in a variety of other brands as well.  It has a 63° AFOV, a 66° eAFOV, 17mm of usable eye relief, and a 27.5mm field stop.  It does get a bit fuzzy right near the edge due to some vignetting.

If you want to stay really cheap, the 23mm 62° Vite/Svbony Aspheric, available on ebay for about $15 direct from China, is really pretty decent.  It has a 63° AFOV, a 65° eAFOV, 18mm of usable eye relief with the rubber eye guard removed (it just pulls off), and a 26.2mm field stop.  It gets a bit fuzzy toward the edge, but it's not that much worse than the 25mm BST.

All of the AFOV, eye relief, and FS numbers were measured by me using a variety of direct and photographic techniques.

Below is a comparison image of my 23mm-28mm eyepieces taken through a field flattened AT 72ED.  It's pretty clear how well corrected the 27mm Panoptic is compared to the others.  I would imagine the 24mm is similarly well corrected.

[Thalestris24]

13 minutes ago, Louis D said:

If the 25mm GSO Plossl is similar to my Taiwan made (GSO most likely) 26mm Orion Sirius Plossl, then it has only 11mm of measured, usable eye relief.  This is much too short to take in the entire, measured 53° AFOV with eyeglasses for me.  The problem is the highly recessed, 23mm eye lens.  Due to edge distortion, it works out to have only a 49° eAFOV.  It has a 22.3mm measured field stop.

The 25mm Starguider BST (Paradigm) has a true 60° AFOV, a 61° eAFOV, 17mm of usable eye relief, and a 26.7mm field stop.  It does fall off quite noticeably in sharpness in the outer 25% of the field at f/6.

The now discontinued 25mm Meade HD-60 actually has a 58° AFOV, a 57° eAFOV, 18mm of usable eye relief, and a 24.9mm field stop.  It remains sharper to the edge, but the FOV and FS values are less than the BST.

The real winner at this focal length for long eye relief and maximum true field of view is the 24mm APM UFF, which is now available in a variety of other brands as well.  It has a 63° AFOV, a 66° eAFOV, 17mm of usable eye relief, and a 27.5mm field stop.  It does get a bit fuzzy right near the edge due to some vignetting.

If you want to stay really cheap, the 23mm 62° Vite/Svbony Aspheric, available on ebay for about $15 direct from China, is really pretty decent.  It has a 63° AFOV, a 65° eAFOV, 18mm of usable eye relief with the rubber eye guard removed (it just pulls off), and a 26.2mm field stop.  It gets a bit fuzzy toward the edge, but it's not that much worse than the 25mm BST.

All of the AFOV, eye relief, and FS numbers were measured by me using a variety of direct and photographic techniques.

Below is a comparison image of my 23mm-28mm eyepieces taken through a field flattened AT 72ED.  It's pretty clear how well corrected the 27mm Panoptic is compared to the others.  I would imagine the 24mm is similarly well corrected.

Hi

I've seen that on CN but I didn't, and still don't, know what I'm looking at! Anyway, apart from the Starguider, they mostly look too bulky and out of my price range. I need a bright, flat field, with large field stop that I can image through afocally without too much distortion. - preferably for less than about £50-£60

Thanks

Louise

[John]

I've owned both these eyepieces in the past 18 months but I can't recall much about them. I guess they were OK for £30 eyepieces (both bought used) but nothing special.

I recall that the edge correction of the BST Starguider 25mm was not as good as I'd hoped it might be. The 27mm EF was nicely made but still a bit ragged at the edge, view-wise. The usable eye relief was about the same I think but I don't wear glasses to observe.

I didn't own them at the same time though so I didn't get a chance to compare them "back to back" as it were.

A decent 25mm plossl (eg: GSO) is probably as good.

 

 

 

Edited February 22, 2021 by John

[Louis D]

1 hour ago, Thalestris24 said:

Hi

I've seen that on CN but I didn't, and still don't, know what I'm looking at! Anyway, apart from the Starguider, they mostly look too bulky and out of my price range. I need a bright, flat field, with large field stop that I can image through afocally without too much distortion. - preferably for less than about £50-£60

Thanks

Louise

Try the $15 Aspheric first, assuming you're not using it in a f/4 to f/5 system.  At 1.4 ounces and tiny in size, it's worth a shot.  If you don't like it, use it as a focuser plug.

[Thalestris24]

2 hours ago, Louis D said:

Try the $15 Aspheric first, assuming you're not using it in a f/4 to f/5 system.  At 1.4 ounces and tiny in size, it's worth a shot.  If you don't like it, use it as a focuser plug.

On the face of it, it does look quite impressive for not very much! Can I ask exactly how you took the above image? 

Thanks

Louise

[vlaiv]

@Thalestris24

I can't help but wonder what are you doing exactly. Sorry if I'm being nosy, I'm just interested in "mechanics" of things - how do you plan to use equipment and to what effect.

You say that you need good eye relief EP - to be used by spectacle wearer but then you talk about flat field and afocal method.

My guess is that you are looking for all around EP for daytime use (digiscoping and observing of sorts) - but does it have to be single eyepiece?

You say that you need flat field EP, and that is ok, however, most of field curvature comes from telescope itself. For this reason, I'm guessing that you are using 80mm F/6 with field flattener - which also happens to be reducer (from your sig). You now have F/4.8 scope. There are very few eyepieces that will be corrected to the edge in such fast system and they are expensive.

For digiscoping - you just need certain field stop - focal length and AFOV are not as important as you can adjust things on camera side - by selection of lens on camera (or simply adjusting focal length in zoom lens). Maybe orthoscopic eyepiece would give best performance in this case even if it has narrow AFOV which might not be suitable for observation?

32mm GSO Plossl has as wide field stop as you can get in 1.25" format. Here is a test I did few days ago when I got my smart phone afocal adapter:

Or this one:

Both images are at 25% scale of original. Phone has 4000x3000 resolution but lens is such that it covers more than 50° of FOV so there is some black area in shot.

Btw this is 32mm F/4 achromat finder/guider scope. It is incredible little lens. Plossl fares quite well regardless of the speed of the lens.

Although this was hand held shot (I was holding complete contraption in my hand when I took this) - sharpness is still pretty good in the center of the field.  Pidgin is a bit out of focus because it is a bit further away. I think phone focused on branches in front of it.

Here is right edge - not looking bad at all

Left edge is worse - but it might have something to do with phone centering - I did not pay much attention how centered it is - and then there is out of focus due to distance blur:

 

[Thalestris24]

3 minutes ago, vlaiv said:

@Thalestris24

I can't help but wonder what are you doing exactly. Sorry if I'm being nosy, I'm just interested in "mechanics" of things - how do you plan to use equipment and to what effect.

You say that you need good eye relief EP - to be used by spectacle wearer but then you talk about flat field and afocal method.

My guess is that you are looking for all around EP for daytime use (digiscoping and observing of sorts) - but does it have to be single eyepiece?

You say that you need flat field EP, and that is ok, however, most of field curvature comes from telescope itself. For this reason, I'm guessing that you are using 80mm F/6 with field flattener - which also happens to be reducer (from your sig). You now have F/4.8 scope. There are very few eyepieces that will be corrected to the edge in such fast system and they are expensive.

For digiscoping - you just need certain field stop - focal length and AFOV are not as important as you can adjust things on camera side - by selection of lens on camera (or simply adjusting focal length in zoom lens). Maybe orthoscopic eyepiece would give best performance in this case even if it has narrow AFOV which might not be suitable for observation?

32mm GSO Plossl has as wide field stop as you can get in 1.25" format. Here is a test I did few days ago when I got my smart phone afocal adapter:

Or this one:

Both images are at 25% scale of original. Phone has 4000x3000 resolution but lens is such that it covers more than 50° of FOV so there is some black area in shot.

Btw this is 32mm F/4 achromat finder/guider scope. It is incredible little lens. Plossl fares quite well regardless of the speed of the lens.

Although this was hand held shot (I was holding complete contraption in my hand when I took this) - sharpness is still pretty good in the center of the field.  Pidgin is a bit out of focus because it is a bit further away. I think phone focused on branches in front of it.

Here is right edge - not looking bad at all

Left edge is worse - but it might have something to do with phone centering - I did not pay much attention how centered it is - and then there is out of focus due to distance blur:

 

Ok, ok - I'll come clean! I'm actually mainly looking for eyepieces for my microscope! I was afraid if I'd said that at the beginning I wouldn't be taken seriously. Just as if I'd raised the same question on the microscopy forum, the purists there might have shot me down! Just by experimenting with astro eyepieces I can get much wider views than standard microscope oculars. I have a trinocular instrument and have been imaging afocally via the BST Starguider and getting good results. Anyway, so using the eyepieces visually is where the eye relief comes in. The standard microscope eyepieces don't have a great deal of eye relief and it drives me nuts having to keep putting my glasses on and taking them off again. The 25mm focal length is equivalent to a standard 10x microscope eyepiece. Dividing the focal length into 250 gives you the magnification so I wanted to stay close to the 25mm. My current 10x eyepieces have a field number of 18mm which is rather small. I've measured the Starguider with a calibration slide  as up to 23.4 and via the trinocular as 24mm - much better, even with some distortion around the edges.  Now, at the moment, I have the Starguider for the imaging side so I need two EPs for the binocular visual. Obviously that doubles the cost which is why I ideally want to keep the EP price down but still for them to be as good as the Starguider. It might be that if I found a very good EP I could use for the oculars and the imaging, in which case I'd need 3 off! I could then use the Starguider for astronomy .  I've ordered one of the recommended aspheric lenses this morning. There is a part of me that says 'something so cheap can't be any good!' but I'm willing to give it a try. If the worst comes to the worst, I can just get a couple more Starguiders and be happy - except, as mentioned, I only just get the eye relief I'd need and a couple of extra mm would be more comfortable! So, there you have my confession!

Thanks in advance for your indulgence! Yes, your images above are quite nice - come out well!

Louise

[vlaiv]

4 minutes ago, Thalestris24 said:

I'm actually mainly looking for eyepieces for my microscope!

I was not aware it is simple 1-1 attachment to microscope as well. It also uses 1.25" barrel size?

7 minutes ago, Thalestris24 said:

My current 10x eyepieces have a field number of 18mm which is rather small.

I guess that is to be expected. My first guess as a good microscope eyepiece in that focal length would be this one:

https://www.teleskop-express.de/shop/product_info.php/info/p8860_Fujiyama-1-25--HD-Ortho-Eyepiece-25-mm---made-in-Japan.html

but you would not get any improvement in field of view. Price is also not suitable, especially if you need 2 or 3 of them.

If you want max AFOV in that exact focal length of 25mm - then there is no much choice. 60°-62° is really the maximum. Pay attention that you might not quite want astronomical eyepieces.

Astronomical eyepieces have certain geometric distortion that is preferred in astronomical use to that of terrestrial (and for that matter microscopic).

You can't have perfect eyepiece - especially wider field of view one. It will either have rectilinear distortion or angular magnification distortion.

For terrestrial use - zero rectilinear distortion is desirable. Straight lines need to stay straight. Look at above image that I posted - first one. There is a wall. Wall is really a straight wall - but in image it looks slightly bent.

Such eyepiece magnifies objects that are away from the center of the FOV. Not desirable in astronomy as planet would "inflate" when nearing the edge of the FOV.

Here is a good comparison between the two:

Left is what astronomical eyepieces do - right is what terrestrial eyepieces often do. Notice how in right image - man standing on the left seems larger than woman standing in the middle of the image.

By the way - there is almost no difference between the two optimizations up to about 42° AFOV. That is why very sharp and very precise eyepieces stop at about that FOV size.

[Thalestris24]

17 minutes ago, vlaiv said:

I was not aware it is simple 1-1 attachment to microscope as well. It also uses 1.25" barrel size?

I guess that is to be expected. My first guess as a good microscope eyepiece in that focal length would be this one:

https://www.teleskop-express.de/shop/product_info.php/info/p8860_Fujiyama-1-25--HD-Ortho-Eyepiece-25-mm---made-in-Japan.html

but you would not get any improvement in field of view. Price is also not suitable, especially if you need 2 or 3 of them.

If you want max AFOV in that exact focal length of 25mm - then there is no much choice. 60°-62° is really the maximum. Pay attention that you might not quite want astronomical eyepieces.

Astronomical eyepieces have certain geometric distortion that is preferred in astronomical use to that of terrestrial (and for that matter microscopic).

You can't have perfect eyepiece - especially wider field of view one. It will either have rectilinear distortion or angular magnification distortion.

For terrestrial use - zero rectilinear distortion is desirable. Straight lines need to stay straight. Look at above image that I posted - first one. There is a wall. Wall is really a straight wall - but in image it looks slightly bent.

Such eyepiece magnifies objects that are away from the center of the FOV. Not desirable in astronomy as planet would "inflate" when nearing the edge of the FOV.

Here is a good comparison between the two:

Left is what astronomical eyepieces do - right is what terrestrial eyepieces often do. Notice how in right image - man standing on the left seems larger than woman standing in the middle of the image.

By the way - there is almost no difference between the two optimizations up to about 42° AFOV. That is why very sharp and very precise eyepieces stop at about that FOV size.

Hi again

I suppose the main difference, visually, with a microscope that the view is almost just 2D so very little depth that's in focus (depending on objective power). In that case it's just rectilinear, chromatic and spherical distortion one wants to minimise. The spherical and chromatic distortions comes largely, if not wholly, from the objective (I think!). So for the EP it's really just the rectlinear distortion one wants to minimise though a small amount is still acceptable. One does want good contrast. It's like with engineering generally - good enough will do! The imaging side is naturally a bit more demanding but, even so, most of the time you end up cropping around the subject of interest which will likely be occupying the central area of the (objective's) fov. Still, having an EP with a largely flat field makes everything nicer and easier. At the end of the day, as with astro observing (which I've done very little of!), one just wants a pleasant experience   Oh, it's easy to 3D print appropriate adapters. For the EPs I can remove the 1.25" barrel from the astro EP and connect the optical part with the microscope via a 3d printed adapter (threads can be challenging and it usually takes me several goes before I get it just right!). The imaging train keeps the 1.25" barrel and connects via a1.25" astro helical focuser, and then a 3D printed adapter that connects it to the trinocular port. All seems to work ok  .

Louise

[vlaiv]

4 minutes ago, Thalestris24 said:

So for the EP it's really just the rectlinear distortion one wants to minimise though a small amount is still acceptable.

Maybe look into eyepieces for terrestrial spotting scopes then?

Or maybe not  - just looked at prices of those - way too pricey to be useful.