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Eyepieces - Focal Length vs FOV vs Eye relief

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#1 rivonia

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Posted 26 June 2019 - 05:00 AM

Eyepiece descriptions seem somewhat confusing:

 

1. I've read that some expensive eyepieces give a wider field of view at the same magnification than cheaper eyepieces. Is this true?

2. Also read about eye relief being measured in mm - How, if at all, does this relate to focal length.

3. What does eye relief actually describe?

4. Does a lower focal length eyepiece always have a smaller lens?

 

All these questions are in preparation of my first eyepiece purchases.


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#2 Starman47

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Posted 26 June 2019 - 06:20 AM

On the Sky & Telescope website there in a page that answers all your questions. Search for “An Eyepiece Primer”

 

good luck. 



#3 markb

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Posted 26 June 2019 - 06:48 AM

1) Yes, absolutely. In the 1980's 82 degree eyepieces became popular (Naglers got it rolling, describing the super wide fields as a 'spacewalk' view), deservedly so. Recently 100-110 degree eyepieces became popular but are super pricey, and the lens sizes often demand 2" eyepiece holders.

 

Plossls are sharp and affordable but 50-55 degrees to the eye, decent eye relief except under 10mm fcoal length. Orthoscopics are usually extremely sharp but 45 degrees or so, almost half of a Nagler view diameter, short eye relief, 4-6mm focal lengths are almost unusable since the cornea nearly touches the lens.

 

Very out of fashion due to abberations (visual defects, usually to the outer part of the visual field) are Erfles and modified Erfle designs, giving wide 70 degree fields. Many are very sharp in the center and quite inexpensive used. Some are great , some no so great. IIRC they are not recommended under f5 focal ratios. I have liked mine, and they were popular for extra wide binoculars through the 1990s.

 

24-28mm eyepieces are often sold with scopes as they are confortable for the eye.

 

Many people upgrade eps so there is a thriving used market,  and Wanted ads are a great way to go.

 

Eps are usually safe to get used, scopes less so.

 

Read Cloudynight reviews to find out what specific brands and models to avoid and what to buy. Forum posts can be unreliable, but over time you can learn what baseless 'opinions' to ignore, from folks that never actually used the equipment in question.

 

Avoid Kellner, abbreviated such as 20mm K, and Modified Achromats, MA, cheap and easy for scope makers to include with scopes.

 

Do all calculations in mm for the following, forget the English system, mostly.

 

Divide the ep field of view angle by the power (scope focal length by ep focal length to get power) to get the field you actually see. Online and app based calculators are available and easy to use. They can also give 'exit pupil', the size of the light 'beam' entering the eye, minimum of .5mm, maximum 7.5mm for the average person. Age matters on the maximum.

 

3) Eye relief is the distance from the outermost lens surface to the cornea, so you can see the full field circle. Allegedly. Users often complain that a manufacturer is not truthful on eye relief figures, usually when they think it is shorter than claimed. If your eye is only comfortable at a longer distance, you will not see the full field circle. Very common on higher power binoculars with short eye relief.

 

Eye relief can seem shorter if the eye lens is recessed. Eyepieces with wide ends may not fit well with the shape of the skull eye socket causing similar complaints. Cone shaped 'volcano tops' were more common years ago and tended to be comfortable, but fell out of favor. Some ep, like the Baader Zoom, have a removable ring around the eye lens to address this. A couple of great eyepieces have finally had barrel redesigns after recognising that fat ends simply don't fit the eyesoket.

 

Most folks like 8mm to 20mm eye relief, and complaints rise as it goes to 4mm or less. Too long, and folks cannot keep the eye in the right spot.

 

You do NOT need to wear eyeglasses for simple near or far sightedness as the telescope focus compensates for you, but may need them for astigmatism or other corrections. High power exit pupils go through a smaller area of the cornea, so some astigmatic eyes can omit the glasses at higher powers, but only experimentation can tell. With eyeglasses, 20mm is often said to work. Televue has a correction system that adds a lens to some of their eyepieces, IIRC. No personal knowledge on that.

 

2) Generally, the shorter the focal length of the ep, the shorter the eye relief. Some designs, like Vixen's Lanthanum series, are designed for a set eye relief across the range. Vixen's Lan. series is 20mm IIRC. Easiest 2.5mm eyepieces to use as a result, but that is a special use ep.

 

4) Yes, shorter eps usually have smaller lenses, and longer ep have larger lenses, but that is within a same manufacturer and type of eyepiece. Generally the wider angle eps have much much larger lenses, some up to 3 pounds/1.5 kg of glass and metal. The Vixens mentioned above stay about the same size, from shortest to longest focal length.

 

Note that the field of an eyepiece is limited by the barrel diameter, so a 2" barrel may allow a 80 degree field in a 3mm ep, but only 40 degrees in a 50mm ep (guesstimate on the latter), regardless or design or manufacturer. As in all else, the laws of physics always win.

 

The there 'legs' of the astronomy money pit are telescope, eyepieces, and mount, so plan carefully as you enjoy the ride!

 

Good luck!


Edited by markb, 26 June 2019 - 07:08 AM.

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#4 Bowlerhat

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Posted 26 June 2019 - 07:25 AM

yes. have you ever have trouble with object drifting away out of the fov too fast? more expensive eyepieces will give similar magnification, but give a much wider field of view so the object stays longer.

 

use this website to see the simulated view

 https://astronomy.to.../field_of_view/

 

also, lower focal length usually means smaller lenses but it depends on the brands. and wide angle lenses with short focal length will have bigger lenses.


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#5 havasman

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Posted 26 June 2019 - 08:54 AM

Hello Rivonia and welcome to the forums!

 

You are doing well to learn a bit before buying eyepieces (or anything else) for your scope.

 

Exit Pupil = eyepiece focal length / scope focal ratio

Magnification = scope focal length / eyepiece focal length
Total Field of View = (eyepiece field stop diameter X scope focal length) / 57.3

Eyepiece field stop diameters for most current production eyepieces can be found here  -  https://www.cloudyni...e/#entry9432811

 

​That simple arithmetic shows how magnification, exit pupil and TFOV are interrelated and calculates each for a given scope/eyepiece combination. The Amateur Telescope Optics website is my top authoritative reference site for all matters of scope optics and Figure 209 near the top of the 1st page of the eyepiece section should illustrate many components of the eyepiece optical system for you including eye relief.  https://www.telescop.../eyepiece1.htm 

 

Eyepiece cost is usually tied to the complexity of the design. Complexity can be required by the steeper light cones of fast scopes and to provide wider fields of view. There are less expensive ep's scoring high on the price/performance scale. To your Q #4: NO. See Televue Delos for example. Eye lens size is designed in and can vary widely as different design parameters are emphasized for a product.


Edited by havasman, 26 June 2019 - 08:56 AM.

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#6 Hesiod

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Posted 26 June 2019 - 11:10 AM

1)the field of view depends on the design (more than cost: a TMB Monocentric is FAR more expensive than most super/ultrawideangle eyepiece); you can guess the relative sizes of the field of view between two eyepieces of the same focal lenght by looking at their apparent field of views: a 10mm/80° will show you twice the field of a 10mm/40° at the same magnification

 

2)again, it is a consequence of the design: some eyepieces have a more or less constant eye relief (e.g. Pentax XW, or BST Starguider), while, with others, it changes with the focal length (in such case it is usually related to the eyepiece's focal lenght: as an example, with typical Plossl eyepiece you may estimate the eye relief by assuming it is roughly 4/5 of the focal lenght). Again, cost is not a factor here: the BST Starguider are rather cheap, but have constant eye relief, while the more expensive TeleVue Plossl or Takahashi Abbe's eye relief is function of their focal length (shorter ones have tighter eye relief, to the point that some judge them almost unusable; but take note that too much eye relief can be just as nasty, making harder to place the eye. This is rather common with 32 or 40mm Plossl)

 

3)markb explained this very well; I would just add that, sometimes, the real eye relief can be slightly different from the specified one since the top lens itself may be somewhat "buried" inside the eyepiece's rim

 

4)absolutely not, the size of the top lens is just a consequence of the design: again, the BST Starguiders' top lenses have all roughly the same size, while in the case of Tele Vue Plossl line the top lens' size decreases as the focal length become lower. On the other hand, it is possible and not so uncommon that "complex" eyepieces (those of more modern design, which were not widely available to amateur stargazers until a few decades ago) become larger and heavier as the focal length become lower, due to the need of more lenses to keep other features (e.g. the Vixen SSW 3.5 is the taller of its line)


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#7 Sky Muse

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Posted 26 June 2019 - 12:07 PM

Eye-relief is how far you hold your eye from the eyepiece's eye-lens in order to see the full field-of-view that the eyepiece offers...

 

]

 

If you have to wear prescription-eyeglasses whilst observing, then you would need eyepieces of at least 15mm of eye-relief if not 20mm, and so to accommodate the thickness of the frame of the eyeglasses...

 

 

 

Plossls are the current minimum standard in performance eyepieces, and Plossls shorter than 9mm to 10mm are going to have tight eye-relief.  For example, with this 6mm Plossl, I have to hold my eyeball right up to its eye-lens, to where my eye almost touches same, and in order to see its full field-of-view...

 

 

 

...and with a scant, tight 3mm of eye-relief; but I can use it as I don't have to wear eyeglasses whilst observing.  Although, it's a bit difficult to use, even for me, but the views through it are great.

 

Performance eyepieces other than Plossls generally have wider fields-of-view, longer eye-relief, larger eye-lenses through which to look, and are more expensive than Plossls.

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#8 aeajr

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Posted 27 June 2019 - 11:25 PM

Eyepiece descriptions seem somewhat confusing:

 

1. I've read that some expensive eyepieces give a wider field of view at the same magnification than cheaper eyepieces. Is this true?

2. Also read about eye relief being measured in mm - How, if at all, does this relate to focal length.

3. What does eye relief actually describe?

4. Does a lower focal length eyepiece always have a smaller lens?

 

All these questions are in preparation of my first eyepiece purchases.

Your questions are answered in this article.

 

Understanding Telescope Eyepieces- There are recommendations, based on budget, but the meat of the article is about understanding the issues when selecting eyepieces.
https://telescopicwa...cope-eyepieces/

 

 

Selecting an eyepiece - Orion telescope
This is a very general discussion of eyepieces and why there are a variety
of designs
https://www.youtube....h?v=m7u9Q5hV7yc



#9 rivonia

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Posted 28 June 2019 - 05:17 AM

On the Sky & Telescope website there in a page that answers all your questions. Search for “An Eyepiece Primer”

 

good luck. 

That was a good read, thank you.



#10 aeajr

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Posted 28 June 2019 - 10:10 AM

Eyepiece descriptions seem somewhat confusing:

 

1. I've read that some expensive eyepieces give a wider field of view at the same magnification than cheaper eyepieces. Is this true?

2. Also read about eye relief being measured in mm - How, if at all, does this relate to focal length.

3. What does eye relief actually describe?

4. Does a lower focal length eyepiece always have a smaller lens?

 

All these questions are in preparation of my first eyepiece purchases.

Note that there is no direct correlation between price, field of view, eye relief and focal length.  They vary based on the design of the eyepiece and the quality of the manufacture.

 

The size of the lens is related to the eye relief and is driven by the design. 

 

Eyepiece Designs -  This is the one I turn to when I am trying to understand
or explain the differences between the various designs.  There are so many
different designs.  Many are named for their original designer, such as
Huygens, Ramsden, Kellner, Plossl, Konig, Erfle, Branden and Nagler.

Others have more generic names like Explore Scientific 82 or Deep View.
http://www.chuckhawk...ece_designs.htm



#11 aeajr

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Posted 28 June 2019 - 10:14 AM

We don't know what telescope you have and we don't know your budget.

 

When I am helping someone shop for eyepieces, here is what I ask.

  • What telescope are you using?  Focal length?  Focal Ratio?  Aperture?
     
  • Do you wear glasses when you observe.   Many people who wear glasses take them off to observe.  I wear glasses, but not when I observe.
     
  • What is your TOTAL eyepiece budget right now?

You can buy very expensive eyepieces and you can buy moderately priced eyepieces, depending on your budget, and still have a very good experience.

 

You can also us zoom eyepieces rather than single focal length eyepieces.   I have both but use the zoom eyepieces much more than the single focal length eyepieces.



#12 aeajr

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Posted 28 June 2019 - 10:22 AM

I always recommend that you post a link to the scope, or any equipment you are asking about, so we are all sure we are talking about the same things.

 

How do you plan your eyepieces?

 

I am going to outline a strategy for you.  Whether you have a Mak, SCT, refractor or Newtonian, the strategy is the same.   I have done this example based on my Meade ETX 125.  This is a 127 mm Maksutov–Cassegrain with a 1900 mm FL for an F15 focal ratio.   This is a good example of a scope with a 1.25” focuser that can’t accept 2” eyepieces.  For this reason I  have left 2” eyepieces out of the discussion.   Just redo the calculations for your focal length to get the eyepiece sizes that work for you.  

 

If you have a scope that can accept 1.25" and 2" eyepieces, let us know.

 

 

Watch the Video - First, some background on selecting an eyepiece - Orion telescope video

This is a very general video discussion of eyepieces and why there are a variety of designs
https://www.youtube....h?v=m7u9Q5hV7yc

 

EYEPIECE STRATEGY SUMMARY

  • One or two low power wide view eyepieces
  • One or two midrange eyepieces
  • Two to four high power eyepieces
  • Or, Zoom plus barlow to cover mid range and high power
  • Planning to use a barlow can save you money

A 127 mm scope will have a maximum mag of around 254X, though many nights you may not be able to reach that magnification level due to atmospheric conditions.    More on that later.    I estimated that based on 2X the aperture in mm.   127 X 2 = 254X

 

This scope has a 1.25” diagonal/focuser.  The best low power/widest view eyepiece would be a 32 mm Plossl eyepiece.  In my scope that provides 59.4X and  .84 degree FOV.  That is about as wide as this scope will go.  If your scope has a different focal length than you can redo the calculations for your scope with a 32 mm Plossl that typically has a 50 degree AFOV.

 

Focal length scope ( FLS)  / Focal Length eyepiece ( FLE) = Magnification

 

Apparent Field of View of eyepiece (AFOV) / Magnification = Field of View (FOV)

( this is a simplified calculation that offers a very close approximation of FOV)

 

Based on a 32 mm for the low power wide view and 254X as the top mag target, here is a sample range of eyepieces and magnifications for my scope.

 

59X     = 32 mm ( low power)

95X     = 20 mm (medium power)

127X  = 15 mm  (medium power)

158X  = 12 mm  (high power)

 

The above mags should work almost any night.

 

Below are mags for this 127 mm scope that would need good seeing and transparency to be useful

190X  = 10 mm  (Some nights I can’t go this high.  This could be the 20 mm in a 2X barlow)

237X  =  8 mm  ( Fewer nights I am able to reach this mag. )

256X = 7.5 mm (15 mm in 2X barlow could provide this which is likely to be infrequently used)

317X = 6  mm ( Might work on the moon on good nights.) 

 

Since I have a 12, I can easily 2X barlow that to give 6 mm to try it without having to buy an eyepiece I would rarely use.

 

Nothing rigid about these mag targets, I am just using them as an illustration of a range of magnifications to work toward.  And there is nothing to stop you from trying to go higher than 2X aperture, as I illustrate.  Do the calculations for your scope.   Note that I have selected eyepiece spacing that would work well if I wanted to use a 2X barlow to help me cover some of the higher powers if I didn’t want to spend the money on magnifications that I might not be able to use often. 

 

How high you can go depends on transparency, the clarity of the air, and "seeing".

 

What is SEEING and why it can be bad.  How it will limit how high you can go.

This is not a problem with your telescope or your eyepiece.

http://www.skyandtel...ing-the-seeing/
http://www.damianpeach.com/seeing1.htm
http://www.damianpea...m/pickering.htm

 

Barlow Lens – I mentioned using a 2X barlow lens.  A barlow is an intermediate optical device that goes between the eyepiece and the objective lens or primary mirror.   It effectively gives each eyepiece two magnifications, one with and one without the barlow.  For visual use, barlows ranging from 1.5X to 3X are common.   So, if you have 4 eyepieces properly spaced and add a barlow, you now have 8 magnifications with only 4 eyepieces.   You can read more about barlows at this link:  http://www.telescope...rlows/99807.uts

 

The alternative method to multiple eyepieces is to use a zoom eyepiece.   This is my primary approach.  An 8-24 mm zoom, in my ETX 125 scope, provides  79X to 237X, and everything in between.  Most nights this is all the range I need based on atmospheric conditions, so the zoom is all I use.   If I want to push higher, I can drop the zoom in my barlow and probe those higher magnifications.   If this were my ETX 80, 400 mm FL, then a 2X or 3X barlow would be in use frequently.   

 

Eyepieces are a tool that you will want to add over time.  Above is just a strategy for where and how to add them.  Below are some eyepiece examples for your consideration.  The good news is that eyepieces are standard sized.  Any brand of 1.25" eyepiece will fit in almost any telescope.  So if you plan to upgrade your telescope some day, or add a larger scope, the eyepieces you buy today will fit.

 

 

32 mm Plossl - I find most of them all to be good.  Celestron, Meade, Orion, GSO are all quite good.   Televue has an excellent reputation, at a higher price.

https://agenaastro.c...de-televue.html

 

Lower Cost Single FL eyepieces - AT Paradigm line has an excellent reputation and offers a 60 degree AFOV, wider than Plossl eyepieces.   I don’t have these but I read so many good reports about them, especially in scopes of F5 or higher focal ratios.
https://www.astronom...pieces_c52.aspx

 

The Agena Astro Dual ED are the same eyepieces under a different label.

https://agenaastro.c...roproducts.html

 

Discussion about Paradigm eyepieces
https://www.cloudyni...s/#entry8229760

 

 

Medium Priced Single FL Eyepieces – Explore Scientific 68 degree, 82 degree and Meade 82 degree
https://agenaastro.c...scientific.html

https://www.astronom...pieces_c75.aspx

 

 

Premium single FL eyepieces – TeleVue – I don’t have any Tele Vue but they are considered, by many, to be among the best available, but they are expensive.
https://www.astronom...pieces_c83.aspx

 

 

THE ZOOM EYEPIECE 

 

This is my favorite eyepiece.  Like a zoom lens on a camera, this single eyepiece effectively replaces a range of eyepieces.  Sounds great, but there is a trade-off.  The apparent field of view of the zoom runs from a narrower AFOV at the 24 mm range to a wider FOV at the 8 mm range.  So, like any approach, the zoom is a compromise.  I find that compromise quite acceptable when weighed against the benefits.  I find I tend to use it mostly in the 18 mm to 8 mm part of the range.

 

Lower cost zoom – Celestron 8-24 – This was my first zoom and still use it from time to time in my scopes – $66

Higher priced Zoom – Baader Hyperion 8-24 mm – Now my main eyepiece in my Orion XT8i, Apertura AD12 and frequently in my other scopes – $290
https://agenaastro.c...lanetarium.html

  • I never expected the zoom eyepiece to become my primary eyepiece, but it has.
  • With a zoom, the eyepiece seems to disappear as you just move in and out at will, no swapping, no thinking about eyepiece changes
  • The Celestron 8-24 zoom is good and comparable to my Plossl eyepieces
  • The Baader Hyperion is great and comparable to my Explore Scientific eyepieces
  • Watching doubles split as I rotate the barrel is wonderful
  • One filter serves over a wide range of magnifications, no screwing and unscrewing to try other eyepieces
  • Moving smoothly from and between small changes in magnification helps when seeing is not the best
  • I am always working at the optimum magnification for this target.
  • Sharing the view with others is easier, especially in my manual tracking Dob - I hand it over at low mag so it stays in the view longer.  They zoom back in to whatever magnification works best for them.
  • My eyepiece case has been greatly simplified
  • Kids love the zoom

In my 127 mm Mak, when I observe, 90% of the time, I use the 32 mm Plossl and then the 8-24 zoom and that is all I use.   But if conditions are really good, I will probe higher with the barlow and zoom. 

 

With my ETX 80, 400 mm FL, the 32 mm Plossl, the zoom and the barlow are all used each evening.  

 

In my Orion XT8i and Apertura AD12 Dobsonians, which have a 2” focuser, I use 38 and 20 mm 2”, 8-24 zoom and may go to a 2X barlow.  Same strategy for each scope.

 

REFERENCE RESOURCES

 

Zoom eyepiece review including the Celestron

An older view, and the Celestron specs have changed, but the discussion  matches well to my experience with my Celestron zoom

http://www.chuckhawk...m_eyepieces.htm

 

Baader Hyperian Mark IV Zoom review   - The current model  -

https://astronomycon...ox-january-2018

 

Baader Zoom User Discussion

https://www.cloudyni...om-for-planets/

 

Explore Scientific 82 degree Vs. Meade series 5000 Ultra Wide 82 degree eyepieces

https://www.cloudyni...e-82-eyepieces/

 

A very good discussion, by Al Nagler, about eyepieces and magnification for those who want to go a little deeper. He discusses our eyes, our telescopes, focal ratios, exit pupils, the atmosphere and things related to choosing magnification.   A good read and not too technical.  It is a general discussion and not specific to any eyepiece or any brand of eyepiece.
http://www.televue.c...page.asp?id=102

 

Eyepiece Designs -  This is the one I turn to when I am trying to understand or explain the differences between the various designs.  There are many different designs, Many are named
for their original designer, such as Huyghens, Ramsden, Kellner, Plossl, Konig, Erfle, Branden and Nagler.
http://www.chuckhawk...ece_designs.htm


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#13 25585

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Posted 28 June 2019 - 10:49 PM

This is a good informative video https://youtu.be/XmRyGsaM4ws



#14 Dave Mitsky

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Posted 29 June 2019 - 01:18 AM

If you want even more information on eyepieces, consult the following:

 

https://www.skyandte...eyepiece-guide/

 

http://sas-sky.org/w...g-Eyepieces.pdf

 

https://www.bing.com...7914B&FORM=VIRE

 

http://quadibloc.com/science/opt04.htm

 

https://www.handprin.../ASTRO/ae5.html

 

http://billsastro.co...iece_design.htm

 

http://www.nightskyinfo.com/eyepieces/

 

http://www.spacegaze...sp?pageid=97152



#15 25585

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Posted 29 June 2019 - 06:50 PM

My first eyepieces were a 32mm Erfle, a 20mm Erfle and a 2x Barlow. Later I added 13mm & 10mm eyepieces, a nebula filter and a 2.5x Barlow.

 

My scopes were Newtonians F7.5 and F5. Those combined served me for years. I replaced the 13mm with another 13, but that was it.

 

I have added more over time, but 32, 20, 13, 10mm & 2x Barlow are to me the keystone focal lengths for any scope, going on magnifications alone.



#16 aeajr

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Posted 01 July 2019 - 11:48 AM

The OP has not been back since June 26.  Either he got his answer or has lost interest in the discussion.




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