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What is your definition for "parfocal"?

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#1 russell23

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Posted 20 September 2020 - 09:54 PM

A current thread has some debate about whether or not a particular zoom eyepiece is parfocal.  In reading threads over the years I've noticed that some people consider eyepieces from a line that only require a small tweak in focus to be parfocal.  However, others may consider an eyepiece line or a zoom eyepiece as parfocal if there is zero focus adjustment required when switching eyepieces or zooming the zoom eyepiece.

 

I'm just curious what "parfocal" means to different people and if you think there can be a slight focus adjustment for a parfocal eyepiece line then how much focus adjustment would you allow?



#2 mkothe

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Posted 20 September 2020 - 10:16 PM

Ok, I’ll bite.

For me, if I refocus because I am not sure if the image still is perfectly focused, the eyepiece is parfocal. I tweak focus routinely, even if I don’t change eyepieces, just to make sure. If I refocus because the image is clearly blurry and needs refocusing, it’s not parfocal.

I think the bigger issue is that some people need to refocus a certain zoom, while others don’t. Sarkikos’s discussions of his Leica vs. Baader zoom come to mind. This is often attributed to the loss of accommodation in older eyes. So I can understand why there may be a difference of opinion regarding a particular zoom.

Michael

Edited by mkothe, 20 September 2020 - 10:20 PM.

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#3 areyoukiddingme

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Posted 20 September 2020 - 11:08 PM

Psychophysics gives a definition of the Just Noticeable Difference (JND), which is the minimum threshold of difference in stimuli that can be detected 50% of the time (i.e., chance).

 

This is useful for finding out how brightness or sound intensity map from physical quantities to perception. It also gets used for minimal intensities--like how little light or sound can be perceived.

 

And if someone were sufficiently perverse they could find out the perceptual JND for a zoom. Keeping in mind that it probably varies with seeing, how hydrated you are, and whether you've eaten enough spinach recently. :-)

 

Of course, this is a process of averaging and will differ for individuals. But it at least provides a framework for understanding when something becomes significant, and it is a quantity that we can agree is meaningful because the threshold of chance tells us when something is perceptually real or not.

 

Psychological mumbo jumbo aside, I consider an eyepiece parfocal if I routinely change the magnification and don't get the urge to tweak the magnification when the seeing is steady.

 

The Leica and Nagler zooms fall in that category. My Svbony 7-21 and Nikon cheapy-thing do not.


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#4 csrlice12

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Posted 20 September 2020 - 11:23 PM

Honestly, I don't worry over this...that's what the twirly knobs are for, and I ain't afraid to use them.


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#5 stevenrjanssens

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Posted 20 September 2020 - 11:33 PM

For me parfocal means same Paracorr setting as that's a much bigger hassle compared to adjusting the focuser (though not a big one). But every time I switch eyepieces or the setting on my Baader zoom, a slight adjustment of focus is needed.


Edited by stevenrjanssens, 20 September 2020 - 11:34 PM.

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#6 Ernest_SPB

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Posted 21 September 2020 - 12:28 AM

Two eyepieces are parfocal when their front focus placed the same distance from end their bodies (plane where barrel and body of the eyepiece are connected).

 

So in case good observer vision change of parfocal eyepieces in focuser does not require change of focusing.

 

Note that it is not absolute true.

 

(1) focusing depends from individual preferences of observer - most of them prefer to get image in 300-500 mm (not in infinity). In case observer prefer near placing of the image some focusing is required even for truly parfocal eyepieces.

(2) for short-sighted observer also some tuning of focusing is required even for truly parfocal eyepieces.

(3) taking into account accommodation (eye ability to get focused in range 4 diopters for mid age observers) tolerance to be focused is as wide  as FL*FL/250 mm (e.g. for 10 mm eyepiece it can be 10*10/250 = 0.4 mm)  


Edited by Ernest_SPB, 21 September 2020 - 11:07 AM.

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#7 Jon Isaacs

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Posted 21 September 2020 - 06:18 AM

My two cents:

 

- I believe Don would say the manufacturers use a definition that allows for a maybe a millimeter of refocusing.  

 

-  The depth of focus of a telescope depends on it's focal ratio.  At F/10, that's about 0.22mm (0.008") at F/5 it's about 0.055mm (0.002")

 

- My own definition:  I would call it "perfectly parfocal."  No focusing is required when swapping eyepieces.  I find that the 16mm Type 5 Nagler plus all the type 6's except the 3.5mm (don't know about the 2.5mm) are perfectly parfocal in my scopes, they're mostly around F/5.  

 

Now there is one caveat.  When going to a shorter focal length, because my eye is better able to resolve the image, I may try to refocus.  But when switching back to the longer focal length, with the above eyepieces, I feel no need to refocus.  

 

- The Baader 8mm-24mm zoom is definitely not perfectly parfocal.  Even relatively small changes require refocusing.

 

Jon


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#8 LDW47

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Posted 21 September 2020 - 07:52 AM

My two cents:

 

- I believe Don would say the manufacturers use a definition that allows for a maybe a millimeter of refocusing.  

 

-  The depth of focus of a telescope depends on it's focal ratio.  At F/10, that's about 0.22mm (0.008") at F/5 it's about 0.055mm (0.002")

 

- My own definition:  I would call it "perfectly parfocal."  No focusing is required when swapping eyepieces.  I find that the 16mm Type 5 Nagler plus all the type 6's except the 3.5mm (don't know about the 2.5mm) are perfectly parfocal in my scopes, they're mostly around F/5.  

 

Now there is one caveat.  When going to a shorter focal length, because my eye is better able to resolve the image, I may try to refocus.  But when switching back to the longer focal length, with the above eyepieces, I feel no need to refocus.  

 

- The Baader 8mm-24mm zoom is definitely not perfectly parfocal.  Even relatively small changes require refocusing.

 

Jon

You are close to what I am trying to say about those Svbony zooms but you are talking about various size eps in a set of the same brand I think ? Obviously I was talking in another post about a single eyepiece, a zoom of great quality, relatively new in the market place. Your quote ‘ not perfectly ‘ but very, very close, minimal if you will rings a bell ! My opinion is that at that point of a minute tweaking needed or not gets into the realm of ‘ splitting hairs ‘, unnecessary nit picking, its beyond anything technical, lol ! When we get to that point its every persons experience for themselves !


Edited by LDW47, 21 September 2020 - 07:56 AM.


#9 Jon Isaacs

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Posted 21 September 2020 - 07:56 AM

Quote ‘ not perfectly ‘ but very, very close, minimal if you will !

 

With the zoom, if you zoom out from 8mm to 24mm, is it still in perfect focus. If you zoom in from 12mm to 8mm, is it still in perfect focus?

 

Focus is about splitting hairs, well may not hairs, but stars.

 

Jon


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#10 CrazyPanda

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Posted 21 September 2020 - 08:23 AM

A current thread has some debate about whether or not a particular zoom eyepiece is parfocal.  In reading threads over the years I've noticed that some people consider eyepieces from a line that only require a small tweak in focus to be parfocal.  However, others may consider an eyepiece line or a zoom eyepiece as parfocal if there is zero focus adjustment required when switching eyepieces or zooming the zoom eyepiece.

 

I'm just curious what "parfocal" means to different people and if you think there can be a slight focus adjustment for a parfocal eyepiece line then how much focus adjustment would you allow?

I mean for me it's simple - if I have to touch the focuser knob after changing zoom level or eyepiece, it's not parfocal.

 

That doesn't mean that pseudo-parfocal isn't useful. For instance, my 19 Pan, 4.5 Delos, and 3 Delite are close enough in focus that I can use them to zero in on small planetary nebulae. 19 Pan to find, 4.5 Delos to center, 3 Delite to observe. Since stars remain mostly points rather than donuts, I don't lose the planetary as easily when increasing magnification.

 

However, if I bought a zoom that labeled itself parfocal for the express purpose of planetary observing, then I would expect it to be *exactly* the same focus throughout the zoom range.


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#11 LDW47

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Posted 21 September 2020 - 10:16 AM

I have asked on other posts but I never got a straight answer, who could, who would design and build a completely, a perfectly parfocal zoom ep, maybe they could but you wouldn’t pay the price. So a tweakingly close zoom is close enough to parfocal for all intents and purposes, as a definition, either that or lets forget this crazy, everlasting argument in relation to zooms as its getting ridiculous and viewing times a wastin’ ! I have been just as bad as the next guy, lol !



#12 LDW47

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Posted 21 September 2020 - 10:20 AM

I mean for me it's simple - if I have to touch the focuser knob after changing zoom level or eyepiece, it's not parfocal.

 

That doesn't mean that pseudo-parfocal isn't useful. For instance, my 19 Pan, 4.5 Delos, and 3 Delite are close enough in focus that I can use them to zero in on small planetary nebulae. 19 Pan to find, 4.5 Delos to center, 3 Delite to observe. Since stars remain mostly points rather than donuts, I don't lose the planetary as easily when increasing magnification.

 

However, if I bought a zoom that labeled itself parfocal for the express purpose of planetary observing, then I would expect it to be *exactly* the same focus throughout the zoom range.

Instead of ‘pseudo’ just use a simple word ‘almost’ this non winning argument could go on forever !  For me I’m going back to looking through my ‘maybe‘ parfocal zooms and I’m keeping my hands off the focuser, lol !



#13 sunnyday

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Posted 21 September 2020 - 10:23 AM

Honestly, I don't worry over this...that's what the twirly knobs are for, and I ain't afraid to use them.

+1 , it is faster to focus, than to take a breath.



#14 mkothe

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Posted 21 September 2020 - 10:49 AM

I have asked on other posts but I never got a straight answer, who could, who would design and build a completely, a perfectly parfocal zoom ep, maybe they could but you wouldn’t pay the price. So a tweakingly close zoom is close enough to parfocal for all intents and purposes, as a definition, either that or lets forget this crazy, everlasting argument in relation to zooms as its getting ridiculous and viewing times a wastin’ ! I have been just as bad as the next guy, lol !

I believe the amount of refocus required for a given zoom differs from person to person (as does the opinion of how annoying it is to refocus). My understanding is that older people are more likely to have to refocus due to their aging eyes losing the ability to compensate for small changes in focus. I see you are in your 70s - maybe you are lucky to have "younger" eyes, or the theory is not correct. 

 

If you need to refocus only a little bit, but others more so, I am not surprised to hear that. Presumably this would not be the case for a truly parfocal zoom, so if I hear comments from others that a given zoom requires them to refocus, I assume it is not parfocal.

 

It should be possible to devise a setup where the amount of refocus required can be measured objectively, maybe by using a camera with fixed focus.

 

Michael


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#15 LDW47

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Posted 21 September 2020 - 11:12 AM

I believe the amount of refocus required for a given zoom differs from person to person (as does the opinion of how annoying it is to refocus). My understanding is that older people are more likely to have to refocus due to their aging eyes losing the ability to compensate for small changes in focus. I see you are in your 70s - maybe you are lucky to have "younger" eyes, or the theory is not correct. 

 

If you need to refocus only a little bit, but others more so, I am not surprised to hear that. Presumably this would not be the case for a truly parfocal zoom, so if I hear comments from others that a given zoom requires them to refocus, I assume it is not parfocal.

 

It should be possible to devise a setup where the amount of refocus required can be measured objectively, maybe by using a camera with fixed focus.

 

Michael

That doesn’t answer my question on parfocal which is ‘ who could even design build a perfectly parfocal zoom eyepiece where no slight tweaking is required’ not always but sometimes under some conditions, if the answer is no one by way of no answers then that leaves it all wide open and everyone is right in their own world of zooms ! At that point the ridiculousness of this whole ongoing  discussion should be dropped, I mean I’m happy with my parfocal Svbony zooms, lol !   PS:  Think about what is happening, two groups firing across each others bow but getting no where ! This isn’t the first time on some subject, lol !


Edited by LDW47, 21 September 2020 - 11:15 AM.


#16 mkothe

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Posted 21 September 2020 - 12:01 PM

That doesn’t answer my question on parfocal which is ‘ who could even design build a perfectly parfocal zoom eyepiece where no slight tweaking is required’ not always but sometimes under some conditions, if the answer is no one by way of no answers then that leaves it all wide open and everyone is right in their own world of zooms ! At that point the ridiculousness of this whole ongoing  discussion should be dropped, I mean I’m happy with my parfocal Svbony zooms, lol !   PS:  Think about what is happening, two groups firing across each others bow but getting no where ! This isn’t the first time on some subject, lol !

 

In this post by Sarkikos, who seems pretty sensitive to this issue, the Nagler zooms are described as perfectly parfocal, and the Leica zoom as "nearly perfectly" parfocal . So maybe the Nagler zooms fit the criterion.

 

In my experience the Swarovski zoom is nearly parfocal, and that was similar with the Leica zoom which I used to own briefly at the same time. The Baader was not parfocal for me, but it has been a while and I don't remember which version I had. 

 

I think I will buy a Svbony to see for myself now (excuse is to give it to the kids :grin: ). Which Svbony zoom were we discussing?  :confused:  :lol:

 

BTW, I don't see this as an adversarial or ridiculous discussion; it helps me better understand the issues involved in testing eyepieces and how to interpret other's opinions.

 

If it's parfocal to you, that's all that matters (to you) smile.gif.

 

Best,

Michael


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#17 martinl

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Posted 21 September 2020 - 12:10 PM

It seems (from this and previous discussions) that the subjective definition of “parfocal” is something like “close enough not to bother the end user”. 
 

Personally, I’m always tweaking the focus. A fraction of a turn of the focuser therefor still counts as “parfocal enough” for me. If the image, after changing EPs, is so out of focus that I cannot easily recognise what I’m looking at then it is most definitely not parfocal. This is particularly important for star hopping where I may step up in magnification several times to zero in on a target. Major refocusing at that stage may throw the whole process out and I have to start from scratch.


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#18 LDW47

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Posted 21 September 2020 - 12:16 PM

It seems (from this and previous discussions) that the subjective definition of “parfocal” is something like “close enough not to bother the end user”. 
 

Personally, I’m always tweaking the focus. A fraction of a turn of the focuser therefor still counts as “parfocal enough” for me. If the image, after changing EPs, is so out of focus that I cannot easily recognise what I’m looking at then it is most definitely not parfocal. This is particularly important for star hopping where I may step up in magnification several times to zero in on a target. Major refocusing at that stage may throw the whole process out and I have to start from scratch.

An excellent to the point comment of what its all about in real life !



#19 LDW47

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Posted 21 September 2020 - 12:22 PM

In this post by Sarkikos, who seems pretty sensitive to this issue, the Nagler zooms are described as perfectly parfocal, and the Leica zoom as "nearly perfectly" parfocal . So maybe the Nagler zooms fit the criterion.

 

In my experience the Swarovski zoom is nearly parfocal, and that was similar with the Leica zoom which I used to own briefly at the same time. The Baader was not parfocal for me, but it has been a while and I don't remember which version I had. 

 

I think I will buy a Svbony to see for myself now (excuse is to give it to the kids grin.gif ). Which Svbony zoom were we discussing?  confused1.gif  lol.gif

 

BTW, I don't see this as an adversarial or ridiculous discussion; it helps me better understand the issues involved in testing eyepieces and how to interpret other's opinions.

 

If it's parfocal to you, that's all that matters (to you) smile.gif.

 

Best,

Michael

If your last statement is the case than why argue what ?  And this has turned into an argument not a discussion and it happens quite often, lol !



#20 sunnyday

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Posted 21 September 2020 - 12:36 PM

belushi.gif



#21 Peter Besenbruch

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Posted 21 September 2020 - 02:03 PM

Honestly, I don't worry over this...that's what the twirly knobs are for, and I ain't afraid to use them.

Twirly knobs, I gotta get me some of those.


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#22 Peter Besenbruch

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Posted 21 September 2020 - 02:41 PM

In my stable of currently used eyepieces, the Baader zoom requires refocusing when changing focal lengths. The Svbony 7-21 requires a bit more refocusing than the Baader. When it comes to parfocal, some of my fixed focus eyepieces from different families are parfocal. The 15mm UFF is parfocal with the 12mm WA. The other WAs are off by about as much as the Svbony zoom.

 

Truly parfocal eyepieces get handier when you have a mount that jiggles when refocusing. The more jiggle, the handier parfocalism is.



#23 Magnetic Field

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Posted 21 September 2020 - 03:24 PM

My two cents:

 

- I believe Don would say the manufacturers use a definition that allows for a maybe a millimeter of refocusing.  

 

-  The depth of focus of a telescope depends on it's focal ratio.  At F/10, that's about 0.22mm (0.008") at F/5 it's about 0.055mm (0.002")

 

- My own definition:  I would call it "perfectly parfocal."  No focusing is required when swapping eyepieces.  I find that the 16mm Type 5 Nagler plus all the type 6's except the 3.5mm (don't know about the 2.5mm) are perfectly parfocal in my scopes, they're mostly around F/5.  

 

Now there is one caveat.  When going to a shorter focal length, because my eye is better able to resolve the image, I may try to refocus.  But when switching back to the longer focal length, with the above eyepieces, I feel no need to refocus.  

 

- The Baader 8mm-24mm zoom is definitely not perfectly parfocal.  Even relatively small changes require refocusing.

 

Jon

I think parfocality cannot be defined in an absolute sense. It is only relative. Because if someone only had 1 eyepiece how would he know it is parfocal.

 

For me it is also relative but within the same manufacturer: say two eyepieces from the same manufacturer (e.g. 3mm, 10mm) that require no new focusing after swapping.

 

Or my Tele Vue Zoom 3-6mm which claims to be parfocal. It think the claim is true.

 

 

But my 6mm Vixen NPL swapping for the TV Zoom 6mm setting has to be refocused or vice versa. 


Edited by Magnetic Field, 21 September 2020 - 03:25 PM.


#24 Starman1

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Posted 21 September 2020 - 03:39 PM

A current thread has some debate about whether or not a particular zoom eyepiece is parfocal.  In reading threads over the years I've noticed that some people consider eyepieces from a line that only require a small tweak in focus to be parfocal.  However, others may consider an eyepiece line or a zoom eyepiece as parfocal if there is zero focus adjustment required when switching eyepieces or zooming the zoom eyepiece.

 

I'm just curious what "parfocal" means to different people and if you think there can be a slight focus adjustment for a parfocal eyepiece line then how much focus adjustment would you allow?

The industry standard is +/- 1mm.  if two eyepieces focus in that range, they are considered parfocal.

For me personally, EXACT parfocality is impossible because the eye focuses slightly differently with different exit pupils.

But, if the eyepiece, when inserted, is close enough to the focus of the previous eyepiece that a small planetary nebula is still identifiable as a nebula and not a star,

then it's close enough to parfocal to consider it parfocal.

 

For a technical answer, see Ernest's.

For a good answer how to use a zoom, see Jon's: focus at the highest power and then move down--the eyepiece will seem to stay in close focus.

Focus at low power and move up and you will need to refocus at the higher powers.

 

That is, of course, if the zoom is more or less parfocal across its range, and many zooms are not.

But so?  The purpose of a zoom is to have an entire kit of eyepieces in one eyepiece so you always find the right magnification.

That doesn't change if a tiny amount of refocusing is necessary.



#25 SteveG

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Posted 21 September 2020 - 04:17 PM

Parfocal is impossible to define between various users, because we all focus our scopes a little differently due to the differences in our vision

 

I'm happy if it's just close.


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