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# What Percentage Of Field Of View Is Lost, When Using A Barlow Lens?

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44 replies to this topic

Viking 1

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Posted 12 January 2018 - 10:42 PM

Can anybody keep the science/math easy on this question? Thanks to you all.

### #2 gezak22

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Posted 12 January 2018 - 11:06 PM

A 2x Barlow will reduce the TFOV* by 2x.

Edit: *of a given eyepiece

Edited by gezak22, 12 January 2018 - 11:56 PM.

### #3 aeajr

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Posted 12 January 2018 - 11:25 PM

Can anybody keep the science/math easy on this question? Thanks to you all.

No field of view should be lost by using a barlow.  As magnification goes up, field of view goes down. That would be true whether you use a barlow or not.

A barlow lens increases magnification by some factor.

A 2X barlow will double the magnification of any given eyepiece.  But it does not affect that apparent field of view of the eyepiece, the AFOV.

For example:

Scope of 1200 mm FL

10 mm Plossl ( 52 degree AFOV)

1200 / 10 = 120X

52/120 = .43 degree FOV

Put that same eyepiece in a 2X barlow

1200/10 = 120 X 2 = 240X

52 / 240X = .215  degree FOV

If we do the same calculation but instead of using a 10 mm in a 2X barlow we use a 5 mm eyepiece the result would be exactly the same.  The barlow does not change the AFOV of the eyepiece, only the magnification that it produces.

Edited by aeajr, 13 January 2018 - 12:16 AM.

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### #4 Mitrovarr

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Posted 12 January 2018 - 11:29 PM

Sometimes barlows vignette the field and you do lose AFOV, but not ideally.

### #5 44ye

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Posted 12 January 2018 - 11:49 PM

Try this with your list of equipment

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

Viking 1

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Posted 13 January 2018 - 02:55 AM

Can anybody keep the science/math easy on this question? Thanks to you all.

No field of view should be lost by using a barlow.  As magnification goes up, field of view goes down. That would be true whether you use a barlow or not.

A barlow lens increases magnification by some factor.

A 2X barlow will double the magnification of any given eyepiece.  But it does not affect that apparent field of view of the eyepiece, the AFOV.

For example:

Scope of 1200 mm FL

10 mm Plossl ( 52 degree AFOV)

1200 / 10 = 120X

52/120 = .43 degree FOV

Put that same eyepiece in a 2X barlow

1200/10 = 120 X 2 = 240X

52 / 240X = .215  degree FOV

If we do the same calculation but instead of using a 10 mm in a 2X barlow we use a 5 mm eyepiece the result would be exactly the same.  The barlow does not change the AFOV of the eyepiece, only the magnification that it produces.

Cool

### #7 B 26354

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Posted 13 January 2018 - 02:57 AM

The barlow does not change the AFOV of the eyepiece, only the magnification that it produces.

Seems fairly obvious that no external optical element will have any affect on an eyepiece's intrinsic AFOV

I had the distinct impression that the OP was asking about the actual field of view (TFOV), as opposed to the eyepiece's AFOV.

As gezak22 pointed out... twice the magnification = half the TFOV.

Edited by B 26354, 13 January 2018 - 02:59 AM.

### #8 DHEB

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Posted 13 January 2018 - 03:11 AM

The barlow does not change the AFOV of the eyepiece, only the magnification that it produces.

Seems fairly obvious that no external optical element will have any affect on an eyepiece's intrinsic AFOV

I had the distinct impression that the OP was asking about the actual field of view (TFOV), as opposed to the eyepiece's AFOV.

As gezak22 pointed out... twice the magnification = half the TFOV.

Minus some potential vignetting.
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### #9 Aleko

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Posted 13 January 2018 - 04:20 AM

As already pointed out, a 2x Barlow reduces the TFOV by half, meaning the diameter of the field is half.  Keep in mind though that the area of your field of view would be only 1/4.

Alex

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### #10 jallbery

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Posted 13 January 2018 - 05:24 AM

Ideally speaking, a Barlow does not the reduce AFoV at all, and so the reduction of TFoV is exactly equal to the magnification factor.  However...

• A 2x Barlow seldom has a magnification factor of exactly 2x; the mag factor can vary by plus or minus 10% or even more.
• Even if a Barlow has exactly a 2x mag factor with a specific eyepiece, it will not have exactly a 2x factor with other eyepieces that are not parfocal with that first eyepiece
• Use of a Barlow can cause vignetting, which is most likely to occur with longer focal length eyepieces

More exotic Barlow-like devices (e.g. Televue Powermates) typically deliver a consistent mag factor and are less susceptible to vignetting.

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### #11 aeajr

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Posted 13 January 2018 - 07:34 AM

The barlow does not change the AFOV of the eyepiece, only the magnification that it produces.

Seems fairly obvious that no external optical element will have any affect on an eyepiece's intrinsic AFOV

I had the distinct impression that the OP was asking about the actual field of view (TFOV), as opposed to the eyepiece's AFOV.

As gezak22 pointed out... twice the magnification = half the TFOV.

Was not obvious to me when I first started.   Someone had to point it out to me.

Everything is easy when you know how.  Everything is hard when you don't.

### #12 Ernest_SPB

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Posted 13 January 2018 - 10:10 AM

Using Barlow we lost TFOV (twice for 2x LB, three for 3x LB).

Using simple (one component) Barlow we can lost certain percent of AFOV cutting it by edge of lens. The percent depends from EP construction and can not be evaluated. UWA and XWA in combination with short LB (especially with mag>3x) more suffer from the cutting.

Using so named telecentric Barlow (two components, like Powermate) yuo are guarantied from such loss in AFOV

Edited by Ernest_SPB, 13 January 2018 - 10:15 AM.

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### #13 SteveG

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Posted 13 January 2018 - 04:36 PM

I believe he's talking about vignetting, from another post discussing barlows. Using a Shorty style barlow on a longer focal length eyepiece will often vignette the afov. The answer is not as easy and will depend on the eyepiece and barlow combination. Typically, when there is vignetting, it is only 5-10 %.

### #14 MCovington

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Posted 13 January 2018 - 05:25 PM

Can anybody keep the science/math easy on this question? Thanks to you all.

Exactly the same as the magnification of the Barlow.

A 2x Barlow cuts your true field of view in half.

The reason is that the field stop of the eyepiece is still the same.  If you double the size of the image there, its diameter only covers half as much distance in the sky.

### #15 Starman1

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Posted 13 January 2018 - 06:17 PM

Can anybody keep the science/math easy on this question? Thanks to you all.

Apparent field--stays the same

True Field--becomes 1/2 as wide and 1/4 the area (assuming a 2X Barlow).

It's the same as using an eyepiece with the identical apparent field and 1/2 the focal length (or 1/3 for a 3X Barlow).

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### #16 MartinPond

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Posted 14 January 2018 - 12:53 AM

"---It's the same as using an eyepiece with the identical apparent field and 1/2 the focal length (or 1/3 for a 3X Barlow).---"

Consequently, the Barlow, at the higher resulting power,

cannot take field from your view when it is the eyepiece that limited

it at the lower power.  At  80x, your TFOV is going to go down anyway

if you started at 40x, for the exact same apparent field of view.

Trying to clarify...hope I didn't complicate.

It's another one of those areas where a rule of thimb doesn't make sense,

except to say it cannot reduce apparent field.

### #17 Jon Isaacs

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Posted 14 January 2018 - 12:46 PM

In this case,  I think it is easiest to understand if one thinks of the Barlow as increasing the focal length of the telescope. A 2X Barlow doubles the focal length which halves the TFoV.

But as Ernest and others have pointed out,  this an idealized Barlow.  A real Barlow can cause vignetting which reduces the AFoV.

Jon

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### #18 MartinPond

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Posted 14 January 2018 - 06:55 PM

True...another issue for short barrels.

BUT:    there is value to testing:

-------------

With an F5 scope, I cannot get

---my thick 17mm or 15mm Plossls, the Thinner Meade Super-Plossl 12.4mm,

or the Astro-Tech High-Grade 17mm Plossl (an asym-dimple thicky)

to vignette at all with the

---Orion Tri-Mag (3X) or a dulled-interior Meade #126 Shorty (2X).

---------------

I did some further research....

there is an interesting instance that depends on the TV Plossl type to happen:

https://www.cloudyni...h-tv-2x-barlow/

(there may be others)

Mike Hosea says:

--------------------------------

Al Nagler said that he could have solved the problem by making the

(Plossl) lenses thicker (presumably so he could also make them wider),

but this would have reduced eye relief. What you have is the trade-off that the designer

chose. To avoid most of the vignetting, you need to use a 2.5x Powermate rather than a Barlow.

----------------------------------

So...now I think I know why that doesn't happen with my "Thick Plossls" with an F5.

I don't use any less than a 15mm FL Plossl anyway....I do not like too-tight

eye relief at 10mm or 6mm.  What's an extra 2mm when your ER at 6fl is 4mm?  Ouch.

---------------------------------------------

So, I can't get the vignetting to occur across a variety of Plossls and Barlows,

but the article indicates that the combination of Televue Plossls and Televue Barlow has the issue.

This seems like far from a universal problem..

Photoist seems to confirm my experience, but says the TV Barlow itself isn't associated with the problem:

------Photoist---------------------------"

did some more tests and I found that when I use the 2x TV Barlow with the Meade series 4000 (Japan) 9.7mm, 15mm, and 20mm there are no issues.

Ditto for 12mm and 16mm Brandons - they work just fine as well.

---------------------------------"

The vignetting issue looks like a hasty generalization error

based on very specific cases.

Maybe there are other eyepieces, but at least at F5,

saying it happens with all eyepieces and all Barlows is trivial to shoot down.

This is not a general rule.

If it's more common at F4, I would suggest a screw on cell at the eyepiece and then a regular Barlow,

to "get round the bend".

Edited by MartinPond, 14 January 2018 - 08:53 PM.

### #19 Starman1

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Posted 15 January 2018 - 12:14 AM

The TeleVue Plössls all vignette slightly, without a Barlow.

So it's not surprising a Barlow reveals it.

### #20 MartinPond

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Posted 16 January 2018 - 12:18 AM

The TeleVue Plössls all vignette slightly, without a Barlow.

So it's not surprising a Barlow reveals it.

.

So far, that is the only eyepiece line

where the "Barlow vignetting"  I have seen reports detecting.

If there are others, they must be quite rare.

Unless...there are special conditions, like with an F4 scope.

Edited by MartinPond, 16 January 2018 - 12:19 AM.

### #21 Starman1

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Posted 16 January 2018 - 12:58 AM

A lot of 32mm Plössls vignette in Barlows.

Aggressive baffling and large field stops don't work well together.

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### #22 Mitrovarr

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Posted 16 January 2018 - 01:05 AM

In my experience, vignetting in barlows is more likely if...

1. The eyepieces has a high FL.

2. The barlow has a high power (more than 2x)

3. The barlow is cheap.

4. The barlow is short.

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### #23 howardcano

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Posted 16 January 2018 - 08:02 AM

The TeleVue Plössls all vignette slightly, without a Barlow.

So it's not surprising a Barlow reveals it.

.

So far, that is the only eyepiece line

where the "Barlow vignetting"  I have seen reports detecting.

If there are others, they must be quite rare.

Unless...there are special conditions, like with an F4 scope.

I have three different 32mm Plossls and two different 40mm Plossls.  All of them vignette badly at F/6 with a 2x shorty Barlow, and they're not much better with a long Barlow.

Edited by howardcano, 16 January 2018 - 08:03 AM.

### #24 DHEB

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Posted 16 January 2018 - 09:19 AM

According to the book "Advanced Amateur Astronomy" by Gerald North, a Barlow will vignette the image reaching the focus when the diameter of its lens is under a certain minimum D:

D = 1/a * (v/F + d)

where

a = amplification factor

v = distance to Barlow's focus

F = focal ratio of the telescope

d = diameter of the required fully illuminated field at the focus.

Those who have the book may find the explanation given in figure 2.2, page 38 (1997 edition).

I made some calculations but I reached the wrong conclusions because could not properly understand which diameter is meant by d. May be more on this later.

Edited by DHEB, 16 January 2018 - 10:04 AM.

### #25 aeajr

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Posted 16 January 2018 - 09:48 AM

In this case,  I think it is easiest to understand if one thinks of the Barlow as increasing the focal length of the telescope. A 2X Barlow doubles the focal length which halves the TFoV.

But as Ernest and others have pointed out,  this an idealized Barlow.  A real Barlow can cause vignetting which reduces the AFoV.

Jon

Jon,

Help me understand this.

AFOV is a specification of the eyepiece.   It would seem to me that that does not change.

True field of view is the actual result of eyepiece and everything in the light path.

If you vignette I can understand that you are reducing the true field of view because part of the light path is cut off, but AFOV of the eyepiece should not change.  How could it change?

Just trying to understand the optics and how this all works.   I am no optics engineer but I did study optics in Physics in college, back when were using punch cards for computer input.

Also, would it matter if it is a shorty barlow vs. a long tube barlow?  It would seem a long tube barlow would be more likely to vignette.

Edited by aeajr, 16 January 2018 - 10:33 AM.

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