<p>Using a Barlow effectively increases the focal length of the telescope, leaving other factors the same.  So, the magnification will be the same as if you were using a 5mm eyepiece.</p>
<p>However, a cheap 10mm eyepiece usually has better eye relief and apparent field of view than a cheap 5mm.  So, you are actually better off with the Barlow than getting a 5mm.</p>
<p>The only downsides are you increase the number of surfaces (and thus loose light to reflections), and there might be some issues with focus if you have short focuser travel.</p>

CC BY-SA 4.0
2022-07-30T23:11:13.283Z
I keep thinking that there's got to be a catch somewhere, somehow. Ignoring reflections and chromatic and other aberrations due to imperfect optics, we can always get a boost in eye relief by using a Barlow with a lower magnification eyepiece without sacrificing anything else? That seems like an actually really good reason to use a Barlow lens!
uhoh
CC BY-SA 4.0
2022-07-30T23:59:09.680Z
@uhoh the main catch is that the Barlow costs money.  If it were free, they'd integrate it into every eyepiece.
Greg Miller
CC BY-SA 4.0
2022-07-31T23:45:41.283Z
@GregMiller IIRC some high end eye pieces do include a built in barlow; used with variable powers so they can gain economies of scale by reusing the same fancy optics for multiple eye pieces within a single series.
Dan Is Fiddling By Firelight
CC BY-SA 4.0
2022-07-30T19:22:07.040Z
2022-07-30T19:22:07.040Z
Greg Miller
<p>So I have been wondering if a barlow (say 2x) will produce the same result of a half eyepiece....
So would a 10mm with a 2X Barlow produce the same result as a 5mm? (Assuming the FOV is the same on the 10mm and the 5mm)....</p>
<p>Since I dont have any combination that would be &quot;equivalent&quot; I cant test the result</p>

CC BY-SA 4.0
2022-07-31T18:42:24.757Z
This site may be of service in the future: https://astronomy.tools/calculators/field_of_view/
coblr
<eyepiece><barlow>
Will a 10mm eyepiece with a barlow 2x produce the same result as a 5mm eyepiece?
Walucas
# Will a 10mm eyepiece with a barlow 2x produce the same result as a 5mm eyepiece?

(Asked by: username_0 on 2022-07-30)

So I have been wondering if a barlow (say 2x) will produce the same result of a half eyepiece....
So would a 10mm with a 2X Barlow produce the same result as a 5mm? (Assuming the FOV is the same on the 10mm and the 5mm)....

Since I dont have any combination that would be "equivalent" I cant test the result

**Tags:** eyepiece, barlow

> **Comments:**
> **username_1**: This site may be of service in the future: https://astronomy.tools/calculators/field_of_view/  

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## 1 answers

### Answer by username_2 on 2022-07-30. Score: 10 | Accepted answer

Using a Barlow effectively increases the focal length of the telescope, leaving other factors the same. So, the magnification will be the same as if you were using a 5mm eyepiece.

However, a cheap 10mm eyepiece usually has better eye relief and apparent field of view than a cheap 5mm. So, you are actually better off with the Barlow than getting a 5mm.

The only downsides are you increase the number of surfaces (and thus loose light to reflections), and there might be some issues with focus if you have short focuser travel.

> **Comments:**
> **username_3**: I keep thinking that there's got to be a catch somewhere, somehow. Ignoring reflections and chromatic and other aberrations due to imperfect optics, we can always get a boost in eye relief by using a Barlow with a lower magnification eyepiece without sacrificing anything else? That seems like an actually really good reason to use a Barlow lens!  
> **username_2**: @username_3 the main catch is that the Barlow costs money.  If it were free, they'd integrate it into every eyepiece.  
> **username_4**: @GregMiller IIRC some high end eye pieces do include a built in barlow; used with variable powers so they can gain economies of scale by reusing the same fancy optics for multiple eye pieces within a single series.  

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