<p>There is two effects which will happen:</p>
<ul>
<li><p>if you magnify beyond the useful magnification you are essentially magnifying the aperture's diffraction pattern (thus the <a href="https://en.wikipedia.org/wiki/Airy_disk" rel="noreferrer">airy disk</a> for circular apertures without obstructions). Thus you make the light sources (or rather everything) larger, but you don't add resolution - which will result in a blurry impression. That's similar to zooming-in to a pixelized image: you don't get more resolution than the pixels, irrespective of how large you make or display those. See also e.g. <a href="https://www.edmundoptics.eu/knowledge-center/application-notes/imaging/limitations-on-resolution-and-contrast-the-airy-disk/" rel="noreferrer">here</a> for some numbers based on the <a href="https://en.wikipedia.org/wiki/F-number" rel="noreferrer">f-number</a> (ratio of aperture to focal length).</p>
</li>
<li><p>every surface in the optical path (a lens has two, a mirror one) adds optical errors due to manufacturing limits in smoothness and curvature. Additionally you add some degree of scattering in impurities within the lenses glass. Both effects reduce the effective contrast and thus the resolution slightly, too.</p>
</li>
</ul>

CC BY-SA 4.0
2022-10-17T13:42:47.317Z
Another point to add is that the object gets dimmer as the magnification goes up, as the same light is spread over a larger area.
Greg Miller
CC BY-SA 4.0
2022-10-16T18:09:51.200Z
2022-10-16T18:09:51.200Z
planetmaker
<p>If one stacked many Barlow lenses/used several focal extenders, what would the resulting image look like?</p>

CC BY-SA 4.0
2022-10-16T17:34:43.570Z
Well it depends on the telescope aperture, but likely the image would be very blurred.
amateurAstro
<telescope><barlow>
How would images with an overextended focal length look like?
2080
# How would images with an overextended focal length look like?

(Asked by: username_0 on 2022-10-16)

If one stacked many Barlow lenses/used several focal extenders, what would the resulting image look like?

**Tags:** telescope, barlow

> **Comments:**
> **username_1**: Well it depends on the telescope aperture, but likely the image would be very blurred.  

---

## 1 answers

### Answer by username_2 on 2022-10-16. Score: 6 | Accepted answer

There is two effects which will happen:

* if you magnify beyond the useful magnification you are essentially magnifying the aperture's diffraction pattern (thus the [airy disk](https://en.wikipedia.org/wiki/Airy_disk) for circular apertures without obstructions). Thus you make the light sources (or rather everything) larger, but you don't add resolution - which will result in a blurry impression. That's similar to zooming-in to a pixelized image: you don't get more resolution than the pixels, irrespective of how large you make or display those. See also e.g. [here](https://www.edmundoptics.eu/knowledge-center/application-notes/imaging/limitations-on-resolution-and-contrast-the-airy-disk/) for some numbers based on the [f-number](https://en.wikipedia.org/wiki/F-number) (ratio of aperture to focal length).
* every surface in the optical path (a lens has two, a mirror one) adds optical errors due to manufacturing limits in smoothness and curvature. Additionally you add some degree of scattering in impurities within the lenses glass. Both effects reduce the effective contrast and thus the resolution slightly, too.

> **Comments:**
> **username_3**: Another point to add is that the object gets dimmer as the magnification goes up, as the same light is spread over a larger area.  

---

