By Alexander Merz · Published on · Details checked on 6 November 2026
Vignetting is the fall-off in brightness towards the corners of an image: the edges of a photo receive less light than the centre. In astrophotography it is more than a cosmetic flaw, because brightening the image later also amplifies the dark corners.
⭐ In short: For daytime photos a slider is enough; for astrophotos you need flats. Stopping down helps with camera lenses; against adapters that are too narrow on a telescope, only a larger connection helps. Whatever remains is corrected by flat frames together with dust shadows in a single step.
Four types of vignetting
Vignetting has four causes, and not every one can be stopped down away. The important distinction is between light that is cut off on its way and light that arrives at the sensor at an angle and is therefore weaker (German Wikipedia: Vignettierung; Wikipedia: Vignetting).
| Type | Cause | Stopping down helps? | Flats correct it? |
|---|---|---|---|
| Natural | oblique incidence of light, fall-off according to the cos⁴ law | no | yes |
| Optical | lens barrels inside the lens cut off marginal rays | yes, usually 2–4 stops | yes |
| Mechanical | lens hood, filter, adapter that is too narrow | hardly | only with slight shading |
| Pixel vignetting | sensor pixels capture oblique light less well | no | yes |
Natural vignetting is pure geometry. Brightness falls with the fourth power of the cosine of the angle of incidence (Wikipedia). At 20° at the edge of the image that means about 22 percent less light, at 30° already about 44 percent (our own calculation: cos⁴ 20° ≈ 0.78; cos⁴ 30° ≈ 0.56).
Why astrophotos are so sensitive
After stacking, an astrophoto is brightened strongly – this is called stretching. In the process, tiny differences in brightness in the background become visible, because that is exactly where the faint nebulae lie. A fall-off in brightness of just a few percent thus turns into a dark frame around the image.
Gradient tools are no substitute for calibration. They smooth out large-scale gradients but easily mistake faint nebulae for background. Dust shadows on the filter or sensor – the typical dark rings – they do not catch at all.
Bottlenecks: 1.25 inch, T2 and full frame
On a telescope, the strongest vignetting often originates in the accessories, not in the optics. Every barrel, every thread and every filter in the light path has to pass the whole light cone for the sensor diagonal. If an opening is smaller than the diagonal, it mechanically shades the corners.
| Bottleneck | Diameter | Suitable for |
|---|---|---|
| 1.25-inch barrel | 31.75 mm outside, less inside | planetary and small astro cameras; tight with APS-C |
| T2 thread (M42 × 0.75) | 42 mm thread, smaller clear aperture | APS-C (diagonal approx. 27–28 mm) |
| Full-frame sensor 36 × 24 mm | diagonal approx. 43.3 mm | needs larger connections than T2 |
| 2-inch barrel | 50.8 mm outside | APS-C safe, full frame depending on the setup |
If you shoot full frame on a telescope, you hit the limit with the T2 thread. A user of the German-language forum asked exactly this question on 27 Oct 2026 for a Canon EOS 6D Mark II on a 14-inch Meade; the advice was wide connections via larger threads instead of the standard T2 (forum.astronomie.de, in German). With APS-C, on the other hand, T2 remains the standard; more on this under T2 adapters.
Correcting vignetting with flats
Flats are exposures of an evenly lit surface through exactly your setup. They show only the unevenness that the optics themselves add: vignetting and dust shadows. During stacking, each single frame is divided by the master flat (Siril documentation).
- Change nothing: focus, camera rotation and filter stay exactly as they were for the lights – otherwise the dust shadows no longer match.
- Even light: a flat-field box or a light panel in front of the telescope aperture.
- Expose correctly: the histogram should sit roughly in the middle and must not touch either end; Siril gives 80 percent as the upper limit for the right edge of the peak.
- Take a series of flats, plus bias or dark flats to calibrate them (Siril).
Tip: With narrowband filters, flats take considerably longer. Users of the German-language forum report 4–8 seconds instead of the usual 2 seconds with a dual-band filter; the goal is a histogram peak at 40–50 percent (forum.astronomie.de, 14–15 Oct 2026, in German). How darks and bias fit in is explained under dark frames.
Flats correct light that is missing – not light that interferes. In the same forum thread, one user traced ring-shaped gradients that persisted despite flats to reflections from a shiny metal edge at the exit of the field flattener. Blackening and baffles help against reflections and stray light, not calibration frames.
Vignetting with camera lenses
With a lens, stopping down helps first. Optical vignetting usually disappears after two to four stops (German Wikipedia). For star photos, even one stop is a good compromise, because it also improves edge sharpness and colour fringing.
For daytime photos, the camera and RAW program often correct vignetting automatically. Many cameras already remove optical and pixel vignetting when generating the JPEG (Wikipedia). For astrophotos, flats replace these profiles, because they also capture dust and filters.
| Situation | Best remedy |
|---|---|
| Daytime photo, slightly dark corners | lens profile in the RAW program |
| Star photo with a lens on a tracker | stop down by one f-stop plus flats |
| Telescope with camera | suitable connection plus flats |
| Dark rings in the image | flats (dust shadows) |
| Rings despite flats | look for reflections, blacken the light path |
Less vignetting starts with the telescope. Refractors designed for cameras, such as the William Optics RedCat 51 Gen 3 WIFDWilliam Optics RedCat 51 WIFD Petzval Apo, come with a built-in field flattener – one adapter fewer in the light path. Which optics suit which sensor is covered under deep-sky astrophotography.
FAQ about vignetting
What is vignetting in photos?
An image that gets darker towards the edges. It is caused by oblique incidence of light, by lens barrels inside the lens or by accessories that cut off marginal rays.
Is vignetting a problem?
In portraits it is often even added on purpose. In astrophotography it is a nuisance, because stretching the image amplifies it strongly.
How do I remove vignetting in Lightroom or Photoshop?
Via the lens correction or the vignetting slider. For astrophotos, the better solution is calibration with flats before stacking.
Where do dark rings in an astrophoto come from?
Usually from dust particles on the filter or sensor window, whose out-of-focus shadows appear as doughnuts. Flats remove them, as long as nothing was moved between lights and flats.
Do I need flats if I remove gradients with software?
Yes, if you want to bring out faint nebulae. Gradient tools smooth out gradients, but they do not remove dust shadows and can also remove faint parts of nebulae.
Read more
- Dark frames, bias and flats – the complete calibration
- Chromatic aberration – the second typical optical error
- Astrophotography guide – including lenses and what matters at the edge of the image
Sources: German Wikipedia: Vignettierung · Wikipedia: Vignetting · Siril: Calibration · German Wikipedia: T2 mount · German Wikipedia: APS-C · Experiences from the German-language forum forum.astronomie.de (threads linked in the text). cos⁴ values and diagonals: our own calculation.
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