Reduction of lens vignetting
In Nebulb you have a tool for correcting the vignetting introduced by the lens.
And as with the reduction of chromatic aberration, the reduction of vignetting is also based on an estimation made directly on the image itself (no preset value is used for each lens). Thanks to this, it is possible to apply the vignetting correction on any lens, even if the metadata of the lens is not saved in the image file.
IMPORTANT: Vignetting correction (although it is located within the "Preprocess" module) can be applied both before and after stacking. In fact, it is usually adjusted after stacking, since Nebulb automatically applies a series of corrections during stacking (in order to always obtain the best possible result), so you don't have to worry about adjusting it before stacking.
Vignetting estimation
Nebulb automatically performs vignetting estimation when importing images, i.e. the software estimates the effect of vignetting on the image based on an analysis of the image.
And this vignetting estimation is represented in a graph, which also serves as an assistance when you want to apply the correction.
This graph shows 3 curves:
- Gray curve: This is the intensity profile obtained by Nebulb when analyzing the image, which is used to estimate the vignetting of the image. If the image had no vignetting at all this curve would be flat, so the downward shape of this curve indicates the darkening caused by vignetting in the image. In case no correction has been applied (the "Correction" slider is set to 0) the orange curve will coincide exactly with the gray curve and hide it, since the orange curve is always above it.
- Blue curve: This is the vignetting estimation curve obtained by Nebulb from the gray curve. As you can see in the previous image, this curve follows perfectly the trajectory of the gray curve, but it is much smoother (as it is a noise-free estimation).
- Orange curve: Represents the vignetting correction, i.e. how the intensity profile of the image has changed as a function of the applied correction. In case no correction is applied, the orange curve coincides exactly with the gray one. And in the case where 100% vignetting correction is applied, the orange curve is flat.
Refresh vignetting estimation
Nebulb uses the sky to estimate vignetting, since, it is the most uniform area of the image.
However, when importing the images, the mask for the sky has not yet been defined, so Nebulb does not yet know where the sky is and, therefore, will probably not estimate the vignetting correctly at first. In that case, strange curves will appear that do not match the vignetting behavior, as you can see in the following example:
The above curves are those estimated by Nebulb when importing the following image:
And as you can see, the fact that there are objects that cover a large part of the sky causes Nebulb not to estimate the vignetting curves correctly at first.
However, this is something very easy to correct, since, in order for Nebulb to always estimate the vignetting correctly, you only have to do two things:
1. Define the sky mask:
2. Refresh the vignetting by clicking the button in the upper right corner of the graph:
As you can see, after defining the mask for the sky and pressing the refresh vignetting button, the vignetting estimation curves now perfectly match the vignetting present in the image.
IMPORTANT: The refreshing of the vignetting curves must always be done before the stacking is executed, since Nebulb uses these curves during the stacking and, therefore, they can no longer be modified once it is finished. For this reason, it is recommended that after defining the sky mask you always check that the curves of this graph are OK. However, in case you forget to do it (something that has happened to me from time to time) and the initial estimation of the curves is not good, nothing happens either (don't worry), it will just happen that you will not be able to apply the vignetting correction after stacking (but there are no more inconveniences than that).
Vignetting correction
Finally, once the vignetting estimate is correct, all that remains is to decide how much correction you want to apply (if any).
By default, the vignetting correction is not applied, i.e. the image is always displayed initially with the natural vignetting it has.
That said, it is very easy to correct the vignetting at any time (both before and after stacking) by simply adjusting the "Correction" slider to leave the image with the level of vignetting you like best.
Here are some example images of how the vignetting correction looks for different settings:
Image with original vignetting | Image with vignetting correction at 50% | Image with vignetting correction at 100% |
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