- Vignetting 1: Overview
- Vignetting 2: Parametric Correction
- Vignetting 3: Correction Images
- Vignetting 4: Photoshop Divide Layer
- Vignetting 5: A Pathological Example
- Vignetting 6: Other Options and Summary (this post)
For a little perspective on what we’ve been dealing with, we can posterize high-contrast versions of our flat fields to better appreciate the topology of vignetting in different circumstances:
Far from exhausting the possibilities for vignetting correction in this series of posts, there are many other options to consider. Here are a few:
Capture One Pro
If you are using Phase One’s excellent Capture One Pro to process your images, the Lens Correction Tab includes tools to not only correct images for lenses in their database but you can create and apply LCCs (Lens Cast Calibration) images. It uses flat field images like we’ve been talking about, to enable you to selectively correct for Color Cast, Dust Removal, or Enable Uniform Light in any combination. It works really well. The tab also includes the more conventional parametric sliders for vignetting.
Hugin is freely-available panorama stitching software that has a number of tools available for image corrections. The fulla command-line tool in the package enables a couple of different ways to address vignetting. One way is to use the –flatfield command line option to specify a flat-field image used for a division operation very similar to what we did using ImageMagick commands in an earlier post.
Another option offered in fulla is the –vignette command-line option used to specify 4 coefficients for a 6th-order polynomial describing the fall-off curve for radially-symmetric vignetting. I’ve used it, it works quite well, but it’s probably not for anyone who recoiled when I mentioned 6th-order polynomial coefficients.
Although possibly worthy of another entire post, it should be mentioned that vignetting can be corrected for video in a number of ways too. Besides the usual parametric slider corrections, Adobe After Effects, for example, provides layer blending modes. You can use the same techniques we’ve been discussing to create a correction layer above a clip and change the blending mode to Divide or Multiply, as we’ve previously demonstrated with Photoshop, to get accurate vignetting control.
Contrary to what a 6-part blog series on destroying vignetting might indicate, I don’t have a visceral hatred of vignetting. But vignetting is a consequence of the camera, lens, and settings you use; it’s not a characteristic of the subject you’re shooting. I generally don’t want it to be a noticeable component of an image unless there’s a specific reason for it. Maybe adding some judicious vignetting to an image can improve it but that added vignetting is likely to be of different character and magnitude than the system’s innate vignetting. Vignetting often seems gimmicky and detracts from an image but at least there are techniques available to fully control it on your terms.
There are, like anything else, numerous explanations, tutorials, and remedies on the Internet that address vignetting. Some are even accurate. Much is (possibly well-intentioned) bad advice. For example, there are discussions here and there about creating radial gradients and applying them in layers using Multiply or whatever. These are often accompanied by some hand-waving about tuning with opacity adjustments etc. (to disguise the damage done to the image). Ignore all that. Radial gradients are linear by nature and, as we’ve seen, real-world vignetting is not linear at all. Making a linear adjustment to an exponential problem just shoves pixel values around, improving one thing, creating problems elsewhere. Bogus, bogus, bogus. If you want quick and easy, stick with parametric vignetting adjustments common in almost all image processing software.
There are a few high-level lessons I’ve learned about vignetting and vignette control.
- The best, most accurate, vignetting adjustment is enabled by your creation of good flat field images. Those flat fields can then be used to either determine the optimal settings for parametric slider adjustments or can be converted to highly-accurate correction images themselves.
- For radially-symmetric vignetting (the most common case) optimal parametric settings are usually “good enough”, creating results that the eye is quite satisfied with.
- For most accurate results, or for the occasional oddball non-radially-symmetric case, flat field correction images are the way to go. They can be used in a number of tools in different ways: command-line manipulation in ImageMagick and layer blending modes in Photoshop being two we focused on. (Under the covers they’re doing the same thing.)
- There’s still a lot more we haven’t covered in this series but we have covered some solid tools to address the most common problems. There’s little practical need to get any deeper into this swamp than we already have.