Using CornerFix to Correct Images

The Problem
What is CornerFix?
So That's It?

The Problem

On both the Leica M8 and M9, you may end up with oddly colored or vignetted images (for slightly different yet similar reasons) and possibly only with particular lenses at varying apertures. Trying to fix this in post-processing is possible, but it's a wildly moving target and nothing corrects this as quickly and accurately as a program called CornerFix. It's an application designed primarily for correcting images thus plagued.

As a quick background, the M8 and M9 (as do most digital cameras generally) have an array of microlenses in front of the sensor. Think of them as funnels for photons. These photons are directed to the individual photosites below through a bayer filter, which essentially splits light into components of red, green and blue. On the M8 and M9, the microlenses are angled towards the center of the sensor, and thus are more angled the further towards the edges of the sensor you go; this is more extreme on the M9 since it's full frame (a bigger sensor). The problem arises when light is altered in some fashion before it reaches the photosites, causing one or more color components (RGB) to not register correctly. This is where the colorful names come from in describing these effects.

On the M8 you need to use UV/IR filters and on lenses generally wider than 35mm - can lead to "cyan corners" unless the lenses are coded. This happens more so with third-party lenses for which there is no correct code - only the closest match to a Leica lens. On the M9 you no longer need the filters, but have a new problem. Since the sensor is full frame, you're no longer cropping out the borders of your images. Using a physically short wide angle (as opposed to longer lenses and focal lengths) causes light hitting the microlenses in this border region to hit at a rather oblique angle to the microlenses. This leads to the "red edge" or "Italian flag" effect.

Two lenses come immediately to mind that are considered "unusable" on the M9 due to extreme red edge. The Zeiss C Biogon 4,5/21 ZM and Voigtländer 21mm f/4 Color Skopar. These extreme wide angles are rather short in physical length - or more precisely - protrude into the camera more, closer to the sensor than most lenses. The result is indeed extreme and essentially impossible to fix in normal post-processing. This is where CornerFix really comes to the rescue!

Even on a body and lens combination that does not cause such discolorations, you can still end up with optical and mechanical vignetting, or darkening of the corners. Especially on fast(er) lenses, wide open - but also wide angles. All of these are detrimental to a correctly and evenly exposed image.

What Is CornerFix?

CornerFix works by taking a reference image which you provide and creating a lens profile with it. For a given lens, you'll want to make at least one such image - often wide open where the effect is most extreme for both discoloration and vignetting at your most commonly used settings. Lens detection set to off works best to eliminate in-camera corrections, which you'll be doing with CornerFix anyway. To be absolutely thorough, you could repeat this process for other apertures and lighting conditions (e.g. daylight and tungsten) as these effects do vary. Let's take one of the most extreme examples of such a condition, using the M9 and Voigtländer 21mm f/4 Color Skopar. Using a grey card in even lighting and shooting at ISO 160 wide open at f/4 we get the following image:

As you can see, there's plenty of red and green discoloration as well as a strong vignetting effect. Now imagine this in your images - most, if not all of them! The vignetting isn't necessarily a bad thing, and many like the effect. One lens famous for this and indeed much sought after in part for this "signature look" is the Leica Noctilux. But let's assume you want a correct, even image that's free of these defects (yes, they're technically defects). Here is a sample shot using the same settings as before in creating the profile:

What you'll notice when looking at the corners is exactly what the profile image shows; red edging to the left and bottom of the frame, greenish edging on the right - and general vignetting in all four corners. Depending on the post-processing workflow you use, correcting for vignetting is very easy and most applications do this. It gets harder with discolorations such as these. Sometimes you get lucky and the effect is only seen on the left edge - or equally in the corners. You can apply an inversely colored, transparent gradient to an image to mask this effect in such cases. But what happens when you have uneven discoloration in different regions of your image? This is very hard to predict or make a generic corrective layer for (in say, PhotoShop or Lightroom). In this extreme example you can see why such a lens is generally considered "unusable" on the M9 - and exactly why CornerFix is such a great tool to correct this problem.

Using the profile created from the grey card reference image and applied to the sample shot - we get this fully-corrected image:

Now that is quite a difference! Both discolorations and vignetting have been completely corrected across the entire image. Once your profile (or profiles) are created, correcting images takes only a second. What's more, you can apply these corrections in batch to an entire folder full of images. Granted, an extra step in your post-processing workflow, but at least it's a painless one that's completely automated. CornerFix will take your uncorrected DNG file(s) and apply the profile - saving the resulting fixed DNG with a "_CF" added to the file name. This is the image you'll apply your normal post-processing routines to. The beauty of it is that the DNG format is preserved, so you can still access all the usual controls for highlight and shadow recovery, color temperature, saturation, etc. and you still have the original, untouched DNG file as well to do with as you please; either for archival or to just delete.

CornerFix also provides a number of preference settings that allow you to tweak various things like file format (including compression and application compatibility settings), luminance and chroma correction intensity, anti-aliasing and more. If you prefer your images somewhere in between uncorrected and fully corrected, you can tweak to your heart's content to keep some of your lens' character.

So That's It?

Other than the aforementioned extra step in post-processing - that's it! You will of course need to create your own profiles, but it's a rather easy process that only needs to be done once, and only for those lenses which exhibit extreme issues that you wish to correct for. Most lenses won't even need this. Now you're probably wondering how much this application is going to cost. Well, the best part is that CornerFix is freely available for download for both the Mac and Windows platform.

You can find more info on the CornerFix home page and download it from SourceForge directly.

We had considered writing up a step-by-step tutorial, but Jeff Hapeman's created a nice one right here and the instructions on the CornerFix home page are pretty thorough as well, if a bit terse.

It goes without saying that we'd like to thank Sandy for writing CornerFix and providing it to the community free of charge!