Zeiss C Biogon T* 4,5/21 ZM – and removing the reds


This lens is perfectly usable on the M240. It doesn’t even take that much work.

The Leica M typ 240 presents some unpleasant choices in terms of 21mm lenses: you can spend $3,000 on a Super Elmar 21mm 3.4 and get the sharpest 21mm ever made for Leica – but suffer complex distortion and red edges. The 21-35mm M-Hexanon Dual (which is not a lot cheaper these days than a used Super Elmar) gives you two focal lengths, awesome sharpness and no color shifts – but it gives you a touch of geometric distortion. Everything else presents varying combinations of bulk, color vignetting, low resolution, and general misery. Here at the Machine Planet, we have a certain inbuilt arrogance to try things that conventional wisdom says should not work. The 21mm f/4.5 Biogon is a case in point. And yes, we made it work with a couple of off-the-shelf tools and less than a couple of hours of trial and error learning the ropes.

The good. If this were the film era, the 21mm f/4.5 would be the champ. It is small (barely bigger than a 40mm M-Rokkor), sharp (testing in some reviews to 3000 lines per picture height), well-made, and has about as close to zero distortion as any wideangle lens ever made (for example, it’s lower than the 35/1.4 Summilux ASPH). It also takes normal-depth 46mm filters common to the rest of your lens collection. Here is basically everything you need to know about its stunning performance:


The bad. In terms of conventional performance, the lens is relatively slow in terms of maximum aperture and has the usual light falloff from the center, often exaggerated by digital sensors. You can see from the chart above that it does not get much better as you stop down.

The ugly. The worst thing is that the lens has color shift at the edges. It’s quite severe at first glance. These are the particulars:

  • The red edge extends a couple of MM into the frame, from top to bottom, green and red on the left and red on the right. In the days of the Kodak DCS Pro 14n, this was called the “Italian Flag” effect.
  • The intensity and intrusion of the edges is dependent on selected lens aperture and focused distance. Closer focus and wider apertures mean that the edges are far less obtrusive.
  • There is an overlay of standard brightness vignetting that is characteristic of any symmetrical 21mm lens.

The variable nature of the color shading – why has no one else noticed this? – may well be the cause of claims that the problem “can’t” be corrected or that conventional tools result in under- or over-correction. Once you understand this, it’s easy to solve the problem. Never declare defeat prematurely!

Fixing things up. All solutions to this problem involve some kind of reference image, which is a test shot you make using a white field. You can shoot a white wall, shoot a ceiling with a flash, or use a diffuser. If you shoot through a diffuser, you need one that lacks texture (at small apertures, the ZM 21 can pick up the texture of the paper, even if you have it pressed right up against the lens. Your resulting references will look roughly like this:


One very good diffusion material is Yupo polypropylene watercolor “paper,” which, being plastic, has no grain. You can find this in most art stores.

  1. Layer Masks. Some, like Lloyd Chambers, advocate the use of Photoshop adjustment layers and masks to cancel out color and brightness shading. Although this demonstrably works, its shortcoming is that it needs a separate template and action for every permutation of shading (you can, most of the time, get away with four settings: f/4.5 and f/8, at 1m and ∞). It also presents a clumsy workflow that involves leaving Lightroom, going to Photoshop, and then back to Lightroom (and at that point, with a TIFF and not a DMG). For your most OCD applications, this is a workable solution; it’s just not the most batch-friendly or space-efficient solution.
  2. Cornerfix. Long the go-to solution for Leica M8 and M9 users, Cornerfix was originally designed to address the green shift that occurred when you put a UV/IR filter on an M8. This green shift was generally uniform and radial. Cornerfix takes the reference imageand then computes a mathematical mask. Cornerfix works with DNGs and exports DNGs (suffixed “_cf”)Unform, and it has a tremendous range of settings for addressing color shift, brightness vignetting, and the artifacts that result from correction. Cornerfix also shows you the effect of the selected mask on the current image. It also supports batch processing. The shortcoming of Cornerfix, though, is that because it does correction via equation, there are some kinds of color shading that it struggles with.
  3. Adobe Flat Field plug-in. The strangely named Flat Field plug-in is available on the Adobe Labs site. This plugin has virtually no controls and seems to be an automated variant of the layer mask technique. You select the image you want to correct, activate the plug-in, and then give it the reference image. The only controls are for “Color” or “Color and falloff,” which lets you leave in brightness vignetting if you want. The plugin is slow and kicks out another DNG, stacked with the first one, suffixed “_ff.” It does work very well – much better with the 21mm than Cornerfix – and it does not require you to exit Lightroom, but it’s a black-box solution that requires you to select your reference image carefully (because you can over- or under-correct by choosing the wrong one).

The winner: Flat Field. As the only solution that (a) works and (b) does not require shifting from program to program, Adobe’s free Flat Field plug-in for Lightroom is the best solution. Here is precisely how to use it:

  1. Shoot your profiles. Take your sheet of Yupo paper, hold it right in front of the lens (the easiest way is to sandwich the paper between your lens and the glass of a window). Pick your reference distances. We used 1m, 2m, 5m and ∞, but you could also pick your favorite hyperfocal distance. Shoot a test at one f/stop, all distances. Then switch to the next f/stop, all distances.
  2. When you are done, import the files into Lightroom. Immediately rename these with a designator that shows lens, aperture, distance. This will result in a name like “2145-80-inf” for 21/4.5, f/8, at infinity. Export all of these as original DNGs to a folder that is easy to find (think about “profiles” in your “Documents” folder.
  3. Install the Flat Field plugin.
  4. When you want to do a correction, select the picture(s) you want to fix. All of the ones you do together should have the same shooting aperture and distance (the M240 records a computed aperture value, and you should be able to tell by the composition where the lens was focused).
  5. Go to File–>Plug-in Extras–>DNG Flat Field–>Apply External Correction. This will pop up a Finder or Explorer window to select the profile from #2 (Lightroom does not let you choose from the catalog).
  6. Choose “color and falloff.” Although vignetting may seem cool in theory, symmetrical lenses need all the help they can get.
  7. Run it.
  8. You will then get a new file adjacent to the original with the “_ff” suffix. You can now manipulate this as if it were the original.
  9. If you get too much correction, try a reference photo shot at a closer distance. If you get under-correction, go for a farther distance.

Upshot. It is tragic that so many people started unloading these lenses based on a red-shift issue that is so simple to correct with modern tools. The ZM 21/4.5 is a fantastic optic that can now make the jump to modern digital Ms. And there is no reason why the same techniques could not be used to adapt other wideangle lenses to Ms or wideangle M lenses to things like the Sony A7 series.



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3 responses to “Zeiss C Biogon T* 4,5/21 ZM – and removing the reds”

  1. Martin says :

    Thanks. Excellent post. I assume this is not an issue with B&W conversion or with the Leica Monochome. Or is it a different issue? Btw – one can always shoot film! This lens is fantastic with film. You can plunk it on an M2 body and get a stupendous super-wide for kit for $1,200 or less.

  2. James says :

    does it work correctly in taking infrared pictures (no hotspot or other defects?)?

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