Friday, May 25, 2012

The Leica M Monochrom's spectral sensitivity

I previously wrote about the Leica M Monochrom's sensor in this post. In summary, what I said was that I doubted that the sensor was simple a "naked sensor"  - in order to get reasonable spectral sensitivity, there would have to be a filter of some sort.

Leica have now published the spectral sensitivity of the sensor - you can find it here. Leica provide both a graph of spectral sensitivity, and a visualization of the Leica M Monochrom versus Kodak T-Max 400 film. It's not clear why they chose T-Max as the reference - not my idea of a film with benchmark tonality(!)

Something that is quite important to realize when looking at the visualization is that, although Leica don't explicitly say so, it clearly excludes the effect of the film and the Leica M Monochrom's tone curve (aka, how it responds to light and dark). The visualization is only about spectral response. So if you actually took a picture of the test chart with a M Monochrom and an M7 loaded with T-Max, what you would see would not be the same as the visualization.

Taking a look at the graphs, it seems pretty clear that there is a filter on the Leica M Monochrom's sensor, as expected. I'd think, by the shape of the curve, that the filter is a simple red/IR filter, as I'd speculated in the original article. It appears to start to cut quite early, at about 550 nm. This is probably to get good IR cut characteristics.

A simple red/IR filter is a good compromise between spectral response and sensitivity - one of the selling points of the Leica M Monochrom is sensitivity, and a more aggressive filter would reduce that.

If you compare the "visualization" of the Leica M Monochrom versus Kodak T-Max, a few things stand out:

  • The Leica M Monochrom's sensor and filter combination is overall less sensitive to red than film, in contrast to what we would expect from the sensor alone. You can see that clearly in the red patches of the chart, which are darker for the M Monochrom than for film. 
  • Because the red/IR filter has a fairly gradual cut-off, the M Monochrom isn't going to be at its best with very deep reds. However, that's the case for pretty much all CCD sensors anyway.
  • It's interesting to look at the blue, green, red patches (fourth row, fifth column on the chart). What you see is blue equal, green lighter for the M Monochrom, red darker for the M Monochrom. So in terms of relative sensitivity versus the T-Max, the M Monochrom is slightly biased towards green sensitivity. That's consistent with the classic luminosity equation as discussed in the original post. It implies that the Leica have balanced the camera to be somewhere between B&W films, and the kind of results you'd get by desaturating a color image in a image editor.
  • The skin patches appear reasonably well balanced, which is important. So while the patches differ from the T-Max, the differences are fairly common across the range of skin tones. Skin rendition is probably the thing that most viewers will be the most sensitive to, because humans have very good "mental maps" of what other people's faces "should" look like. However, the variation in red sensitivity does cause some deviations between "caucasian skin" patches. I would be a bit concerned that you might see some patchiness in skin tones from the M Monochrom -  a carefully chosen light green filter may help in such situations. Once some production M Monochrom's are available, hopefully someone will do trials with commercially available filters.
Overall, the visualization looks like a reasonable compromise. Slightly more even skin tones would be nice, but Leica have to work with the reality of the filter materials that are actually available and are compatible with the manufacturing process, and the need to keep good sensitivity. Slightly more even skin tones at the cost of several stops of sensitivity wouldn't be a good trade-off.

What I do find encouraging here is that Leica have published this document so quickly after the questions were raised. It's no secret that I was highly unimpressed that none of the "professional reviewers" that Leica invited to the launch and/or gave preproduction cameras to do testing seemed to even realize that in a monochrome sensor, spectral sensitivity is important. But Leica's willingness to answer questions like this quickly with real data is a real positive. Update: I've since found out that Sean Reid, at Reid Reviews did address spectral sensitivity/filtering. My subscription to the site wasn't current when I originally wrote this post so I hadn't yet read his review.

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