Dynamic range and exposure of Leica M and M Monochrom
The OECF analysis (opto electronic conversion function) determines the property of digital camera to transfer luminance values into digital values. The setting uses a standardized chart with 20 patches that run from very low luminance to very high luminance, covering a contrast range of 1:10000 or 13.5 stops (EV values).
Below is an example.
The recorded values are not exact because of stray light in the patches and the lens, but as an indication they will do. The procedure is rather simple: you photograph the chart with the camera at different ISO values, read out the RGB values of every patch and plot a graph. Below is a graph of the Monochrom at ISO 320 and 3200 and the M at ISO 200 and 3200.
The obvious difference is the lower densities of the Monochrom compared to the M and the higher dynamic range of the Monochrom compared the M. The exact value of the classic grey chart is not defined in the digital realm, but there is a rule of thumb that pixel value 118 would correspond to the mid grey value with a latitude from 100 to 150. For the M at ISO 200 this would correspond to patches 9 and 10, for the Monochrom at ISO320 the patches are 10 and 11. The M at ISO 3200 covers the patches 8 and 9, for the Monochrom at ISO 3200 the patches are 10 to 12. Compared to the M the Monochrom slightly underexposes by 0.5 to 0.7 stops. The reason for this behavior is the wish to capture the full tonal range from 0 to 255. The M captures the patches from 2 to 20 (a range of 10 EV-stops) with pixel values from 0.1 to 254 and the Monochrom has pixel values from 0.78 to 253.34. It is evident that the dark areas are better preserved in the Monochrom than the M. But this is theory! Experience shows that pixel values between 15 and 248 are the maximum range for a clear recording of textured black and textured white. Now the M has a range of patches 4.5 to 17 or an EV range of 6 to 7 stops and the M has a range from patches 5 to 19 or an EV range of 6 to 7 stops too. This implies that the Monochrom is better at highlight preservation, but has no over exposure latitude. At ISO 3200 the situation is this for the M: patches 4 to 16 or an EV range of 6 stops and the Monochrom covers patches 5 to 19 or an EV range of 6 stops.
The upshot?
The theoretical tonal range of 12 stops is indeed recorded with both cameras, but the practical range shrinks to 6 to 7 stops, but hardly changes when the basic ISO values are changed to ISO 3200. The Monochrom has better separation of the highlights and a higher density in the shadows, very important characteristics in digital imagery and the M is best in the mid tones at every ISO value. Under- and overexposure by 0.5 to 0.7 stop changes the tonal scale significantly and one should experiment with the parameters to get the result that suits the expectations.
OECF revisited
The first graph shows how the several digital M cameras record luminance differences between white and black in twenty steps.Note that the M9 (red) has the best differentiation in the highlights and the M is in fact on the brink of over-exposure. The M8 has surprisingly (?) one of the best curves.
The second graph shows how sensitive the M reacts on a difference of 0.4 stops. Blue line is basic ISO but with a compensation of -0.3 stops. The red line shows the curve with a compensation of 0.7 stops: you lose somewhat in the shadows but wins in the highlights: make your choice.
The third graph shows the M and MM at ISO 3200 and the M9 at 1250. Again the differences and choices are obvious.
The fourth graph shows the M and MM at ISO3200. It is clear that the MM has a wider range, but the M shows the classical behavior of a push development with an extended differentiation in the midtones.
The fifth graph shows the MM at ISO3200 and ISO6400. Note that there is no discernible difference.
The sixth graph shows the difference between the M8 and M9 at ISO 1250.
This collection of graphs indicates that the M8 and M are most critical in the highlight area and might benefit from some under-exposure. It is however the behavior in the critical midrange where the differences are greatest. You can not say that one is better than the other: it depends on what you want: a long tonal range with subtle differences in brightness values or a shorter range with strong differences in brightnesses of the mid-tones. It is interesting to note that Leica with the M is slowly moving to a quite visible midrange differentiation, but reduces the highlight differentiation. The best shadow differentiation we find with the M8.
