Leica M9, part 4, vignetting and automatic shading reduction.
Introduction.
Some readers said it was unfair to compare the Nikon D3X with the Leica M9 as the Nikon is bulky, very feature-rich and has a totally different handling. The M9 on the other hand is compact and intuitive to use and should be compared to a camera in its own league. Let me note again (I said it in the review, but repetition is sometimes welcome), that I did not want to compare the Nikon asa camera system to the Leica as a camera system. It did a review with this goal some time ago and do not wish to repeat this story. I used the Nikon as it is universally acclaimed, like it or not, as the benchmark and point of reference for image quality and performance in the high-end professional digital camera market. There are camera systems with more and with less performance, but the benchmark is just this: a point of reference. Some readers got the impression that I favored the deployment of AA filters or low pass filters. I do not. I think the Leica approach is inherently the better option, the less thickness of the cover glass the better! What I noted is the effectiveness of the Nikon low pass filter in combination with the post processing software inside the camera. While the Leica option is theoretically sound, in direct comparison with the D3X one has to conclude that at this moment in time the true potential of the thin cover glass strategy of Leica can be improved to get an decisive edge compared to the main rivals in the high-end professional 35mm market.
A comparison between the M9 and comparable cameras is quite difficult as there are none! I am not aware of a current digital CRF with a sensor that can be compared to the M9 sensor. I do not consider the MFT models for Olympus and Panasonic valid competitors to the M9. The only candidate is the Leica M8 and M8.2 and I have compared these models with the M9. Some readers did comment that the advantages of the M9 are not big enough to warrant a migration from the M8 to the M9, because their demands are fully satisfied with the M8. I try to give a balanced analysis of the virtues and possibilities of both cameras and I fully agree that the potential capabilities of the M8 in expert hands can comply with many user demands in normal practice. But the M9 has additional advantages that may convince M8 and non-M8 users to buy and use this camera. There is a second approach to compare M8 and M9. In the previous part I used the same lens and distance and thus did not exploit the M9 sensor size to best advantage. I neglected on purpose the classical Leica rule to fill the negative size with the maximum amount of information.
The M8 has a crop factor (better is the reduction of angle of view) of 1.33 and it is common practice for Leica users to use a 35mm on the M8 to compare with the 50mm on film-loading Leica bodies. It is interesting to make the same comparison with the M9. So I put the SX 50 asph on the M9 and the SX35 asph on the M8 and made the same pictures on tripod at the same distance. The exposure meter was on A as was the white balance. I did not correct for color bias as this is not my goal here. Both lenses were stopped down to the optimum aperture of f/4.
Below you see a smaller image of the full size digital negative. Note that the M8 image is smaller but has only a minute amount of detail loss
The M9 images show a higher level of definition and a crisper reproduction of very fine detail. It is up to the reader to decide whether this advantage is worth the upgrade or even a first buy into the digital M world. I got many queries from readers who are actively involved in AgX photography with M7, M6 and/or MP and are now wondering whether the moment has come to make the transition. I will devote a future part of this report to answer this question, but as a start I can say that the M9 pictures are very close to and may even surpass the quality of the best of the modern medium speed films of ISO 100 to 125.
Shading
Shading is also known as vignetting. One of the main arguments of Leica for the smaller M8 sensor was precisely the additional vignetting that was introduced by the combination of the steep ray angles at the exit pupil and the shape and position of the micro lenses on the individual pixels of the sensor. The M9 sensor has to cope with the same ray angles but has an optimized micro lens shape.
In addition Leica has incorporated software in the camera that can reduce the amount of vignetting. This technique is not new: many post-processing programs have this option. In the Leica M9 you can select the lens used manually and this also offers third party lenses the same luxury. Of course the corrections are optimized for the Leica lenses, but many third party lenses share a comparable optical design and might benefit form the correction. As a matter of interest we can note that in many cases vignetting or shading is not a big problem. The eye can easily accommodate and ignore moderate vignetting as long as the transition is smooth from center to corner and below 1.5 stops. Measurements of shading often report figures of 1 stop and more and most readers assume this to be a problem. But shading becomes troublesome only above 1.5 stops and should not be overrated. The Leica M9 can handle oblique rays as long as the angle at the exit pupil is below 35 to 40 degrees. Above this angle the shading cannot be compensated. That is the reason why classical lenses like the Angulon class will exhibit significant vignetting.
But shading in digital cameras also implies color shading. As the rays of light have to pass obliquely through a cover glass of some thickness, the length of the path of travel increases and so does the color shift with some loss of the red light, increasing the cyan part. In most cases this is not detectible, but it can be noticed in critical conditions.
It is also worth noting that the same amount of measured vignetting will be less noticeable on film than on the sensor.
The main question however is how effective the on-board shading compensation is in the M9. I used a dedicated piece of equipment with extremely uniform luminance distribution and checked the amount of vignetting with and without the M9 lens detection on OFF and Manual (where appropriate) of AUTO. Generally one may say that the results are quite good, with an improvement of between a half and a full stop. In extreme cases the vignetting is not gone completely but reduced to minor proportions.Note too that the shading algorithms also correct the color shift. This is quite visible with the E24/3.8
There is considerable interest in the behavior of the Zeiss ZM lenses on the M9 with the lens detection on Manual selection.
I checked the following range of ZM lenses.
ZM 2.8/35; ZM 2.8/28; ZM 2.8/25; ZM ZM 2.8/21; ZM 4.5/21; ZM 4/18; ZM 2.8/15.
With the exception of the 4.5/21 which showed significant vignetting all ZM lenses improved when the lens selector was on the comparable Leica lens. This is no surprise as many ZM and L lenses have a comparable optical design and then should react to the same correction with the same behavior. With the ZM 15 you need to experiment a bit with different settings to get the best result. It is advisable to use this approach to the other lenses too. Generally we may conclude that almost the full range of ZM lenses is useable on the M( with good results as far as shading goes. I had no access to Voigtlander lenses, but this experience shows that it is worth a try, but results may differ significantly.
In the next part I will look at ISO speed and noise and some other topics.