In another forum we started a discussion about how the Zone System fits into the DR of our camera sensors. Ansel Adam's Zone System consists of 11 zones named 0 to X, where 0 is supposed to be pure black, X is pure white (no information in any of them thus), and zones I through IX are the ones containing information with 1 f-stop apart one to another, being zone V middle gray.
At the same time, camera exposure metering systems are supposed to translate the luminance of the measured area (no matter which measuring method is used) to a middle gray tone, i.e. to zone V.
Is this true? to which real linear level are measured areas translated to? and once the RAW file is developed, gamma corrected, and opened in Photoshop, what level in the range 0..255 means neutral gray? the 128 level?
Let's find out...
Me and another guy have run the same test on a Canon 350D and on his Fuji S3 Pro. The test consisted in just one shot: using a tele (300mm) over a uniformly lighted white surface, deliberately defocused to obtain the most regularly distributed image possible.
Under these circumstances, the final luminance produced by the camera should be the middle gray we are looking for on each camera, i.e. that gray tone to which the light metered areas are translated by the camera metering system.LINEAR HISTOGRAM PRIOR TO WHITE BALANCE
Developing the image without applying any white balance (DCRAW -r 1 1 1 1 option) we can see what's the gap between the middle gray and the highlights right before starting to blow them up. This gap is how much we could overexpose the metered area before blowing it.
Now we will repeat the test for the Fuji S3 Pro, which has a very particular sensor, the Fuji super CCD which is actually two sensors in one (S captors: the regular ones, R captors: smaller and less sensitive photosites to prevent highlights from being blown in high DR scenes). So two images can be obtained from a unique RAW file.LINEAR HISTOGRAM PRIOR TO WHITE BALANCE
Very interesing figures can be obtained from these histograms:
1. Under the same real exposure, the relative exposure of the S sensor over the R sensor happens to be 3.6 f-stops
(just count the divisions). I have calculated accurately this figure over 3 different RAW files, so I think is a standard design value of how the Fuji Super CCD works.
2. Now, the security gap before starting to blow the highlights in the S sensor (not in the R sensor) is only about 2.3 f-stops
. This means that the middle gray areas of the scene are allocated by the Fuji metering system over 2 f-stops higher than the 350D does.
3. This reduced security gap does not mean we cannot capture properly the 4 needed f-stops for zones VI, VII, VIII and IX, since the R photosites will do that job as soon as the information from the S photosites blowns up. In fact, the security gap for R captors is: 2.3+3.6=5.9 f-stops
! almost 2 f-stops more than the Canon. That's is why it is so difficult to blow the highlights in a Fuji camera if high DR mode is activated.NEUTRAL GRAY LEVEL AFTER GAMMA CORRECTION
Let's find out which value in the 0..255 range is the neutral gray located. Again we perform a totally neutral development on ACR getting the following image:
as we can see it's quite brighter than the gray produced by the 350D. With the colour checker we measure it to be about 112 (I measure the middle area of the image since my friend didn't manage to have a very uniformly lighted surface; that's why these histograms are a bit wider than desired. However as he spot mettered in the centre of the scene, that is the neutral gray (zone V) for the Fuji, and yields a value of 112
, quite closer to 128 than that of the Canon.
- Both cameras can register properly at least 4 f-stops higher than the metered area, which is needed to allocate zones VI, VII, VIII and IX of the Zone System
- Not all cameras provide the same gamma corrected value once in the range 0..255 for their neutral gray, and consequently it is wrong to say 128 is the neutral gray level in Photoshop. This will depend on the way the camera was designed (Canon 350D produces level ~74, and Fuji S3 in high DR mode produces level ~112).
- Of course a simple curve could readjust all levels making the true neutral gray (zone V) match level 128; but the actual starting point is a different value.