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digital sensitometry (part II)

 

 

 

Digital Sensitometry
part I

 

by Henri Gaud

 

I unveil here a few tests I made recently in order to better understand the sensitometric response of a silicon photographic image sensor.

This is not a scientific demonstration, simply an example of what we can understand about how digital image sensors work.

The sensor is Canon's 1 Ds MkII sensor; as a starting point we'll focus on the characteristic input/output curves. In a future article, examples of images will be presented.

Note : this test is not influenced by flare, thanks to a simple trick, so no flare here.

 


Characteristic curve ISO 100, canon 1Ds Mk II

 

Admissible tonal range Delta E = 6 f-stops

Characteristic curve of the sensor plotted from an input file in RAW format, namely the new Canon format RAW.CR2. The file has been opened as a 16-bit file and converted to the LAB format. What we are interested in, is to know whether our detector meets our expectations ; values from 0 to 100 are the output and values from 1 to 41 are the input luminance steps of the subject, in increasing luminance values.

An input reference scale of grey levels was created (like in conventional photography), with an increment of 0.15 i.e. 1/2 f-stop. Output values in the digital image file are expressed from 0 to 100 in units of the LAB file format. My personal choice was to invert those values in order to get a curve similar to a slide film (vs. a negative, for which densities increase). The LAB format is a "theoretical" format so the zero value corresponds to an absolute black allowed by the format ; 100 corresponds to an absolute white. Presented like this, the characteristic curve is close in shape to a conventional colour slide film, in terms of film we would qualify this curve as being rather weakly compensating, similar to a film that would record luminance values "mechanically" without any contrast compensation.

What can be said about this curve : the maximum admissible luminance range is easy to read ; on the chart, 12 steps can be read (corresponding to a total of 12-1/2 clicks), i.e. a range of 6 full f-stops.

Conclusion : if we compare the sensor to a conventional slide film, we can say that it can record a tonal range of 6 f-stops. We object that the curve does not have the proper shape that would please us, no problem, the 16-bit LAB format allows us to do whatever suits us very easily by post-processing as long as we did not lose information about extreme luminance values. In other words, anything outside the tonal range of 6 f-stops is lost, nothing new with respect to what we know from a conventional film.

The goal of the test

of course, testing the sensor, the range and shape of the characteristic curve, the maximum admissible tonal range Delta E i.e. the maximum luminance range that the sensor can record from a real scene where large variations of luminances exist.

This test did not take flare into account. In the real world, since flare tends to reduce the contrast in the optical image, we reasonably expect that our system is able to record scenes with a luminance range of about 7 f-stops.

The other goal is to determine what could be the digital equivalent of the well-known zone-system-like exposures N-2 ; N-1 ; N . N+1 ; N+2 and what can be done by digital post-processing. Does it make sense to define this for a digital sensor? does it make sense to expect some digital contrast management, like we do for film by varying our developer and processing time?

Possible characteristic curves : 
Canon 1Ds Mk II

 

 

Let us examine what can be done. Same sensor but 5 possibilities of post-processing : N-2 ; N-1 ; N . N+1 ; N+2 and a wide range of equivalent ISO sensitivities. The standard ISO range goes from ISO 100 to ISO 1600, plus 50 and 3200 taking into account corrections by the Canon RAW file software manager. Doing so we get different curves parallel to each other. Unlike silver halide films, those digital curves do not change so much when the ISO setting changes. Extreme curves are not too bad but the post-processing is very efficient (+1, +2, -1, -2 f-stops). When ISO 50 setting is processed like 25 or 12, eventually some important 'digital saturation' appears but we have pushed the system to its limits so we get what we deserve. In more standard conditions, it is hard to see differences in the curves from one ISO setting to another. In order to determine the right exposure time, we have used a 18% grey card for which the output is placed at L=50 just in between 0 and 100. Slight variations in sensitivity are probably due to the experimental setup more than due to the camera itself. We would dream to be able to get such a set of response curves with a single type of film processed in various developers !

So now we are all set as far as calibration is concerned, we can start our photographic work in confidence.

Note. Many users complain about the Canon Viewer Utility, but for the 1 Ds II you do not have the choice if you want to keep the advantages of the RAW format. So we use it because we have to, we admit that it is very slow but not as bad as the previous one.

For this article I had to process 203 files 15 times each ; one minute and a half per process with a Mac G4 bi-processor 2x1GHz, altogether 75 hours of machine time...

What is really impressive is the smoothness and regularity of the curves and the ability of the sensor, with some post-processing, to record all required image information. A tonal range of 6 f-stops, a possibility to adjust the ISO setting by 4 f-stops, an additional 2 f-stop scale plus a possibility of +-2 f-stops by post-processing, all this makes a total allowed tonal range of 9 f-stops (at N+1 and N+2, increased 'saturation' induces some loss of performance).

The maximum overall performance of this sensor in terms of all acceptable speed/aperture combinations is 6+9 f-stops. Of course it is very difficult to properly exploit all values ; however the ability to record 6 f-stops +4 additional by post-processing is very usable and will allow to "rescue" many difficult shots, by a combination of N+2 and N-2 images, for example using a tool like Photoshop®.

Curves at 50 Iso

 

 

Additional information

N+2/G-2 = pushed processing N+2 i.e. +2 f-stops ; gain in relative sensitivity = 4X ; G-2 = correction factor for the gamma contrast factor gamma-2. The vertical scale is Canon's and is arbitrary but is the same for all post processing and all ISO settings.

N-2: choice of N-2 post-processing i.e. loss of 2 f-stops in terms of sensitivity, relative sensitivity divided by 4.

This set of curves represents the whole possibilities of the sensor at ISO 50 setting. The results are easy to use, the contrast parameter allows to adjust the compensating effect. But this is not really a miracle taking into account all the possibilities allowed by a 16-bit file, the maximum recordable Delta E is still 6 f-stops and does not change significantly.

The inflexion point is always located at L=50 in the output for the 5 curves and for all sensitivities.

All N+- post-processing are perfect except for N-1 and N-2 where a huge increase of 'saturation' is visible. To be used for rescue purpose only, however there is actually recovery of lost data. In principle taking into account the excellent exposure meter of this kind of camera, it is hard to be wrong when you have a look at the histogram while taking the picture. In extreme cases you can even have your laptop with you and check, simply open the file in 16-bit, examine the 16-bit LAB histogram with Photoshop®.

It can be seen that pushed post-processing are correct and that N-2 precisely gains 2 f-stops, this degree of precision cannot do any harm, even if it is difficult in practice to estimate the error you made on your f-stop setting.

Curves at 100 Iso

 

 

Curves at 200 Iso

 

Curves at 400 Iso

 

Curves at 800 Iso

 

 

Curves at 1600 Iso

 

 

Curves at 3200 Iso

 

 

Conclusion

From a photographer's point of view, one has hardly ever encountered such sensitometric curves with film. The expected possibilities are superb and hard to believe. We have to live with this reality: digital sensors offer us the moon ;-)

Now we need to complete this lab experiments with some real shots on real objects in order to check whether some unexpected factors could occur...

In other words, we can dream to get a perfect image rendition from ISO 12800 to ISO 25 : this is something uncommon, to say the least.

Not so good are the N-1 and N-2 post-processing results. The idea is to use the sensor at N+1 and N+2 and to post-process at N-1 or N-2, for example instead of ISO 100, set ISO 50 or ISO 25 and post-process to rescue the shot. Well this is not a good solution, doing so we get a terrible increase of 'saturation' and highlights are burned, see the next article.

However when exposed N-1 or N-2 and post-processed N+1 or N+2, things should go smoothly, this is what we expect from our curves.

Last conclusion : long life to digital sensors providing such exemplary sensitometric curves, but the affair has to be followed carefully.

HG - 2005

 

read also :
digital sensitometry (part II)

 

dernière modification de cet article : 2005

 

 

tous les textes sont publiés sous l'entière responsabilité de leurs auteurs
pour toute remarque concernant les articles, merci de contacter henri.peyre@(ntispam)phonem.fr

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