LibRaw Repository on GitHub

The copy of LibRaw internal SVN repository has been created on GitHub. All changes made to the master branch through Git will be incorporated into the main Subversion repo.

So, if you wish to participate in LibRaw development you may get full sources from GitHub, add your changes, commit, and send us a request to merge your changes into the main source tree; all this using just standard GitHub tools. Also you can report a bug or make a feature request using GitHub interface.


Chasing a Gray Cat In a Gray Room: the level of middle gray and the headroom in the highlights for Canon 5D Mark II

The current methods used to determine the sensitivity of digital cameras are not based on the RAW data coming from the sensor; rather they are based on the results of processing the RAW in a converter (be it an external converter or in-camera).

This approach, with all its simplicity, is in fact based on the properties of the RAW converter and on the transformations it applies to RAW data. In particular, the converter can introduce hidden exposure compensation, change the tone curve, and so on. The sensitivity of the camera resulting from such a procedure is a pretty arbitrary value. The matter is discussed in good detail in Wikipedia, in the article explaining ISO 12232 standard.

This approach allows the camera manufacturers to enjoy all sorts of tricks while stating the sensitivity, say cameras from different manufacturers but having the same rated sensitivity behave wildly different when it comes to photographic latitude. This means that switching between different camera bodies one often needs to re-adapt, changing the way he applies exposure compensation.

A simple experiment that takes no additional equipment other than already existing camera and lens allows to accurately determine how the camera exposes, that is:

  • which level of signal (in terms of RAW data) is obtained while setting the exposure by the exposure meter;
  • what headroom in highlights is left, i.e. how many exposure stops are between the middle gray and sensor saturation.

Two Paths Leading Nowhere

Why are photographers compelled to suffer the discord of RAW formats?!

During the last 10 to 15 years, digital photography forced the film out of nearly all the domains. End users purchased hundreds of millions of digital cameras; and that is not including the cameras sold integrated with cellular phones. Such a huge industry can't exist without standards and such standards appear to exist. They cover the storage media (various flash cards), and image format which happens to be JPEG. Currently JPEG is the most widely used image format and its image quality and size satisfy the overwhelming majority of users.

However it is not always what professionals want. By professionals we do not mean just professional photographers. The list includes designers, pre-press staff, archivists, photo banks and many others. It often happens that JPEG format is also deemed less than appropriate by advanced amateurs. That is why nearly all digital cameras that are positioned by manufacturers as professional or semi-professionals models (as well as all current dSLR cameras) suggest an alternative format, the so-called RAW. For a casual onlooker it may appear that RAW is also some kind of a standard format that delivers better quality quality for pros.

This small article is to show that the matter is much more complicated. At the current stage the situation with RAW format is not just bad but really dreadful and continues to spiral downwards rapidly. This affects mostly professionals while less demanding amateurs simply enjoy the progress of digital.

Dan Margulis on RAW module

The question that needs an answer is, for what purpose is the module designed. I can think of four different approaches to acquisition.

  1. I want the module to do nothing more than open the file without damage. I understand that it will probably be flat and colorless if I do this. I intend to fix the problems in Photoshop.
  2. Although I will refine the image in Photoshop, I would like to be able to make quick, obvious moves in the acquisition module to make life easier later.
  3. I am not interested in the very best quality. I will do whatever is possible in the raw module but I will not work in Photoshop afterward. So I would like to be able to get attractive color from the module.
  4. I want to make the image as perfect as possible in the module. I will intervene later in Photoshop only as necessary.

You need to decide which group(s) you appeal to.

Random And Groundless Thoughts On Color Control In a Raw Convertor

After finally finishing reading Fairchild s Color Appearance Models I started to get deep into thought and some empirical things became clear.

Color Contrast

In the photographical community it is pretty much a common place that if you show the viewer two pictures, one with normal colors and one with an increased saturation, the viewer will in most of the cases pick the one with the higher saturation (if it is, for example a landscape scene) as the more natural one (of course if saturation is increased in the reasonable limits).

I could not quickly find the wording of this effect in books by Margulis, although I was almost certain that it was there in one way or another: in his book Photoshop Lab Color) this rule (increasing the a-b axis contrast) is used starting with the very first example.

Canon: How To Get 400/5.6 In Five Different Ways

While preparing for my summer trip to Altai, I attended to choosing the telephoto lenses I was going to carry. I needed a regular telephoto lens for shooting water attractions (Katun is very large river and the target can be very far away) and an extra long reach lens for shooting the full phase of the solar eclipse.

The goal was to select a 400 mm as a long lens, and as much as possible for the extra long one. After looking around and asking some friends I found the following possibilities for a 400mm with Canon:

  • EF 400/5.6
  • EF 300/4 + TC1.4
  • EF 200/2.8 + TC2
  • EF 70-200/4 + TC2
  • EF 135/2 + TC2 + TC1.4

Surprisingly, even the last option on the list (the one with two teleconverters stacked) did not seem hopeless: I tried that option for film a while ago, and it was comparable with a 70-200/2.8 with a 2x TC.

Peace in Lights

As it was shown earlier the direct application of exposure to the right method (ETTR) without preliminary scene analysis and evaluation often results in a major underexposure which in turn causes poor details, noise, and artefacts. In order to evaluate the necessary depth of tone correction for such underexposed shots we will use the following natural method we will return the zones containing details of high visibility (those are zones from IV to VI) to target density values. Such a move makes sense because in order for those details to be really visible, they need to be in those zones.

How to stretch 2.5 eV to 5; and what would be the price

In the photographic community, the common perception is that the exposition scale is symmetrical. The perception dates back to Adams: the Zone V is in the middle of the range, with 5 zones above and below it, top and bottom being symmetrical: there are 2 zones each way containing details, then one more each way with traces of textures, with the final two (one each way) which are respectively completely void white and solid black. The 2x (1eV) changes in brightness correspond to the transition between the zones.

The photographic zone step wedge is not visually (perceptually) even. For instance in the Kodak Q13 grey wedge (the top scale on the image to the left), where the distance between the two neighboring patches is 1/3 eV, we can clearly see that in the lighter part, this distance between the patches seems visually large, and in shadows visually small, as if the steps are compressed. The Q13 wedge encompasses Zones I through VII, although not representing the brightest part (further on, we will see that they cannot be represented at all by an evenly distributed wedge).

Headroom in Highlights : Where is Zone V in The Digital World?

We will try to demonstrate here why there is no more then 3 stops headroom in highlights between the exposure suggested by the spot-meter and the clipping point of the raw data. We presume the camera follows the current ISO standards; the sensor is single-structure (not like those Fuji S5 cameras have); the capture is obtained in 1 shot; and that the raw data is linear.

Using Magenta Filter for Shooting With a dSLR Camera Under the Daylight

In the previous article and during lengthy discussions on various forums, we promised to demonstrate the usefulness of magenta filters. Those filters compensate the imbalance between the sensitivities of color channels in digital cameras. Promises should be fulfilled, and having this filter, it takes nearly no time to prepare the demonstration given that you have that sunlight...