This blog is really about my experience with the Olympus E-P3 camera (and to some extent E-P2 before it). This is also about the advantage, that is obtained when shooting in RAW format, and always exposing as much as possible. The expression "as much as possible" does not mean here overexposing. What, then, is overexposure?
Gamma
Because of this issue of exposure I´m not going directly to the E-P3, but we need to discuss a few things. First a look at the the crucial difference between the RAW file and the film. Ignoring this difference leads into blurred concept of overexposure or correct exposure. The difference is gamma curve.
In practice, in almost all digital cameras, AE follows the old film era definition. It is based on the average reflectivity of the object. The camera may be set to expose by either 12% or 18% reflectivity. Film has a gamma curve, which is kind of a ski hill-shaped curve. On the top it is bright and curve is gentle, the middle is steep, even slide and down hill evens out into shadows. Most pictures appear to be good, when its central tones have a good contrast. Dark colors must not suddenly change into black and light shades must not go into white all of a sudden. Such sharp edges do not seem natural. Therefore, tonal changes are slowed down at each end with gentle arc. For film photographers it is most important to use the contrasty middle tonal range as completely as possible. According to this need for visual naturality different types of film require somewhat different exposure. Slide films need less exposure to prevent highlights from becoming white (to be overexposed), and negative films must not be exposed too thin and prints appearing too dark in shadow areas (would be underexposure).
For the majority of subjects, all goes well, if it is exposed according to the camera's meter. Only experience tells when, in what direction and how much it is necessary to deviate from the camera´s value.
RAW
RAW file is different from the film in that it does not have the same gamma curve, it is linear. From white to black, there goes a straight line with no curving at either ends. The requirement of naturalness (to the human eye) in the image, is carried out in the RAW conversion when RAW file is converted into pixel file and saved for example as TIFF or JPG format. Then the final image gets a gamma curve, and this curve is adjustable by the user in every good converter. However, this curve is not always visible as itself in most converters, only the effect in the image is visible.
Now, one could ask where's the difference. The subject is just exposed at the middle (or somewhere there) of that straight line, gamma curve is included in converter and all is well. Actually that is just what most photographers are doing - and at the same time they are wasting a significant part of tones from their images.
Tonal values in a RAW-file
In the case of RAW file we need to discuss bits in images. Bits tell us into how many tiny levels our direct line is divided. And it must be divided, otherwise we would not have a digital file. The overall impression is still a straight line, because there are so many levels. Typically, a modern digital camera has a 12-bit (per channel) converter, which converts the sensor's analog (linear) signal into digital (small steps in straight line). The best cameras can have a 14- or 16-bit converter. This does not change the issue at all, the same scale from white to black is only divided into larger number of smaller steps.
A 12-bit image consists of 2 ^12 = 4096 possile steps or tonal values from white to black. At this point, we unfortunately must say goodbye to the linearity of tonal distribution: tone values are not evenly distributed into bits. Starting from white towards black, half of the ALL tonal values are inside the first bit. For simplicity, we can speak in photographic terms, ie steps in aperture or shutter speed scale: One full step in opening the aperture (eg, F5.6 -> F4) or an equivalent increase in exposure time (eg, 1/60s -> 1/30s) increases the amount of light to double. Then the first step starting from pure white includes half of all possible tones in a RAW file! In a 12-bit digital camera, the first step starting from white has thus 2048 possible tonal values (per channel), the next step has 1024 possible tonal values, again the next step has 512 possible tonal values, and so on.
Now you may (hopefully) no longer think that it is indifferent to where on this scale of tonal distribution your RAW file is exposed, if you want the best possible tonality in your image!!!
Above we have two RAW images as they are opened in Lightroom 3. The upper one is exposed according to what camera meter (AE) says and the second is exposed to the right. The change in exposure is +2 steps. Below images we have their histograms in Lightroom with default settings, left camera AE and right my ETTR version.
Next pair of images has the same images normalized. Mainly I clicked Auto plus changed slighty sliders. Again above is camera´s version and below ETTR version. Histograms also in the same order as above. The most obvious differencies are 1,8 steps more exposure in camera´s version and more blacks for ETTR version. There was no need for any change in exposure for ETTR image. A quick glance reveals not much difference in images, but they have. The real clou is seen in the next pair of images.
The clou is the amount of detail and noise. These 100% crops are in the same order as above, and neither has any luminance noise reduction. ETTR is one of the most efficient ways of minimizing noise in shadows and elsewhere. Note also how two stops more exposure did not diminish detail in light ares, it only gives more detail as s/n ratio gets better.
Histogram and exposure
The histogram reflects the distribution of tones in the image. The histogram has white on the right side and black on the left. Therefore, we can say that in the histogram the first step from the RIGHT has half of the possible tonal values of the picture.
What does a histogram in a digital camera show? It does not show the tones of a RAW image, but tones of a JPG preview calculated from a RAW image. These tonal values are dependent on the camera's contrast and saturation settings. The histogram might show over- or underexposure (or both), even though the sensor itself would have plenty of exposure latitude left. In addition, the histogram is affected by the selected color space. In particular, the sRGB color space is narrower than the sensor color space. When it is selected, the RGB histogram displays too often saturated colors that do not actually exist in the RAW file.
Worse still, the histogram is not even a good ETTR-exposure meter, because it reflects the image globally and not its individual tone areas. It does not directly tell what is the exact area in the image, which is now going blocked. ETTR-exposure requires the use of each pixel in the image area as a separate light meter and display.
Finally, the worst of all: a traditional reflex camera´s exposure metering (prior to taking a photo) is even more problematic in terms of shooting in RAW. It does not understand anything about the behavior of the sensor. They are two completely different things.
Expose To The Right, ETTR
As much as three quarters of possible tonal values of a RAW file are in the first two steps from the right. This should be a sufficient reason for every RAW photographer, to expose as to the right as possible, without burning the light tones of the image, of course. This method deviates from the traditional concept of "correct" exposure. ETTR RAW file can appeear be too light when opened into a converter. The converter is used to extend the tonal range into dark direction until the tonal range is as desired. In most cases, successful exposure means that the brightest tones of the image do not need to be adjusted at all. The resulting image has richer tones and is even significantly cleaner regarding noise.
ETTR has been also criticized or denigrated in online blogs. The problem is not the idea itself, but the cameras. Only few of the cameras provide photographers with a good tools for reliable control of ETTR. Without control, there is danger of overexposing the sensor, and the image is beyond repair. ETTR does not always mean more generous exposure than the camera's exposure meter indicates. Exposure may be also less when the subject has lots of contrast, but even then the exposure goes for the lightest tones.
ETTR means that you should set the ”weakest" settings that can be found in your camera: lowest contrast and saturation as well as a widest color space. With Live View cameras exposure control with sensor feedback is possible before exposure. With reflex cameras exposure can be seen only after the exposure. The quality of information received depends on how well or poorly the camera settings in imitate RAW files. In addition, all the automatic adjustments (Highlight, Shadow ...) which shape the gamma curve must be switched off.
The second ETTR example is again exposed for the lightest area in image and that is the lightest details of relief. We have now almost a one step increase over camera´s meter. Histogram and sliders are here at Lightroom´s default. Lightroom´s warning colors show ares where at least one RGB channel is blocked at Lightroom´s default gamma (2.2).
This before/after pair shows details from the lightest are in the image. On the left are default settings and on the right you can see how two tiny moves in sliders fix the toe and shoulder of gamma curve so that nothing is blocked. ETTR does not lead into more work at post, vice versa
Shooting JPGs
As a clarification: ETTR is not applicable when shooting JPGs. It is not possible to ETTR a JPG file and darken afterwards unless subject contrast is so limited that it can be included in the straight part of gamma curve. Even then, most likely, light tones show banding
ETTR for me is always observing the lightest non-specular tones in image. Specular highlights and reflections may and should go white to give the image the right tonality and contrast. In this example ETTR has led into one step "over exposure" compared to camera AE. In reality this exposure is correct and the engineer who designed my camera´s AE has it wrong. It is a matter of taste where is the limit or start of specularity. It can be tweaked with a tiny change in Lightroom, if needed. Otherwise the image is OK.
Olympus E-P3 and ETTR
So, using ETTR might lead into struggle with inferior equipment. I learned ETTR however during my profession as advertising photographer in the late 90's. The tools then were Leaf three shot digital backs and Leaf software. With them and studio flash equipment it was possible to set the exposure at 1/20-stop precision exactly where it was supposed to. Later PhaseOne CaptureOne allowed the same in other cameras. But doing so demands shooting tethered.
Beyond these devices only the Olympus VF-2 viewfinder gave me a new hope toward guaranteed ETTR shooting anywhere, anytime. With E-P2 I learned to observe the viewfinder image and see when exposure at highlights was as wanted. It demanded these camera settings:
Shooting Menu 1 > Picture Mode > 4 Muted > Contrast -2, Saturation, -2, gradation: Normal
Custom G > Live View Boost: Off, Color Space: Adobe RGB
Setup Menu > VF-2 Brightness: Experimenting until I learned the appropriate setting. After that Custom D > Info Settings > Histogram: Off
This was just a start, because human eye is really not a good instrument, it is affected by the overall brightness of the scene. I had yet to check the captured image in the VF-2.
The Olympus E-P3 finally fixed these deficiencies, because it has an adjustable highlight and shadow warning by colors in viewfinder image, that is, before the exposure. This control is set at:
Custom D > Histogram Settings > Highlight: 245
Auto white balance must be set to neutral:
Custom G > Auto WB, Keep Warm Color: Off
Now, I adjust the exposure until the red warning color just disappears from the critical tone area. In principle, this allows a 1/6-stop accuracy at best, but in practice, the reference tone is very close to the value of 96% when the image opens in Lightroom. Same area will be at RGB value 245 in the final image with very little need to adjust as seen above. I do not need to use histogram, I do not need to check captured images, at least not because of the exposure.
I do not know of any other camera body, which gives me such a perfect exposure control with the same precision and certainty. E-P3 is my ETTR King!
-p-
May 3, 2012 at 14:36 
