Raw Exposure reference page

Introduction

A raw file is a camera sensor data snapshot with a minimum of processing applied. In essence, it is a rectangular array of raw values read from a corresponding rectangular array of sensor pixels. A raw value is proportional to the amount of light gathered by a pixel. As a result, raw values and their distribution reveal a lot about how the sensor was exposed to light. Raw Exposure provides an interface to inspect and analyze raw values and their distribution.

Note on raw format support: raw formats vary from camera to camera, so new camera support requires an application update.

Interface

Once you load a raw file into the application, it will show a raw image preview on the left half of the window, with a raw value histogram and statistics on the right half. The histogram and statistics are displayed separately for every color channel¹.

To inspect an individual pixel, move your cursor over the image. Pixel coordinates, along with a color filter array pattern in its neighborhood, are shown in the bottom left corner. Raw values of the closest pixels to the cursor are pinpointed on the histogram.

Raw value histogram

The raw value histogram shows the distribution of raw values on the photographic exposure scale. The numbered vertical lines designate exposure stops. One stop corresponds to a doubling of the raw value: 0, 1, 2, 4, 8, …, 2^(raw bits), where “raw bits” is a bit count per raw value used in a particular raw format. The total range is divided into histogram bins (subranges of fixed width on a logarithmic scale). A rectangular bar is drawn over the bin with height proportional to the average count of raw values falling into the bin range. The bin width can be configured from 1/3 EV to 1/64 EV.

Histogram bars are clearly visible if bin width is set to coarse 1/3 EV.

The vertical axis scale can be compressed, if needed, to reveal bars of small height. Select the vertical axis settings “Square Root” or “Logarithm.”

Overexposure and underexposure

The largest theoretically-possible raw value is the raw saturation level. It is marked by the dashed line in the last stop of the histogram. The red and blue dots on the horizontal axis designate the overexposure and underexposure levels. Note that the overexposure level is different from the raw saturation level².

If enabled in settings, the image preview highlights overexposed and underexposed pixels with the colors red and blue.

Example of a heavily-overexposed image.

The overexposure and underexposure levels can be shifted by the exposure warning offset. This allows you to model what under and overexposure would look like, if the photo was taken with a different exposure. The offset affects both image preview warnings and the statistics. Enter the offset value in the corresponding settings entry. Note that this modeling will be inaccurate if the raw file is heavily overexposed or, to a lesser degree, underexposed.

Statistics

The following statistics are collected and shown separately for every color channel:

Designation	Meaning
Min	Minimum raw value
Max	Maximum raw value
Avg	Average raw value
OvExp	Overexposed pixels count
UnExp	Underexposed pixels count

Settings

Click the “gear” button to change the settings.

The following settings are available:

Turn the underexposure and overexposure warnings on or off in the image preview.
Enter exposure warnings offset.
Select histogram bin size.
Select histogram vertical axis compression.

Export

Raw exposure allows export of the raw value histogram, both as image and source data. Click the “export” button or select the export option in the menu to export the histogram.

Raw samples

Raw files used in the screenshots above are available for download.