This is one of a series of articles that cover important qualities of digital images that any industry professional dealing with digital photography must know and understand.
Qualities of Digital Images:
- Bit Depth: 8-bit Vs. 16-bit
- Sharpness vs. Focus
- Dynamic Range
- Image Noise
- Image File Formats, Resolution & Size
In photography, dynamic range is the ratio between the least and the highest measurable intensity of light. Simply put, it is the range of light and tonal detail from the darkest shadows to the brightest highlights. A good understanding of this concept will not only make us better photographers and visual artists, but also enhance our perception of the real world.
The concept of dynamic range can become highly complex and technical. How to address the subject depends on whether we are describing a capture device (such as a camera or scanner), a display device (such as a print or computer display), or the environment and subject itself.
Dynamic Range ultimately relates to the tonal detail and tonal range of an image. The differences are due to various factors, such as angle of view, resolution, detail, and light sensitivity. Our eyes, for instance, are able to look around and automatically adjust themselves depending on the subject matter, while digital cameras capture still images. This is what gives the human eye an advantage over cameras and film. In terms of resolution, the human eye has the ability to resolve the equivalent of a camera with 52 megapixels. However, we are unable to remember every detail of what we see. The dynamic range of our eyes is instantaneous in that we are able to adapt quickly depending on the light condition of a setting. That is because the iris and pupil corrects the sensitivity of our vision based on the lighting conditions almost instantaneously.
More Than Meets the Eye
The camera cannot adjust as quickly as the complex human eye. A camera sensor will either capture all the tonal detail in the shadows, and therefore overexpose or ‘blow-out’ the highlights, or expose for the brightest portion of the scene and lose the detail in the darkest shadows. This is why when you see an individual shoot for HDR (High Dynamic Range) they either bracket (take multiple shots at different exposures) manually or utilize a camera that has an automated exposure-bracketing (AEB) feature. Bracketing allows you to layer a composite of all the images, retaining the detail present in each exposure setting, and therefore bringing out the details that would otherwise be lost to under or over-exposure.
In a camera, the dynamic range is determined by the sensor and the way light interacts with it. If the sensor has larger pixels, the higher the overall dynamic range will be. This does not mean, however that the more pixels you have, the more dynamic range you will have. Pixel count and pixel size are two different things – for example, a crop sensor versus a full-frame sensor DSLR. This means that cameras with sensor technology that possess a larger dynamic range based on pixel size are able to capture both highlight and shadow detail simultaneously.
The dynamic range of a digital camera can be described as the ratio of maximum light intensity measurable (at pixel saturation), to minimum light intensity measurable (in relation to image noise).
Increasing Dynamic Range – Exposure and Processing in RAW
Like mentioned before, bracketing multiple exposures together can increase your dynamic range. This is great for landscape and architectural photography but is not a great idea when working with a model or other live or moving subject. Slight movements between each shot creates what we call ‘ghosting’. Ghosting is highly difficult to remove in post-processing and can give the subject a ‘hazy’ look.
Variations in shutter speed and aperture also have an impact on the dynamic range of an image, given their relation to overall image exposure.
RAW files preserve the dynamic range and tonal details, whereas JPEGs often clip details. Working with images shot in RAW format will allow you to successfully adjust luminance in post-production.Because of this, ask your client to submit RAW files when possible rather than JPEG. Also, since the data has not been compressed already, the exposure of a RAW file can be adjusted slightly. Exposure compensation can correct for metering errors, or can help bring out lost shadow or highlight detail, increasing the dynamic range of the final image after the photo has already been taken.
[wp_lightbox_prettyPhoto_image link="http://www.retouchingacademy.com/wp-content/uploads/2013/07/DOC_8128z_ed_final.jpg" description="HDR high dynamic range" source="http://www.retouchingacademy.com/wp-content/uploads/2013/07/DOC_8128z_ed_final.jpg" title="HDR high dynamic range"]Images in this article courtesy Doru Oprisan
Additional Factors – Pigment or Color Density and Contrast Ratio
Pigment or color density affect the dynamic range in the same way they affect tonal value. This is the same for contrast ratios. Curves and Levels adjustments in Photoshop can be used to bring out detail in slightly clipped areas, or suppress it.
The dynamic range of images relate to the range of light to dark which the camera can capture without becoming either pure black or blowing out the highlights. We can compensate and bring out lost detail in post-processing by adjusting the exposure of the image or creating layered composites of the same image shot with the bracketing technique. RAW files retain the most uncompressed detail and allow for more adjustments to the luminosity and tonal range of the image. Increasing contrast will also help bring out detail in shadows and highlights, enhancing the overall dynamic range of the image.