Today’s Question: How does JPEG compression affect the dynamic range available on the overall image and how the display used affects each characteristic of the final image?
Tim’s Quick Answer: JPEG compression doesn’t technically affect the dynamic range potential of the image, but does affect overall image fidelity and detail. The characteristics of the display being used have a potentially significant impact, though with modern displays this isn’t generally a major issue.
More Detail: In the context of photography, dynamic range is most applicable in the context of capturing an image in the first place. In other words, dynamic range mostly relates to the range the camera is able to capture from the darkest shadows to the brightest highlights.
Once the image data is captured, the dynamic range potential of the digital image is somewhat fixed based on how the image is processed and the bit depth. And even the bit depth is more applicable to the smoothness of gradations rather than a strict dynamic range at this point. After all, a digital image can always have a dynamic range that extends all the way from pure black to pure white. What’s more important is how pixel values were mapped to the digital image based on capture data.
One of the significant issues with saving an image as a JPEG is that the image will only support an 8-bit per channel bit depth. That translates to only 256 shades of gray for a black and white image, and almost 16.8 million colors for an RGB image. This compares to 65,536 shades of gray for a 16-bit black and white image, and over 281 trillion possible colors for a 16-bit RGB image.
The reduced bit depth reduces the potential for detail in the image and increases the risk that gradations of tone and color won’t be smooth. Furthermore, JPEG compression reduces detail and perceived sharpness in the image.
When it comes to the digital display of an image (whether JPEG or something else), the attributes of that display play a key role. Display resolution determines the perceived quality of the image. For example, a display with a 4K resolution will provide a crisper view even if set to a lower resolution. The dynamic range and color space capabilities of the display can also affect perceived image quality. For example, just because an image contains excellent shadow detail does not guarantee that the shadow detail will be visible on the monitor display, depending on the specifications and configuration of that display.