MegaPixels and Pixel Peeping – A Layman’s Primer
There are many sources of information describing the scientific and technical aspects of modern digital camera functionality and design. And there is also a fair amount of proprietary technology used by various camera manufacturers which is patented intellectual property and not readily available to the public. Certainly the vast majority of camera users who simply strive to make lovely images with a digital camera can still learn to do so without multiple advanced degrees in optics and digital electronics. But a grasp of certain fundamental digital imaging concepts can still help us to improve the results we get from our cameras. Suffice to say, this article is written for the layman and intentionally sacrifices technical depth for the sake of simpler understanding. I hope it will be helpful to some of our readers.
What are Megapixels?
At the heart of every digital camera, is a relatively small electronic sensor chip which contains a vast array of tiny photosensitive elements called pixels. This sensor array plays essentially the same role that a frame of film does in film cameras. These tiny photo-sites collect the light coming through the camera lens, which is then recorded as data and processed electronically to ultimately become our digital photos. One of the most commonly misunderstood specifications of digital cameras is the quantity (measured in millions) of these tiny pixels contained on the sensor… aka the number of megapixels (mp). Camera buyers often assume that a 12mp camera is better than a 10mp camera, or that one with 24mp is superior to one with 16mp. And while there certainly can be advantages to higher resolution pixel arrays, the size and spacing of these pixels and the precision of the data they record is also crucial to the quality of the images ultimately rendered. Various classes and models of cameras use different sized sensors as well as different numbers of megapixels, and larger sensors generally offer superior image quality and dynamic range at the pixel level. So a full-frame DSLR camera with its large sensor will typically record a higher quality image than a Micro Four Thirds (MFT or m4/3) camera using a medium sized sensor, and a much higher quality image than a small-sensor compact or camera phone. These differences in sensor sizes are generally much more significant to image quality than the number of megapixels they contain.
What is Pixel Peeping?
A lot of images are never viewed at the full-resolution recorded by the camera sensor however. Indeed the vast majority of digital photos are viewed only on relatively low-resolution displays like computer monitors, TV or cell phone screens. These display devices typically have resolutions of 2mp or less, so to display the images in these smaller media formats a re-sampling of the image is done essentially discarding a large percentage of the data originally captured by the sensor. Under these circumstances it may be difficult to detect the image quality differences between the outputs of various cameras despite major differences in their size or cost.
So why do so many photographers and even casual camera buffs continue to long for that next “better” camera model with the superior “image quality”? Well there’s more to a quality image than pixels of course, but that’s what this article is about… pixels.
There are endless disagreements amongst photographers about the usefulness of viewing images at full resolution, (often referred to as pixel peeping). Many people feel that pixel level quality is unimportant since their images will never be presented at 100% size. Many also know that the quality of the images their cameras produce at full size is not too good, and so they avoid examining the quality of their photos at the sensor level. But there is a powerful case to be made for pixel peeping, and I’d like to share some thoughts on that now.
First, the sharpness and focus of any given photo taken is often not easily discerned when viewed at reduced resolutions. If a series of shots is taken of the same scene (especially hand-held), the focus and motion blur caused by camera and/or subject movement will often vary significantly amongst them. Here’s an example…
At reduced resolution these consecutively shot frames may appear to be of equal quality:
But examining these seemingly identical shots at full resolution reveals a different story:
Choosing which frame of any such series to process and/or print is obviously better done at high-res. The same is true for assessing variations in color, exposure and artifacts like noise and lens induced aberations (eg: corner softness and fringing). All of the qualities and/or deficiencies in any given photograph are better and more easily evaluated at high resolutions. This is why slide film photographers have always used a loupe or magnifier to analyze and select their best shots for processing.
For bird and wildlife photographers (or anyone shooting distant objects at longer focal lengths) there can be another important reason to pixel peep… cropping. Even when using the longest telephoto lenses, wildlife is often still too far away to compose a shot where the subject bird or animal is large enough in the frame to present them as we’d like. In these situations cropping is frequently used to allow the target object to be shown larger in the finished image. Using post-processing software to remove significant portions of the captured frame lets us present distant subjects larger in the processed photo, but doing so also means displaying the kept portion significantly closer to the full resolution of the sensor. The high-resolution image quality of the sensor thus becomes much more important and evident as deeper crops are employed. Less detail is lost to re-sampling, and any artifacts or imperfections in the pixel level image is also revealed.
Conversely, down-sampling the images captured by high resolution sensors reduces both the fine detail and the artifacts seen when viewed at reduced sizes.
Finally, for those who post-process their images, nearly every adjustment that is performed in software can be made more judiciously and with greater precision when done and evaluated at high resolution. The results of better selection and processing afforded by working on your photos at higher resolutions is most beneficial when printing or displaying them at larger sizes. But it can improve the quality of your images at whatever size you view or present them, in any media format, and no matter which digital camera was used to take them.