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With the ever-increasing number of pixels manufacturers can cram onto a single digital image sensor, the optics themselves are beginning to become the limiting factor in image quality. This is making it all the more important to stop down our lenses in an effort to squeeze as much sharpness from them as possible. Unfortunately, while stopping down is a great way to sharpen pictures, if we go too far, we end up with deteriorating image quality. This optical effect is called diffraction.
Diffracted light creates a softer image. Non-Diffracted light creates a sharper, more detailed image. (Click to zoom)
A lens utilizes an aperture to help control depth of field, one of the most important tools in photography. However, leaving the aperture wide open will often result in slightly soft images, due to the lens's lack of ability to focus the light rays at that aperture. On the other hand, if you stop down too much, diffraction will also soften images because the extremely small aperture opening will bend the light in a different way, resulting in rays that aren’t accurately captured.
Light entering the camera from a single, small point has the potential to spread out at
sharper angles, causing diffraction and softer images.
We could always just ask that manufacturers work on creating optically perfect lenses. However, this is simply impractical, if not impossible. They may be able to come close, but not at a reasonable price point or physical size. So, in order to get the best possible performance from a lens, we have to find its sweet spot. There are many factors that go into finding the sweet spot of a lens, perhaps the most important of which is the camera’s image sensor.
This is why you're always told to stop down to get the best, sharpest images from your lenses. Also, with zooms, it's useful to find the best focal length in the range, usually somewhere in the middle—although it can easily skew to one side or the other. But, there are issues if you stop down too far.
"So, what is the major downside of shooting closed down? Softness."
So, what is the major downside of shooting closed down? Softness. Basically, each pixel on the sensor is receiving a cone of light, delivered by the combination of glass and aperture blades. The smaller the opening, the larger these cones become, because the light now has to be bent in order to cover a larger area than the aperture opening itself. These cones, and the circle, are called "airy discs," due to the disc-like shape that hits the pixels.
Where is the problem with this? When the discs become so large that they start to overlap one another, softness is more apparent. Each pixel should, theoretically, receive a single cone of light to resolve an image perfectly. This is rarely the case in real life. The points at which you will see the effects of diffraction are dependent on both the lens and the sensor. Having smaller pixels means that you may notice diffraction sooner, compared to a camera that has fewer megapixels on a sensor with the same physical dimensions. However, this isn't always the case. New sensor technology, such as "micro lenses," allows manufacturers to extract greater sensitivity, by guiding the light into the "pixel wells." However, sensors aren't perfectly covered in pixels, since there are very, very small gaps between them.
Knowing how the image sensor is designed is important, because it tells you how much detail the camera can realize before you take into account things like diffraction. For example, most color image sensors utilize a filter array, so each pixel will only capture a single color: red, green, or blue. The most common of these is the "Bayer array," although some manufacturers, such as Fujifilm, have designed their own. Having arrays like these means that each pixel won’t have the complete picture, and that software interpolation is required to produce a final image. Since each pixel is not independent, you will be seeing slightly less resolution than you would imagine.
This is why some camera manufacturers have taken steps to fix problems caused by the use of a color filter array. Fujifilm did this by rearranging the pixels, and using new algorithms. Sigma took a different approach and stacked the different colors in order to get full RGB data at each pixel. Or, you could go back to black-and-white with something like the Leica M Monochrom, or the Red Epic Monochrome, which eliminate the color array altogether in order to provide increased resolution by focusing purely on luminosity data.
A digital image will also commonly be subjected to an anti-aliasing filter. A simple explanation of this layer is that it reduces the sharpness of the image. Now, why would you want to do that? Well, when working with pixels, you sometimes encounter unwanted artifacts, such as aliasing, and moire. These can be seen in images of fine lines that aren’t quite resolved, or when banding is created on a pattern in an image, or when dark areas simply appear blotchy. A recent trend is for camera manufacturers to remove the anti-aliasing filter altogether. It appears that many photographers agree that it's worth risking aliasing and moire, in order to get more out of the sensor.
These physical characteristics of sensor design will have a great impact on the overall sharpness of an image, more than the lens itself will. And, you may not see the effects of diffraction until much later than you technically should. Since you're already fighting against pre-softened images, it's important to get the best performance out of your lenses.
"Understanding how diffraction affects your camera can help you pick out the best lens to use for a certain shoot..."
Consumer technology is gradually getting better, but completely changing all of these factors are important considerations when you need to get the best image quality possible from your 24-, 36-, and 50-megapixel sensors. Also, the difference in depth of field between f/4 and f/16 could mean a lot when shooting a landscape or macro shot. These considerations must be taken into account when taking carefully planned photographs.
Of course, everything can vary greatly, depending on your equipment and subject matter. Some softness may be tolerable, in order to get the composition you require. Composition, lighting, and choice of depth of field make a much larger difference in the quality of an image than it being a little bit soft.
Understanding how diffraction affects your camera can help you pick out the best lens to use for a certain shoot, on a certain camera for optimal image quality. For example, if you need more depth of field for a landscape image, an ultra-wide-angle lens will help, because it will have a much greater area in focus, even at larger apertures. If you're shooting a focus stack for macro photography with that telephoto lens, you may think about shooting more images at f/5.6 than just shooting at f/22.
Softness will forever be an issue. Wide open you will see it, and too far stopped down you will see it. Always make sure to test out and know your gear, to determine its optimal performance. And really, just stop worrying about all the little details and go shoot. Knowing this will help you choose the best settings with more confidence, and help you get the best photographs you can.