Maybe this will be too technical a story, but I will try to explain it as simply as possible.
Modern cameras with often more megapixels are going to have a serious problem that cannot be easily solved.
One aspect of photography is the loss of sharpness caused by diffraction.
What I can still remember during my studies is that light works like a wave.
A light wave travels in a straight line, but can also bend when it passes through or around objects.
When light shines through the lens of a camera with a large aperture, the light does not or hardly diffuse.
However, something strange happens when the light passes through a small hole, like a camera with a small aperture (high F-number) because then it bends and interferes with itself. (The light will spread and rays of light will collide).
You can compare it a bit with a garden hose, if you squeeze it, the water jet will spread more.
This interference is called diffraction (refraction).
This effect is normally negligible because smaller apertures often improve sharpness by minimizing lens aberration, however with sufficiently small apertures this strategy becomes counterproductive and reaches a point where diffraction takes over.
When diffraction occurs, it has an effect on the sharpness and details in a photo, because they decrease.
Diffraction is therefore an optical effect that limits the overall resolution of photography.
Knowing the limit allows you to maximize the detail in the photo and an added benefit is to avoid unnecessarily long exposures or high ISOs.
Diffraction in simple terms is a phenomenon that occurs with light when it “interacts” with an obstacle such as an objective.
Most of us are probably familiar with the light diffraction patterns on the back of CDs.
Diffraction can therefore also occur in a camera, which can be a big challenge if you want a lot of depth of field, but especially sharpness.
It can cause photos to lose their sharpness, something you don’t normally want.
Below is a picture with a simple diagram of what I found on the internet and shows how light particles hit the sensor of a camera at large and smaller apertures.
In the drawings above you can see that at a small aperture (high F-number) light rays travel different distances with some moving out of phase and then starting to disturb each other.
This interference produces a diffraction pattern with peak intensities where the amplitude of the light waves add up.
For an ideal circular aperture, the 2D diffraction pattern is called an “Airy disk” (after its discoverer George Airy).
The width of the “airy disk” is used to define the theoretical maximum resolution for an optical system.
Due to, among other things, the sensor’s anti-aliasing filter, an “airy disk” can have a diameter of about 2 pixels before diffraction limits the sharpness. (Assuming a perfect lens). However, diffraction will have a visual impact before this diameter is reached (at ~ 1.5 pixels).
The size of the “airy disk” is especially useful in the context of pixel size.
Below are some “airy disk” drawings compared to the pixel size of a few camera models (I have only used full-frame cameras here with a different number of pixels on the same sensor surface for convenience):
Canon 5D MK3 (24MP):
In the pictures, each “diamond” is a pixel. Now you see above that with F/16 the “airy disk” already covers a total of more than 2 pixels, so the sharpness will decrease here. So F/11 is the limit here if you assume 1.5 pixels.
Canon 5D (13MP):
In the pictures above you can see that the “airy disk” at F/16 takes up about as many pixels as with the 5D MK3 at F/11. So with this camera F/16 is the limit.
Nikon D800 (36MP):
In the pictures above you can see that the “airy disk” at F/11 already just exceeds the limit of 1.5 pixel, so a lower F-number is the better choice if you don’t want to run the risk of diffraction.
So when the diameter of the “airy disk” becomes large in relation to the pixel size in the camera, it starts to have a negative visual impact on the photo.
Diffraction therefore sets a fundamental resolution limit if you want to stop down.
Because the size of the “airy disk” also depends on the wavelength of light, each of the three primary colors (Red, Green and Blue) will reach its diffraction limit at a different aperture.
The above calculations assume light in the center of the visible spectrum (approximately 550 nm).
Most digital cameras can capture light with a wavelength of anywhere from 450nm to 680nm.
The color green will lose sharpness first and blue the last.
Theoretically, you can stop down more with just the color blue without loss of sharpness than with the color green.
Obviously, diffraction is only a concern at small apertures (high F-number) in combination with the number of pixels and the area of the sensor (read size of the pixels).
Larger sensor with fewer pixels also suffers less from it, so you can stop down more.
In other words, a 24MP crop camera is more likely to suffer from diffraction than a 24MP full-frame camera purely because the pixels of a full-frame are larger …
So smaller pixels are more likely to suffer from diffraction and what do we see happening in recent years? Sensors that have remained the same in size, only the number of pixels has increased.
This applies to both crop and full-frame cameras.
So what does all this mean for photography in practice?
I once did a few tests with my Canon 5D MK3 and Canon 16-35 lens to see what the real effects of diffraction are during landscape photography.
I did this test with a fixed setup, heavy and stable tripod, remote control and with the mirror folded up to eliminate variables that can cause blur as much as possible.
I took some pictures with different apertures. The photos (24 MP RAW) showed that from aperture F/11 it resulted in a less sharp image.
Shots made with aperture F/9 and F/10 gave me the sharpest images in combination with more than sufficient depth of field.
It is important to know that I did the comparison between the photos at 100% zoom.
When zooming out it will be less visible.
It is possible that during landscape photography the available light is so much that you have to stop down more to avoid overexposed photos.
(Sometimes F/11 at ISO 100 is not enough).
In this case, it is better to use an ND filter than to stop down more to avoid less sharp photos.