When I was starting with photography I always wanted to get the best out of my camera. I love to shoot macro images and I always tried to shoot the sharpest photos with a wide depth of field (DOF). By setting a small aperture of f/18 I was not pleased with the result! The image lacked sharpness! My logic reaction was to shoot at even smaller apertures of f/22 and f/32. Now I was really disappointed! Something must have been wrong with my expensive lens; it just didn’t produce sharp pictures! It took me quite some time to find out what was wrong. Without knowing I had crossed the borders of what was possible in photography. Due to the nature of light; a wide DOF doesn’t always lead to more sharpness! Diffraction can lead to a loss in sharpness and diffraction increases when using small apertures.
What is diffraction and how does it influences image sharpness
Light rays that make contact with an object will be bent. This also counts for rays that pass trough your diaphragm and make contact with the edge of the diaphragm opening. Smaller diaphragms have a relatively larger edge surface leading to relatively more rays to be bent. This is called diffraction and leads to loss of sharpness.
By changing your aperture you will be able to adjust your dept of field. At small apertures of f/22-f/45 your will be able to shoot images with a wide depth of field. But by using these small apertures there will be relatively more diffraction resulting in an overall loss of sharpness.
To achieve optimal sharpness you will need a certain degree of depth of field without having too much diffraction. Most lenses are sharpest at apertures about two stops above its maximum aperture. Depending on the lens, this will be in the range of F/4-F/11.
If you don’t need the depth of field you shouldn’t use diaphragms smaller than f/11. Portraits should be shot at lower apertures (f/1.4-f/8). When taking pictures that need a wide depth of field it is best not to stop down to the smallest aperture. For landscapes or macro I mostly stop down until f/11 or f/16 and I adjust my point of focus to achieve optimal use of the depth of field.
Differences in diffraction between cameras
The amount of diffraction varies from camera to camera, as it is influenced by resolution and sensor size.
The higher your camera resolution the sooner diffraction will show. With an aperture of f/16 a full frame camera with 12mil pix will show less diffraction than a full frame camera with 20mil pix. However when you produce same size prints out of these two camera types the amount of visible diffraction will be more or less the same. The advantage of the higher resolution sensor than lays in the possibility to print larger and the lower noise. The difference in diffraction will only show at prints with high enlargement. This also means that there is a limit to the maximum resolution for our cameras.
The smaller your camera sensor the sooner diffraction will show. When you compare a full frame camera with a cropped sensor camera of the same resolution and at the same aperture you will notice more diffraction in the small sensor camera. However this will be compensated as the smaller sensor camera doesn’t need apertures as small to achieve the same depth of field.
Testing my macro lens for diffraction
I have performed a test with my Sigma 105mm f/2.8 macro lens on a Canon 5DII. Although this is a very good lens I never use it at its smallest aperture of f/45. To avoid diffraction I always use apertures of f/16 or wider.
The test is performed with the camera mounted on a tripod, mirror uplock and a cable release. The images are shot in raw and are not sharpened in post processing. I will show you 100% crops of the point of focus. It’s an image of a straw hat with lots of small detail.
The test clearly shows that the smaller your aperture the wider your DOF will be. At wide depth of fields the image looses overall sharpness. Optimal sharpness is achieved at an aperture of f/8. Diffraction starts to become visible at f/16 but the image is still acceptably sharp. At f/22 quite some detail/sharpness is lost and at f/45 the image really lacks detail. Images shot at apertures of f/22-f/45 have a wide depth of field but become unusable due to the lack of overall sharpness.
Every lens-camera combination has its optimal apertures and results for other combinations will be different. When using this lens-camera combination I should shoot at apertures of f/5.6-f/11 to achieve optimal sharpness. Wider apertures (f/2.8-f/4) give acceptable sharpness but are a little softer; these apertures are perfect for portraits. When I want to shoot images with an optimal combination of sharpness and depth of field my aperture should be in the range of f/11-f/16. Using apertures smaller than f/16 will lead to a significant loss of detail and these small apertures should be avoided.
Diffraction has a significant influence on the sharpness of our images. The smaller the aperture the more diffraction will occur. Every lens-camera combination has its sweet spot where the diaphragm will lead to the sharpest images possible. In order to find the sweet spots of your camera-lens combinations you can easily perform a simple test.