Diffraction in Photography
Introduction
Diffraction is an effect that causes a loss of sharpness across the image frame.
This article will illustrate that it is largely irrelevant in practice and all sensor formats are effected to the same extent and suffer from the same limitations, contrary to popular belief.
Diffraction in Photography
Diffraction - General
When light passes through any size aperture, diffraction occurs.
Therefore, the focusing of light always produces diffraction, regardless of the size of the aperture.
The diffraction pattern is circular, with a bright center and concentric rings of decreased intensity around it. This pattern is called “Airy Disk”, named after George Biddell Airy.
Different wavelengths of light produce differently sized Airy disks.
Airy Disk
Size of the Airy Disk
For all practical purposes, the f-number or f-stop determines the size of the airy disk.
The smaller the f-number, the smaller the Airy disk.
Airy disk diameters
Resolving Power and Diffraction Limit
The diffraction phenomenon limits the theoretical resolving power of every lens at a given aperture. However, as there are no ideal lenses, the resolving power of the lens will always be below the theoretical upper limit.
The larger the f-number, the lower the resolving power of the lens. The size of the Airy disk will reduce discernible detail in the projected image.
Diffraction and Depth of Field
Stopping down a lens leads to increased depth of field and increased diffraction. Both effects scale linearly.
Diffraction and depth of field - a linear relationship.
Diffraction in Practice
Stopping down a lens leads to more depth of field and better sharpness, as it reduces aberrations.
Note: This is a rule of thumb. Some high performance lenses will deliver the best image quality wide open or almost wide open.
At a certain point, however, the negative effect of diffraction will decrease the resolved image detail and overall sharpness of the photograph.
Click the images below to enlarge.
Recommendations
1) Always pick an aperture that gives you enough depth of field. It is better to have less overall sharpness than having an important part of the image out of focus.
2) Depending on print/output size, the effect of diffraction might not be visible.
3) Using a lens wide open will theoretically give the best result, but because of aberrations stopping down will improve overall contrast and sharpness.
4) Be aware of the phenomenon, but do not worry about it.
Diffraction and Pixel Density
The maximum resolving power of the lens and the accompanying optical limitations are characteristics of the lens. They do not change with the image sensor’s size or pixel density.
Regardless of pixel density, the effect of diffraction is the same. Because a sensor with more pixels records more overall detail, it will also record the effect in greater detail. However, a sensor with less pixels will loose sharpness and detail resolution to the same extent as a sensor with more pixels.
The misconception that high resolution sensors are more prone to diffraction stems from the problem that people tend to examine large MP photographs at a higher magnification. Compared at the same output size, there is no difference.
Photographed at f/22, click to enlarge.
Diffraction and Sensor Format
The effect of diffraction does not change with different sensor formats. It depends solely on the f-number and the lens.
Note: An f/1.4 lens produces the same amount of diffraction regardless of sensor size.
Because images of smaller sensors are magnified to a greater degree, the effect is more noticeable.
Smaller sensors exhibit more depth of field at the same angle of view and f-number as larger sensors. Therefore, smaller f-numbers can be used to achieve the same depth of field as on larger sensors.
As the crop factor essentially determines the enlargement, the effect scales with the crop factor.
Conclusion: Depth of field and the adverse effect of diffraction scale with the crop factor. Therefore, the impact of diffraction at a certain depth of field is exactly the same across all sensor/film formats.
A larger sensor has no advantage over a smaller sensor when it comes to diffraction.
Important!
As pixel density is irrelevant and the diffraction effect scales proportionally with depth of field, a small format camera is not at a disadvantage when it comes to diffraction
Recommendations
1) The amount of diffraction which photographers find tolerable is subjective.
2) I recommend shooting a series of test pictures at all apertures to determine the subjective sweet spot.
3) If a specific lens was not tested, the following table can serve as a guideline for apertures that provide a generally acceptable balance.
