Under what circumstances does f/22 cause diffraction?

Question

Today I went out to get a photo of a historic building with a lake in front and opted to shoot at f/22 with a tripod. To my surprise, the images aren't sharp. I also used the Olympus high res mode and had a similar outcome. I asked around, and have been told that anything over f/11 I'm going to suffer from diffraction which will reduce sharpness.

Is diffraction a common symptom of a small aperture? Or are there other factors that contributed to the lack of sharpness in my images?

I was shooting on an Olympus OM-D EM1 Mk iii with a 12-45 Pro lens. ISO 100, f/22 aperture priority. IS off. WB cloudy, as it was a very overcast day.

Answer 1

Under what circumstances does f/22 cause diffraction?

Pretty much all of them. All apertures cause diffraction due to the interaction of the wave nature of light with the edges of the aperture.

But the real question is, "When does f/22 cause noticeable diffraction?"

The aperture at which diffraction is first detectable at the pixel level is the point we define as the Diffraction Limited Aperture, or DLA.

On digital cameras the Diffraction Limited Aperture (DLA) is determined by the size of the sensor's pixels.

This is because it is related to the size of the circle of confusion for a given aperture. With a digital sensor the DLA is the aperture at which the size of the circle of confusion becomes larger than the sensor pixels and begins to visibly affect image sharpness at the pixel level. Diffraction at the DLA is barely visible when viewed at 100% (1 image pixel = 1 display pixel) on a display with pixels large enough for the viewer to resolve individual pixels. As sensor pixel density increases, each pixel gets smaller and the DLA moves to a lower f-number.

DLA does not mean that narrower apertures should not be used.

It is where image sharpness begins to be compromised for increased DOF. Higher resolution sensors generally continue to deliver more detail well beyond the DLA than lower resolution sensors until the "Diffraction Cutoff Frequency" is reached (a much narrower aperture). The progression from sharp to soft is not an abrupt one.

DLA can vary greatly from one camera to the next. Among Canon's current lineup the highest DLA is f/10.6 for the 1D X Mark III and R6 Mark II, both with 20MP on a full frame (36X24mm) sensor and ≈6.6µm pixel pitch. The lowest DLA in Canon land is currently f/5.2 for the 32.5MP APS-C R7 and M6 Mark II with pixel pitch of 3.2µm. Your OM-D EM-1 Mark III has a 20.4MP Micro Four-Thirds sensor with pixel pitch of 3.32µm. That puts the DLA for your camera at around f/5.3.

What happens when you select an aperture beyond the DLA?

At the DLA diffraction begins to negatively affect sharpness at the absolute point of focus. Diffraction at the DLA is barely visible when viewed at 100% (1 image pixel = 1 screen pixel) on a display with pixels large enough to be at the threshold of the viewer's eyes ability to resolve.

In exchange the narrower aperture increases the depth of field that is in nominal focus on either side of the absolute point of focus. There are techniques that allow you to maximize depth of field using the widest aperture possible. Learning how to calculate hyper-focal distance (or carrying a chart for each focal length you use) allows you to place the point of focus as close to the camera as possible while allowing for everything beyond that point all the way to infinity to remain acceptably in focus. At close distances and wide apertures the depth of field is about equally in front of and behind the point of focus. As the subject distance increases and/or the aperture narrows, a larger and larger percentage of the DOF is behind the point of focus. Here is a link to a DOF calculator you can use to illustrate this.

When does using f/22 become a noticeable issue?

It depends.

• On a camera with larger pixels it will be less of an issue than on a camera with more pixels crammed onto a smaller sensor.
• If the resulting image is going to be sized for web viewing at relatively low dpi and high compression it won't be much, if any of a factor.
• If the image is printed at relatively small sizes it won't be very much of an issue.
• If, on the other hand, the image is going to be used for a high resolution large sized print or cropped heavily when displayed on a monitor it will become much more of an issue.
• If you're pixel peeping at 100% while holding a magnifying glass above the screen, it will be an extreme issue.

Answer 2

All apertures cause diffraction... what you are asking is when does diffraction limit resolution.

The moment the airy disk (lens projected point of light) is larger than the photosite (pixel) recording it that reduces the MTF/sharpness (contrast). But the general consensus is an overlap of 2-2.5 times the size of the sensor photosites/pixels is when the image resolution is diffraction limited.

See pages 10 and 17 of this paper: https://www2.uned.es/personal/rosuna/resources/photography/Diffraction/Do%20sensors%20outresolve.pdf

But resolution loss due to diffraction isn't necessarily "unsharp"... you loose smaller/finer details due to the diffraction but larger details are still well resolved. E.g. a lot of macro imagery is significantly diffraction limited, but you likely wouldn't otherwise know that.

Answer 3

Diffraction:

When we study the behavior of light, we discover it sometimes behaves in a similar way as water and sound waves. It is well known that a hill or a building shadows sound, we can hear shouts coming from the other side. Now these same obstructions block light waves but not completely. We can’t see around corners however light is observed to fill space behind sharp edges by spreading around them. The technical name for the phenomena is diffraction.

Now a camera lens is a converging lens that causes light rays approaching from tiny points on the subject to be reproduced as tiny points of light on the surface of film or digital chip. It is the size and shape of these image circles that we are taking about. In perfect optical system they would be points. Now a point is too tiny to measure. Sorry to report that they are not points but tiny circles. Often called circles of confusion because under the microscope they seem somewhat fuzzy. One principal investigator was George B. Airy, British 1801 ~ 1892, Director Cambridge Observatory. In his honor these image circles are called Airy disc.

Under magnification the Airy disc has a central core of light surrounded by concentric circles of light with dark bands in between. About 84% of the brightness is in the central disk, 1.75% in the first ring, 0.42% in the second, 0.16% in the third. After that the rings are too dim to be of concern. Because the Airy disk is the least possible image element we are concerned with its size and spacing. If it overlaps with its neighbor, it is impossible detect separation. This is the stuff of resolving power and acuity. The key point is, the resolving power of a lens decreases with the aperture, because this increases the diameter of the Airy disc.

We know that light rays passing near the blades of the camera, iris are shadowed but not completely blocked. Some of the blocked rays bend and some of these bleed into the image forming rays.

Studied at length by Lord John Rayleigh 1842 ~ 1919 British Nobel prize physics 1904. His work is the standard. Rayleigh tells us that the resolving power of the lens decrease as we close down the aperture.

Table of resolving power using Rayleigh equation, wavelength 589 millimicrons

f/1 1392 lines per mm

f/2 696 lines per mm

f/2.8 487 lines per mm

f/4 320 lines per mm

f/5.6 249 lines per mm

f/8 184 lines per mm

f/11 127 line per mm

f/16 87 lines per mm

f/22 63 lines per mm

f/32 44 lines per mm

Answer 4

Under what circumstances does f/22 cause diffraction?

Taking "cause diffraction" literally, any and all circumstances! :-) All apertures diffract; the only time there's no diffraction is when you have an infinite plane wave.

But the question is really asking about detectable diffraction effects in digital images due to near-circular apertures.

tl;dr: above f/8

A half-century ago when I first started reading about photography, I think the answer was the same; going significantly above f/8 we'd run into diffraction assuming you had some nice ASA 125 monochrome film where the grains were roughly evenly distributed over areas averaged over several microns. https://cool.culturalheritage.org/videopreservation/library/film_grain_resolution_and_perception_v24.pdf

The Airy disk is the diffraction pattern of a circular aperture in the far field. It doesn't have a specific size, but traditionally we use 1.22 λ / D for the half angle in radians, where λ is the wavelength and D is the diameter of the aperture.

If the aperture is at the pupil of your lens system with a focal length f, then the corresponding radius (using the small angle approximation) is 1.22 λ f / D.

So if the width of the Airy disk is one pixel, the effect will be slightly noticeable if you really really analyze carefully, so let's add a factor of 2 and say that the easily detectable diffraction effect is when the radius is equal to a pixel size d.

d = 1.22 λ f / D

D = f / f/no. and f/no. in this case f/no. is 22. So

d = 1.22 λ f / D = 1.22 λ f.no. = 1.22 λ 22

With λ = 0.55 microns, then the circumstances will be when the pixel size is 15 microns.

I think your pixel size is about 3 or 4 microns, so you'll start to see detectable diffraction (if you look very carefully) at about f/5.6 or f/8.