If you enjoy reading about the science behind photography, you might have come across the term diffraction.
Diffraction usually comes up in conversations about an optical instrument’s ability to render scenes sharply. It is frequently brought up as a serious flaw to overcome in your photography. But is that true? Should you take care to avoid diffraction? And what exactly is diffraction, anyway?
In this guide, I’ll try to answer all of these questions directly. Without further ado, let’s get into it!
What is Diffraction?
Let’s start with definitions. Diffraction occurs when light waves pass through very narrow slits or openings. To understand how and why this happens, try to imagine the physical structure of light.
As you might know, light exists in photons, which are tiny particles. However, these particles travel together and behave as waves in motion. Confusing, I know!
Light waves generally travel indefinitely in one fixed direction radiating from their source.
However, sometimes, a light wave will encounter a semi-passable obstacle, like a small hole or a slit. In this case, the light wave traveling through the slit can actually bend and curve.
Displaying Diffraction With a Ripple Tank
This effect is actually not unique to light alone. Anything that travels in waves behaves the same way, including sound waves and water, for example.
That’s why physics classrooms often use an experiment involving visible water waves to demonstrate how a light wave propagates through the air.
The water waves are contained in a shallow basin called a ripple tank. By agitating the water mechanically, the tank doesn’t just make it easy to view the various wave patterns. The operator can also manipulate many individual diffraction features to alter the resultant wave.
The interesting thing about diffraction is that it comes in many diffraction patterns produced by different combinations of physical factors. This includes the wavelength of the light. Longer wavelengths behave differently from shorter ones.
Another is the slit that the light passes through.
By altering the shape and size of the slit in a ripple tank, different patterns emerge. But what is really fascinating is what happens when you switch to two narrow slits instead of one.
When the light has to pass through more than one slit, the resulting ripple patterns converge at certain points, overlapping and forming new waves. Eventually, these ripple waves blend together so much that they’re hard to make out individually.
Applying the Ripple Tank Principle on a Wider Scale
The very same design based on slits and ripples is what powers a certain kind of device known as a diffraction grating. Diffraction gratings are very common in physics labs, where they might be used in spectroscopy or to calibrate optical microscopes and other optical instruments.
These gratings use arrays of tiny etched slits, usually on a glass pane, to force light waves to diffract. The diffraction grating does this in such a way that the diffracted wave of the light scattering becomes a visible spectrum to the naked eye.
If you’ve ever handled a prism in physics class, the principle is similar, but the effect is much more impressive first-hand! Red light transmitted through the diffraction grating will separate from blue, green, and other elements of the spectrum, creating stunning effects.
Though usually applied in a strictly scientific setting, as I said, the light produced by diffraction gratings can also be utilized for photography. With the right light source and some creativity, a diffraction grating can make for truly impressive abstract shots.
While this can be stunning to watch in a live experiment or as a deliberate aesthetic effect, it can really be ruinous for general photography.
The Meaning of Lens Diffraction in Photography
As you can imagine, there is one part of the optical system in your camera that can create so-called single-slit diffraction. Of course, I am referring to your aperture.
The reason why lens diffraction can be a big deal is that it can completely throw off your focus. It doesn’t matter if your lens and camera both indicate perfect snap focus.
If the conditions for diffraction apply, it will modify your image and make fine details appear washed out. And none of us want that to happen, do we?
Let’s take a closer look at how diffraction occurs at a base level.
How Different Apertures Diffract Light Waves
The main determiners of the diffraction pattern in a camera’s optical system are the wavelength and aperture size. We have no awareness of the wavelength of the light hitting our sensor (unless you are shooting infrared photography).
However, we can control our circular apertures to a great degree. By adjusting the lens aperture width – or how we commonly refer to it, our f-stop – we can either totally eliminate or deliberately accentuate diffraction.
Wide open, a camera lens will often struggle to achieve sharpness. This is because light will enter the lens across such a large area that focus is too difficult to acquire.
However, a similar effect occurs at very small apertures due to diffraction. With the lens opening acting as a single slit, light entering at a sharp angle has to curve to reach the sensor, resulting in different focus points compared to the light entering in a straight line.
Just like in the ripple tank experiment, the diffracting light travels in a non-uniform manner. Individual waves will intersect, creating interference.
On top of all that, you also need to take into account that different wavelengths of light will diffract differently, with longer wavelengths diffracting more than shorter ones!
These variances further contribute to something called phase difference. Phase difference exists where each light beam landing on your camera sensor will be in a different phase (think of it as a sort of “alignment” of the light wave).
You can visualize this effect in the form of light and dark bands, called a visual diffraction pattern. Each of the bright parallel lines represents a point on the sensor where the light hit. The dark lines represent “gaps” created by the diffraction.
You will find that, at small apertures, the diffraction pattern will show huge amounts of overlap around the middle third of your sensor surface, with varying degrees of dark lines parallel to it.
This uneven distribution is what causes the varying, localized blur effect that’s the telltale sign of lens diffraction.
Handling and Overcoming Lens Diffraction Flaws
So once you’re capable of diagnosing it properly, what do you do to fight against lens diffraction? There are a couple of things you can do, but by far, the easiest and most straightforward method is to open up your lens aperture.
When the exact resolving power of your lens tops out varies among different optical designs. Different lenses will also each have their own diffraction patterns, naturally.
To test the diffraction intensity pattern of your lens, I suggest mounting your camera to a tripod and carefully focusing on a highly-detailed subject.
Textured household items with written labels on them usually work well for this purpose. Textiles and plants with lots of small details (especially when viewed under a macro lens) also make for great candidates.
While keeping focus and exposure locked, shoot a series of photographs of this subject. Between each shot, alter the aperture by one stop until you’re through the whole range.
Compare each of these photos to find out at which f-stop your lens is at its sharpest and most diffraction-free!
Now, let’s go over some more general baselines to get you acquainted with the kind of performance you can expect from your lenses in most shooting situations.
Knowing The Limits of Your Lens
There are a few lenses that won’t display some degree of diffraction past f/11 or so. If you find yourself stopping down that far, you might want to consider increasing your ISO or choosing a slower shutter speed.
While numbers can vary quite a bit, expect the resolving power of most camera lenses to reach their peak performance between f/5.6 and f/8.
Don’t just dial in that magic number and leave it at that, though! Choosing the aperture setting where your lens peaks in sharpness are not always ideal.
Sometimes, you might actually need the large depth of field that a small aperture can give you. Short of using bellows movements in large-format photography, there’s usually no simple way to circumvent that.
Applying The Stop-Down Strategy to Real-Life Situations
You should try to find a compromise that suits the requirements of the shot in front of you. In portrait photography, for example, it’s an easy concession to stop down slightly from your maximum aperture to achieve better peak sharpness.
However, in a landscape shot, a large depth of field might be a much more important consideration instead, so you might want to stop down further than ‘optimal’ to get the shot you want. In the end, minimizing any and all diffraction is sometimes impossible, so getting it right is a fine balancing act!
Other Ways to Prevent Diffraction
There’s also more that you can do apart from stopping down. For example, many cameras nowadays feature an auto-correct feature for diffraction. This can work via the camera’s firmware or even as a sort of hardware implementation at the sensor level.
However, it usually requires you to use first-party lenses: Nikkors with Nikon, Canon lenses on Canon bodies, and so on.
On zoom lenses, you might find that your diffraction pattern can also change depending on the focal length you’re working with. Work your way through the zoom range, testing out the lens sharpness as we went over earlier to find out where your zoom performs the best.
How Sensor Resolution and Size Affect Diffraction
In digital photography, it’s important to note that the effects of lens diffraction do not just depend on your lens nor its circular aperture by itself. Your camera also plays its own role.
Camera sensor designs can resolve objects by incident light or reflected light, each in its own way. How much diffraction appears in different lighting conditions primarily depends on two factors: the resolution of the sensor and its pixel density.
Small pixels that are tightly packed together are much more sensitive to diffraction errors. On a high-resolution camera with an APS-C sensor or smaller, you might begin to see some kind of diffraction pattern already at f/4 or even wider!
Directly opposed to that, sensors with large, more widely distributed pixels tend to fight diffraction a lot better.
This makes full-frame and medium-format digital cameras the best to pick to minimize the chances of any possible diffraction pattern emerging.
Attaining Pin-Sharp Images Free of Diffraction
Just like water waves, light reflecting off of elements in our environment and traveling through the objective lens of our camera can and will diffract based on certain factors.
Most of the time, a little bit of diffraction is unavoidable. Just like other small flaws of any optical instrument, the diffraction pattern of your camera lens might not even be strong enough to affect the outcome of your images.
It’s when it does lead to a serious loss of sharpness that even a small diffraction element leads to frustration. Fortunately, DIY techniques for fixing diffraction are anything but rocket science, as you’ve learned today!
Make sure to keep your lens aperture away from extremes. That is to say, neither stop down too far nor go completely wide open. This way, you can really minimize the appearance of diffractive errors. Your camera might also feature built-in software or hardware optimization to correct images with diffraction automatically.
If you can afford to and can justify it, switching to another lens design or even an entirely different camera body can also improve performance. Aim for sensors with lower pixel density and larger photosites for the best possible results.
I hope this guide gave you a satisfactory understanding of why it’s important to be aware of diffraction in photography. Make sure to apply what you’ve learned today in bite-sized chunks so it sinks in with practice. Have fun shooting, and until next time!