What Is Lens Diffraction?

When landscape photographers use small apertures, they are gaining a very wide depth of field. However, there are problems that come with a smaller aperture, such as lens diffraction.
This causes a photograph to lose sharpness at small apertures.
So what can we do about it? Read on to find out.

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What Is Diffraction?

Diffraction is all about optical physics. We need to look at this to be able to talk about how diffraction affects your photography.
Light waves can interfere with one another. This is especially true when light pushes through a small opening, such as a lens.
To make this easier to visualize, think of ripples made by dropping a pebble into a pond. These waves spread out in concentric circles as you can see below.

If a barrier were to block these waves, it would stop their movement. However, if there was a small opening in the barrier, the water could pass through it.

The patterns made come from the wave bending around the corners. The waves hit each other and cancel themselves out, which is called destructive interference.
In some areas, the waves add together, creating constructive interference. Here, additional patterns form off to the sides.
The patterns are determined by how big the hole in the barrier is. The smaller opening results in a larger spread of waves, while the smaller hole causes less spread.
When we say small hole, it is relative. The opening only causes diffraction when it is similar in size to the wavelength that passes through it.
Light has a tiny wavelength, so if it passes through a five-foot hole, it will not diffract significantly.

What Is Diffraction in Photography?

Diffraction is important in physics and equally important in photography. But how does it affect your every day when capturing?
When we talked about small and big holes in a barrier, we can easily change these to the opening sizes in a lens. These big and small apertures work in the same way.

When the light enters the lens and hits the sensor, it looks like this Airy Disk above. This is the appearance of the diffracted pattern.
The central area is the brightest and has the largest effect on your photos.
We can see easily why an Airy Disk can cause our images to lose sharpness. A small opening/aperture causes the waves to spread out.
At smaller apertures, the Airy Disk becomes much larger.
Now, think of how any given scene that your photograph is composed of hundreds of thousands of these tiny sources of light. Every pinpoint of light travels through the aperture hitting your sensor.
What happens is, these Airy Disks start to overlap, causing blurrier images. You wouldn’t get the same problem with wide apertures as there is no overlap or spill.

High and Low-Megapixel Cameras

By looking at the above example, it begs the question. If the pixels were larger on the sensor, wouldn’t there be less bleed?
This is true. Larger sensors hold larger pixels which can fit in the bleed caused by the Airy Disk. Larger sensors are not affected as much as sensors with smaller sensors.
If I was using the Nikon D700 12-Megapixel camera, I could stop it down to f/11 before any diffraction was present. When using the D800 36-Megapixel camera, I can spot diffraction at f/5.6.
To find out how affected your camera is by diffraction, you need to test it yourself.
A high-resolution sensor will always capture more detail than a low-resolution sensor of the same size. Having more pixels will never lead to lower detail, even with tiny apertures.
If you print your images at the same size, the larger sensor camera will always have more detail.
To get the best possible sharpness from a larger sensor camera, you need to pay attention to apertures smaller than f/8. For your camera, you will need to test the boundaries.

Small and Large Sensors

We need to answer another question. Do crop sensor cameras present more diffraction than full frame sensors?
What we do know is that at any given aperture, the Airy Disk will always be the same physical size. This is independent of sensor size, as it only depends on the aperture used.
If I use a 50 mm f/1.8 lens on a full frame sensor or crop sensor, the size of the Airy Disk will be identical, when you use the same aperture.
The problem is that the Airy Disk takes up a larger percentage of a crop sensor camera vs a full frame camera.
Unsurprisingly, the amount of additional diffraction is the same as your crop factor. If your crop sensor is 1.5x, multiple the aperture used to find the equivalent diffraction for a full-frame camera.
For example, the Airy Disk at f/11 on a crop sensor is equivalent to f/16 on a full-frame sensor.
You can’t print crop sensor images the same size as the full frame images. This means there is no practical difference.

Lens Diffraction and Depth of Field

Lens Diffraction decreases a photograph’s sharpness at small apertures. At the same time, using a smaller aperture increases the depth of field.
[example of f/22 and f/5.6]
As you can see, the f/22 image has much more of the scene in focus. If you want the subject to be sharp, then a smaller aperture is what you want.
However, this is where it gets tricky. When zoomed in, we can see the f/5.6 image is sharper. Significantly sharper.
This doesn’t mean that you should photograph every scene at f/5.6. There will be times when you want a larger or smaller depth of field.
The slight reduction in sharpness is worth being able to see more of the scene.
[close-up example of f/22 and f/5.6]

Finding the Sharpest Aperture

There is diffraction at every single aperture of your lens. This is true because light always needs to bend through an aperture, large or small.
At wide apertures, such as f/2.8, the Airy disk is much smaller than the pixels in your sensor or photograph. This means diffraction is impossible to spot at large apertures.
This doesn’t mean that large apertures are the sharpest on any given lens. A lens is at its sharpest when the aperture is slightly stopped-down.
For example, my 85 mm f/1.4 is sharpest in the center at f/4. Below you can see the sharpness chart.

So why is the peak at f/4 and not f/1.4? At larger apertures, more light travels through the edges of the lens.
Since the center of a lens is the most-correct region, this decreases the sharpness of the entire image.
A smaller aperture actually blocks light that has traveled through the edges of a lens, increasing the sharpness of an image.
This effect, balanced with the decrease of sharpness from lens diffraction is the reason that f/4 gives you a greater sharpness on an f/1.8 lens.
These charts are useful when looking for a new lens to buy, but they are misleading. By looking at the chart above, the corners of the lens are actually sharpest at f/5.6.
Depending on your subject, you may prefer sharper edges rather than the sharpest possible center.

How to Avoid Diffraction

Now that you understand lens diffraction, it’s time to look at how to avoid it. The simple answer is you can’t.
Lens diffraction, as well as all kinds of diffraction, is a result of physics. Your images will be robbed of their sharpness regardless of what you do.
There is one way to avoid it; use larger apertures. If you require the sharpest photograph, don’t head straight to f/22. Try the scene at f/5.6-f/8 first.
If you find your images need to be sharper overall, then try focus stacking at mid-range apertures. You will place the scene in focus while keeping diffraction to a minimal.
Perhaps you already have a few images that you wish were sharper, then try to boost the sharpening by post-processing. It won’t eliminate the effects but is a stable way to improve your images.
There is a possibility to correct for diffraction through a sharpening process known as deconvolution sharpening. This is used best when you have the lens characteristics.
This is the process NASA uses to increase the sharpness of the images taken by the Hubble telescope. Some camera manufacturers, such as Pentax, have a diffraction-reduction menu option.
To test your images in post-processing, increase the ‘Detail’ slider as much as possible. You can see a difference in your images sharpness through its details.

Considerations

• Light with large wavelengths will diffract more readily than those with shorter wavelengths.
• Red light (650 mn) gives you a larger Airy disk than blue light (475 mn) at the same aperture. Due to this, you will see less blur from blue light diffraction.
• Pixels on camera sensors do not detect the same wavelengths of light. In the Bayer array of pixels, there are double the amount of green-sensing pixels than red or blue-sensing pixels.
• The Airy disk in our example is a simplified version of what actually happens. We show them as perfect circles whereas most lenses have 8 or 9 aperture blades, which aren’t perfect circles.

Conclusion

Lens diffraction is a confusing topic, yet you can see the effects it has on your images. If they are significantly present, then it is worth knowing when you capture your scenes.
If you need to photograph scenes with a large depth of field, it is worth knowing about. This is so you can access the trade-offs between larger apertures and a smaller depth of fields.
Lens diffraction will always be present, and unless you are careful, your images will lose out in sharpness. Once you see it and understand how it works, it will become second nature to handle it in your photography.

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