Diffraction in Photography Part 2

3) High- Versus Low-Megapixel Cameras

The comparison above, showing an Airy disk hitting the pixels of your sensor, might prompt a question: if the pixels were larger, wouldn’t the Airy disk be less likely to bleed over?
In fact, that is completely true! Large pixels – those which are bigger than the Airy disk – do not show diffraction at the same apertures that a small-pixel camera would. Perhaps I can stop down to f/11 on the 12-megapixel Nikon D700 before noticing any diffraction, while the 36-megapixel D800/D810 would show visible diffraction at any aperture smaller than f/5.6. These numbers aren’t set in stone, though; I recommend testing your own camera to see when diffraction begins to grow noticeable (and, more importantly, when it begins to grow objectionable).
However, this isn’t a problem with high-resolution sensors. In fact, if all your settings are the same, a high-resolution sensor will always capture more detail than a low-resolution sensor of the same size. More pixels will never lead to lower detail, even at the tiniest of apertures. This means that, if you print your photos at the same size, a Nikon D800/D810 photo will always have more detail than a Nikon D700 photo, all else equal.
That said, if you buy the Nikon D800/D810, chances are good that you want to print large or pixel-peep. If this is the case for you, diffraction absolutely is a bigger issue than it would have been with a low-resolution sensor! To get the best possible sharpness from a D800/D810, you should pay attention if your aperture is smaller than about f/8. Again, I recommend testing the exact boundaries of your camera yourself.

NIKON D800E + 105mm f/2.8 @ 105mm, ISO 100, 1/3, f/7.1

4) Small Versus Large Sensors

It is often said that crop-sensor cameras (i.e., DX Nikon cameras) show diffraction more easily than full-frame cameras (FX Nikon). Is this a myth, or does it hold true?
Let’s start with what we know. At a given aperture on a lens, the Airy disk will always be the same physical size. It doesn’t matter what sensor you use; this is a property of physics that only depends upon the aperture itself. For example, whether I put a 50mm f/1.8 lens on the full-frame D750 or the crop-sensor D3300, the size of its Airy disk projection will be identical (assuming the same aperture).
So, where’s the confusion? The issue irises from the fact that the same Airy disk takes up a larger percentage of a crop-sensor camera than a full-frame camera. Take a look at the example below:


In fact, at an equal print size, a DX camera will show more diffraction than an FX camera. This is because the DX sensor is essentially a crop of the FX sensor; in other words, it magnifies everything in your photograph – including the diffraction – just like cropping in post-production.

The amount of additional diffraction is the same as your crop factor. So, for a 1.5x crop-sensor camera, multiply your aperture by 1.5 in order to see the equivalent diffraction on a full-frame camera. For example, the Airy disk at f/11 on a DX camera takes up roughly the same percentage of your sensor as the Airy disk at f/16 would on a full-frame camera.

Of course, if you use a DX camera, you may not print quite as large as you would with an FX camera. For many photographers, then, there is no practical difference; the smaller prints from a DX camera cancel out the additional diffraction. If you do print at large sizes with a DX camera, be aware that diffraction will be more significant at a given aperture.

NIKON D7000 + 24mm f/1.4 @ 24mm, ISO 100, 1/250, f/5.6

5) Diffraction and Depth of Field

Diffraction decreases a photograph’s sharpness at small apertures. Yet, at the same time, small apertures increase the amount of depth of field in a photograph. This is not a contradiction, although it can be confusing at first. Look, for example, at the comparison below:


As you can tell, the f/22 photo has much more of the scene within its depth of field. If I want this entire subject to be sharp, it is far better than the photograph at f/5.6. However, let’s look at the point of focus more closely:



As you can see, the f/5.6 photo is significantly sharper. (Click on the image to see it more clearly.)
This, of course, does not mean that you should shoot every photograph at f/5.6. If you need a large depth of field, feel free to use smaller apertures; sometimes, it’s worth the slight reduction in sharpness from diffraction.

6) Choosing the Sharpest Aperture

There is always diffraction at every single aperture of your lens. This has to be true; light always needs to bend through an aperture, even if it is very large. However, at wide apertures like f/2.8 or f/4, the Airy disk is much smaller than the pixels in your photograph. This means that diffraction is essentially impossible to see at such small apertures.
However, this doesn’t mean that large apertures are the sharpest on a given lens. As you likely know, a lens tends to be at its sharpest when its aperture is slightly stopped-down. For example, my 20mm f/1.8 lens is sharpest in the center at f/4. Below is a sharpness chart for such a lens:

Nikon 20mm f/1.8G MTF PerformanceCenterMidCornerf/1.8f/2f/2.8f/4f/5.6f/8f/11f/1601,0002,0003,000Lens ApertureImatest Score

So, why is the peak at an aperture of f/4 rather than f/1.8? That is slightly beyond the scope of this article, but the essence is that – at larger apertures – more light travels through the edges of a lens. Since the center of a lens is the best-corrected region, this decreases the sharpness of the photograph (and increases its spherical aberration). A smaller aperture actually blocks light that has traveled through the edges of a lens, which improves the sharpness of a photograph.
This effect, balanced with the decrease of sharpness from diffraction, is the reason that f/4 gives the greatest sharpness on a lens like the 20mm f/1.8.

How do you tell which aperture is sharpest on your lens? Simply look at the tested results online. However, don’t stress too much about always shooting at the “perfect” aperture. For one, even these test results can be ambiguous. In the chart above, for example, the corners of the lens are actually sharpest at f/8. So, depending upon your subject, you may prefer sharper corners rather than the sharpest possible center.

At the same time, even suboptimal apertures aren’t horribly blurry. I have made a few large prints from photographs taken at f/16, and their quality is more than enough for my needs. If you need an aperture like this – generally to increase your depth of field – don’t be afraid to use it.

(If you need the largest possible depth of field in a photograph, like many landscape photographers, I recommend reading about hyperfocal distance. There are many similarities between these two properties of photography.)

NIKON D800E + 24mm f/1.4 @ 24mm, ISO 100, 6/10, f/16.0

7) Avoiding Diffraction

Now that you understand diffraction, how do you make sure to avoid it in your photographs? Unfortunately, the simple answer is that you can’t. Diffraction is a result of physics. It doesn’t matter how good your lens is; diffraction will rob sharpness at smaller apertures no matter what.
Even though you cannot circumvent the laws of physics, there is one way to avoid diffraction in your photographs: use a larger aperture. If you need the absolute sharpest photograph, this is the only way to avoid the effects of diffraction. Are you photographing a scene that needs a large depth of field? Try focus stacking at an aperture of f/5.6 or f/8, where diffraction is minimal.

At the same time, if you did use a small aperture (say, f/16 or f/22), you can improve a photograph’s apparent detail by sharpening in post-processing. This doesn’t actually eliminate the effects of diffraction, but it is a simple way to improve photos taken at small apertures.

In theory, it is possible to correct for diffraction through a sharpening process known as deconvolution sharpening. This type of sharpening is most effective when one has a perfect model of the lens in question, including its exact optical characteristics.

For this reason, generic deconvolution sharpening does not reduce the effects of diffraction to a meaningful degree; NASA, however, is known to use such a method to improve the sharpness of Hubble Telescope photographs. (Some camera manufacturers, including Pentax, may have a diffraction-reduction menu option; however, this is nothing more than a standard unsharp mask cooked into your RAW file.) If you want to test deconvolution sharpening, increase the “Detail” slider as much as possible in either Lightroom or Camera Raw. Of course, it will not be specific to your lens, which would be necessary for true diffraction reduction.

However, although you can sharpen your photographs in post-processing, the best way to decrease diffraction is simply to use a larger aperture.

NIKON D7000 + 105mm f/2.8 @ 105mm, ISO 100, 1/40, f/6.3

8) Extra Information

Aperture is a technical topic; so is the interaction between light and your camera sensor. Some of the information above is presented as a best-case scenario, and the reality can be slightly more complex. Most of the following information will not affect the actual appearance of your photographs, but it is worth covering some of these special cases.

For example, light with large wavelengths will diffract more readily than light with shorter wavelengths; this means that red light (with a wavelength of about 650 nm) leads to a larger Airy disk than blue light (about 475 nm) at the same aperture. So, in theory, you will see slightly less blur from diffraction if you are working in extremely blue light; in practice, this effect is small enough that it has little impact on your photographs.

Also, in most cameras, the pixels that combine to make a photograph do not all detect the same wavelengths of light. For sensors with a Bayer array of pixels (including Nikon, Canon, and Sony DSLR/mirrorless cameras), the number of green-sensing pixels is twice the number of red and blue pixels. This means that the pixel diagram presented earlier is a slight simplification; however, this does not change the fact that blur from diffraction increases due to the size of the Airy disk.

Finally, the depiction of the Airy disk in this article is bit simpler than it would appear in the real world. Above, I showed it as a series of concentric rings; in reality, though, that would only occur if the aperture were perfectly circular. Most lenses have seven, eight, or nine aperture blades, which (even when curved) are not quite circles. So, the “Airy disk” becomes an “Airy octagon.” However, there is no practical difference in the appearance of diffraction in your photographs; your photos will be just as blurry as you stop down the lens.

If you have any questions about the finer points of diffraction, please feel free to ask a question in the comments section; a single article is too short to explain everything that there is to know about such a complex topic.

9) Conclusions

Given all of these technical caveats, diffraction can seem like an out-there, unusual topic to be discussing. However, its effects are clear and significant in your photographs, and they are well worth considering while you are taking pictures. Especially for landscape and architectural photographers – or anyone who wants to take sharp photos with a large depth of field – it is important to understand the tradeoffs that come from shooting at a small aperture.

Diffraction is present in all your photographs, and – if you aren’t careful – it can rob some sharpness from your favorite images. However, once you see its effects in practice, diffraction will become second nature.

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