Lens Diffraction Limit Explained:

What is meant by lens diffraction limit and why does it vary:

Diffraction is a physical characteristic of all lenses and is an optical effect that will limit the resolution you can achieve from your camera and lens. A lot of photographers starting out believe that the more you stop down or close your aperture the more depth of field you get and the sharper the resulting image. This unfortunately is not correct. Each camera and lens combination will have a diffraction limit or diffraction range and once you go beyond this in terms of aperture the less sharp your images will become.

There is a huge amount of mathematical information and theoretical explanations of optical diffraction on the internet and a lot of this is quite technical. As Steven Hawking was once advised, readership will halve as the number of mathematical formulae in an article increases, so I prefer to keep the explanation of diffraction as simple and as sentence-text-friendly as possible.

As we explained in our article on bokeh the optical characteristics bokeh and sharpness are at either end of what you could call the “lens” spectrum when large apertures are used. In the case of small apertures however, it is the optical characteristics depth-of-field and diffraction that are at play. Landscape photographers or long exposure photographers are constantly looking for the maximum depth of field in their images and maximum sharpness and resolution. But unfortunately both of these characteristics are optical opposites and both require compromise when capturing optimal high resolution landscape images.

In its simplest form, diffraction is a function of sensor size and aperture, and by aperture, we mean f-stop. As you close down the aperture on your lens you are making the hole through which the light passes smaller and smaller. As you go smaller the light waves deform as they pass through the aperture diaphragm blades. They spread out in a fan like shape as the obstacle (being your lens aperture) interferes with the light waves. This fanning is known as diffraction and will have serious consequences on the sharpness of the image you are trying to capture.

Let us take the case of a single point of focus in our image, take the symbol “®” for example. At wider apertures, the light passing through the diaphragm blades will be linear exhibiting no diffraction whatsoever. As you begin to close your aperture you will see changes in effects of sharpness of the symbol “®”. You may find that sharpness is in fact better at f/5.6 or f/8 than it is at f/4 (this is not diffraction but the quality of your lens). On cropped or full-frame DSLR’s you should start seeing diffraction effects above f/8 to f/11. The image of our “®” will begin to exhibit diffraction effects or “fanning” as it passes through the smaller apertures. The smaller the aperture the more pronounced the fan. So now, because light is not travelling linearly between your aperture and your sensor but instead it is fanning out, it means the points of light in the image being captured are also fanning. This fanning effect means that the edges within the image will first start losing definition and clarity because the fanning effect spreads the light and spreading the light from a crisp clean edge can only mean it will reduce the definition of that edge. As you continue to reduce the size of your aperture, edge sharpness will continue to degrade, lines will blur together and as you go lower still the whole image will start to lose sharpness and fall apart. Have a look at the six images below. Looking at the ® symbol you can fairly quickly see how the edges begin to deteriorate initially at apertures around f/11. Sharpness then simply falls apart as the lens aperture is closed. As a side observation, you can also very easily see how stopping down your aperture has awful consequences on image quality when it comes to lens and sensor dirt. In this case the camera was a Nikon D800 and since I got the camera it has had oil splashes on the sensor, a known problem with the D800 apparently.

Lens Diffractions Limit Lens Diffractions Limit Lens Diffractions Limit

Most full frame cameras start to show these diffraction or fanning effects at between f/8 and f/11. Smaller sensors such as cropped sensors or the smaller sensors found in point and shoot cameras or mobile phones exhibit diffraction effects at much lower f-stops such as f/4 and f/5.6. On the other end of the spectrum, medium and large format cameras can handle much smaller apertures before diffraction effects are visible. The reasons for this are complicated and are related to what is called an "airy disc". Simply speaking, when this disc size exceeds the size of a pixel in your cameras sensor then you will have diffraction or your camera is then regarded as being diffraction limited. Bigger sensors have bigger pixel sizes and as a result can handle bigger airy disc sizes before being diffraction limited. That is why large format cameras can handle the smallest of apertures producing the maximum depth of field and the smaller sensors require larger apertures to avoid diffraction. Apertures of f/180 are not uncommon on large format cameras, whereas f/3.5, f/4 and possibly lower are what are required on point-and-shoot cameras for maximum sharpness. For a top article on diffraction and its effects on sensor size check out an article on the Cambridge In Color website. It is somewhat technical but if you would like to know more about the subject of diffraction and its effects then it is the best article out there on the subject.

So what is the correct aperture to use? The quick answer to that is there is no one-size-fits-all aperture. It all depends on your camera type, your lens quality and also what you are photographing. The best way to decide what is right for your equipment is to test it yourself. But as a general rule, for cropped-frame or full-frame DSLR’s you are best sticking to a maximum of f/8 to f/11 for optimum sharpness and optimum depth of field for landscape images. Any higher and you will start to notice diffraction effects. For long exposure photographers, again, stick to the f/8 to f/11 mark and if this is not giving you the exposure time you need, then you can add ND filters to give you the flexibility you need with your shutter speed.

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