With such an optical phenomenon as diffraction, photographers meet quite often. Unlike other distortions such as distortion or chromatic aberration, diffraction is equally common to any lens and cannot be completely eliminated. In this article, we will look at the diffraction problem in photography practice.
Diffraction is an optical effect that reduces the sharpness of a photo no matter how many megapixels your camera has or how sharp the lens you are using. If usually the rays of light propagate in a straight line, then passing through a small hole (closed aperture of the lens), they begin to scatter. In this case, the beam forms in the photo not a point, but a circle – the so-called Airy disk. The more we close the aperture, the more noticeable the diffraction becomes, since each point in the image ceases to be a point.
It is possible to notice diffraction with the naked eye when the size of the Airy disk becomes comparable to the size of one pixel on the matrix – this is the diffraction limit.
Specificity of The Action of Light
Any photographer who is actively involved in his business will soon begin to notice unpleasant light, color, and other effects in the frame.
Diffraction manifests itself in a decrease in the detail and clarity of the image, regardless of its resolution. They also talk about diffraction rings or fringes that appear near objects. What is this in accordance with physical laws, how can such a phenomenon be explained?
Diffraction refers to light. This is an exception to the rules of the section of geometric optics. Translated from Latin means “broken”, “bending” is a very close description of the existing process.
In fact, this is what happens: the light beam, which has the character of a wave, travels in a straight line. But if he encounters some obstacles in the form of dense, opaque objects, then he “bypasses” and partially penetrates behind them, into their shadow areas. Here the main “danger” lies in wait for us – as a result, even the borders of the shadow (and what is in the shadow) soften, become not quite sharp.
Moreover, I can tell you that the principle of light bending around objects is also applied to the camera aperture itself, because its petals fit into the category of “dense obstacles”. Thus, we are dealing not with one, but several waves of light, which interact and superimpose on each other.
It is hard to imagine? And try to squint your eyes for a moment, so everything around you turns out to be vague, as if behind a fog or veil. The camera is about as difficult.
An unpleasant phenomenon, creating additional complications, but inevitable. As much as you would like, it will not be possible to completely get rid of this or other kind of distortion. Therefore, you need to remember them and try to minimize the available means.
Effect on The Image
I told you not just interesting facts from the field of theoretical physics. This is a real drawback that can be clearly visible in the photograph and reduce its attractiveness to the viewer and the author himself.
You can clearly understand what diffraction is by going to various forums of amateur photographers or by doing a series of camera tests yourself.
Mount the camera on a tripod, eliminate vibration, select manual or aperture mode and shoot one subject with excellent f settings.
Do not change the other exposure variables. Some people use a special scale or sheet of paper with a picture of the target (in pdf format). Pictures with a difference in aperture are judged by the clarity / blur of the entire target (numbers, letters, lines or a specific object) or its edges.
Diffraction is such a defect in the work of optics, with which practically nothing can be done in photo editors. The only way out is to minimize its impact during the filming phase.
Let’s examine a series of test photos taken with the Nikon D3400. This inexpensive device with a matrix of the APS-C format has a high resolution (24 megapixels), so we will be able to evaluate all the details in the photo, and at the same time we will clearly see the appearance of diffraction. Below are fragments of the image at 100% magnification.
It is convenient to evaluate the sharpness of frames, for example, by the sign of a pedestrian crossing. You can see that detail starts to decline after f/8 and drops critically towards f/14. The more the aperture is closed, the less detail remains in the picture.
The test photos show that the diffraction limit on the Nikon D3400 is around f/8; further diffraction is noticeable.
Let’s look at the Nikon Z 7 as an example of how full-frame sensors behave in such situations. It would seem that diffraction should have a very serious effect on the 45-megapixel Nikon Z 7 sensor, because it has a very high resolution. However, the pixel density of this matrix is still lower than that of the 24-megapixel crop. It’s easy to calculate – just divide the number of megapixels by the sensor area.
So, let’s compare in practice a pair of test images taken at f/8 and at f/16 on a crop and a 45-megapixel full frame.
It can be seen that even at closed apertures, the full-frame camera provides better detail. The example at f/16 is especially indicative: on the crop matrix, the picture is actually inoperative, and the full-frame picture can still be used.
Photography Practice to Avoid Diffraction
It may not be clear to a beginner photographer why you should close the aperture so hard that diffraction begins to affect the photo. However, there are a number of reasons why you have to use closed diaphragms.
The first and main one is to increase the depth of field. In many areas of photography, it is necessary to show sharp all objects in the photo, placing them in the depth of field. The depth of field is controlled by the diaphragm, and sometimes you have to close it tightly in order, for example, to sharply show a flower in the foreground and mountains in the background in the frame. In such cases, a slight reduction in sharpness due to diffraction is better than just a non-sharp foreground or background.
But do not rush to close the diaphragm all the way. First, learn to calculate and use its values rationally. So, in landscape, architectural and interior photography, various methods of focusing and working with depth of field are used. One of them is aiming at the hyperfocal distance.
Perhaps every photographer must determine for himself the level of de-sharpness to which he is ready to go. For example, when photographing with a full-frame 45-megapixel camera, I determined for myself f/16 as the maximum value. Shooting at a crop where diffraction takes its toll first, I prefer not to cover more than f/11.
A similar problem occurs when shooting small objects and in macro photography, where after f/8 the working range of apertures only begins. Since the shooting is carried out at a short distance, the depth of field is also extremely shallow. Therefore, in order to show the object completely sharp, you have to seriously close the aperture. It is better to neglect diffraction here, but achieve the desired depth of field than to get just a blurry frame due to lack of depth of field.
But the depth of field cannot be expanded indefinitely. Often, when shooting small objects, instead of closing the aperture too much, focus stacking is used. Sometimes this technique is used when shooting interiors and landscapes. Its essence is that several shots are taken with focusing at different distances, and then the result is brought together on a computer. This technique helps to avoid the negative effects of diffraction and at the same time expand the depth of field all the way to infinity. The Nikon D850, Nikon Z 7 and Nikon Z 6 cameras have built-in focus stacking.
The second reason why the aperture is tightly closed is the desire to achieve perfect sharpness and no distortion in the frame. At open apertures, lenses can produce a picture that is far from ideal – with blurry edges and an abundance of chromatic aberrations. Defeating this is easy – just cover the diaphragm. If the lens initially has an aperture of, say, f/5.6, which is typical for budget optics, then it will have to be twisted up to f/8 – f/11 to get decent sharpness. It’s better to let diffraction affect the photo a little, but at the same time we will remove all distortion from the side of the lens.
When not to close the diaphragm?
Often photographers close the diaphragm all the way in an effort to shoot blurry car lights or water in a mountain stream at long exposures. And they get low-quality images. Yes, the shutter speed lengthens when the aperture is closed, but even at f/22 in daylight it is unlikely to get long enough to work effectively with motion blur. To work effectively with long exposures, ND filters are needed. It also helps a lot when the camera has very low ISOs. For example, Nikon D810, Nikon D850 and Nikon Z 7 have the ability to lower the sensitivity to ISO 32.
And you also don’t need to overly close the aperture in pursuit of stars around point sources of light – lanterns, the sun, since spectacular rays on most lenses are formed already at f/11 – f/14.
How to Avoid Diffraction in Photography?
Diffraction has a serious impact on image quality. How to avoid it in photography?
1. First, you need to skillfully work with the aperture, avoiding both too low values and as large as possible, even if your camera allows them to be set. In the first case, diffraction can appear due to the smoothing effect, that is, the natural bokeh around the main object; in the second, because the total amount of light entering the lens decreases, hence the drop in detail. It is believed that after f = 11 the diffraction probability is high, not to mention the limiting aperture values of 22 and beyond.
2. Secondly, a polarizing filter on the lens will be useful, it will remove the need to close the aperture in some situations (in bright sun, priority of long exposure for shooting water, etc.).
3. Third, and importantly, the choice of good photographic equipment. Expensive optics are made differently: better and more correctly selected lenses, high aperture ratio, which is sensitive to light and perceives it better regardless of the time of day. Pictures taken with a professional camera are bright and sharp.
4. Fourth, I would like to point out the general awareness of the photographer. Specifically, I mean the user’s knowledge of the depth of field scale, as well as the ability to calculate the hyperfocal distance, where the focal length of the lens is not the last.
Let’s summarize. On modern crops, the sharpness of the image decreases after f/11, on a full frame – after f/14 – f/16. Due to the lower pixel density on the matrix, full-frame cameras (even multi-megapixel ones) provide a high-quality, detailed image even at closed apertures, so you should not be afraid of some particularly terrible diffraction in practical work. Working with closed diaphragms is possible and sometimes necessary. You just need to do it consciously and not abuse shooting with the aperture closed to the limit.
Frequently Asked Questions
Q1: What is diffraction limit?
Answer: Diffraction limit is a fundamental limitation of the accuracy of measurements of the geometry of objects in traditional optical systems. Due to diffraction, the high-frequency components of the image completely attenuate, and groups of small-sized elements of the boundary cease to be resolvable. Estimation of the object boundary by known algorithms becomes smoothed in relation to its true shape.