Often when we start out as photographers we hear more experienced folks talk about the importance of not stopping a lens down too far as to avoid the effects of diffraction.
We may find this confusing because from a composition standpoint we often want to achieve deep depth-of-field, especially with landscape photography. Stopping a lens down to achieve this degree of depth-of-field seems quite logical.
This begs the question whether we really need to care about diffraction. Some photographers are almost fanatical about avoiding the effects of diffraction, while others are not very concerned about it at all as their objective is to achieve an overall look with their composition. They don’t concern themselves with pixel peeping and detailed examinations of their images.
What is diffraction?
The full explanation of diffraction can be a highly technical exercise. Basically it is an optical effect which limits the total resolution of your images, regardless of how many mega-pixels your sensor may have. Diffraction happens when the light coming into your camera body begins to disperse or ‘diffract’ before it hits the sensor in your camera. This is caused by the light passing through a small hole, or aperture, in your lens. The amount of dispersion of the light landing on your camera’s sensor can cause softening of your image.
Does it affect all cameras identically?
The effects of light dispersion (diffraction) are affected by the size of the pixels on your camera’s sensor. Higher pixel density sensors will have smaller sized pixels so diffraction will be more noticeable than with a lower pixel density sensors which have larger pixels. For example, diffraction will become noticeable on a Nikon D800 at f/11 but it would not be apparent when shooting at this same aperture using a Nikon D3 or D700, even though all three cameras utilize full frame sensors.
Cameras with smaller sized sensors like Nikon 1, M4/3 and point and shoot models will suffer the effects of diffraction at lower f-stops when compared to cameras with larger sensors, assuming of course that the size of the pixels on those larger sensors is bigger than those found on the smaller sensors.
What does diffraction look like?
Rather than get into a lengthy technical discussion about diffraction I’ve taken some test images to help demonstrate the effect. These images were shot with a Nikon 1 V2 with the Nikon 1 18.5mm f/1.8 prime lens. The camera was tripod mounted and the shutter was activated via remote.
I chose this lens for three reasons. First, it is one that is very commonly owned by many Nikon 1 users, secondly it is one of the sharpest Nikon 1 lenses, and finally it provides an equivalent filed-of-view of approximately 50mm. This is one of the most common focal length lenses owned by photographers regardless of camera format. As is often the case with lenses, the 18.5mm appears to be at its sharpest when shooting a couple of stops down from wide open.
The following is a series of out-of-camera jpeg images taken at f/4, f/5.6, f/8, f/11 and f/16. These are 100% crops taken from an area roughly in the centre of the frame.
Note: Click on images to enlarge.
As you look at each image pay special attention to the amount of detail and how crisp it appears to you. First let’s look at the f/4 image.
The next one was taken at f/5.6. If you examine it closely you’ll likely be hard pressed to notice any significant difference with the f/4 image.
Now an image at f/8. You will begin to see some softness in the image.
Now at f/11, it becomes more apparent.
And, finally at f/16 it is quite noticeable.
Is is possible to eliminate diffraction with post processing?
Let’s see what happens when using the RAW file from the f/16 image and run it through DxO’s auto lens corrections.
Not much of a difference…let’s add some sharpening by taking Global to 1.20 and Detail to 70, then add some micro contrast to +10 to see what happens.
Let’s remind ourselves what the image looked like when shot at f/5.6 as an out-of-camera jpeg.
And finally, how would the image look when shot at f/5.6 with the same DxO adjustments made to it that we did with the f/16 file.
As we look through these images in succession it becomes pretty clear that we can try to hide the effects of diffraction in post but we can’t eliminate them.
Should you care about the effects of diffraction?
Depending on subject matter and image use you may, or may not, find the level of image softness that is created by diffraction objectionable. As mentioned earlier, many photographers are quite prepared to have a bit of softness in their images as long as they can achieve the overall composition they desire. As we did in some of the previous images, they often apply some sharpness to their images to try to minimize the effects of diffraction.
Practical shooting considerations
It is critical that you understand your camera body and lenses in order to get the best results from them and to minimize the effects of diffraction.
It may be useful for you to do your own hands-on testing with your camera and lenses. Set up your gear on a tripod, lock the mirror up if you are shooting with a DSLR, and use a shutter release or shutter delay. Then, shoot a series of identical images with each of your lenses beginning with each of them fully open (i.e. the lowest f-stop number) then at each standard aperture setting as your lens may allow (e.g. f/1.4, f/2.0, f/2.8, f/4, f/5.6, f/8, etc.). By doing this you will learn where each of your lenses is at its sharpest, and also where diffraction begins to set in and how bad it gets.
This will lead you to some practical understanding of your gear. For example, when I shoot landscape images with my Nikon 1 gear I typically shoot at f/5.6 as I find this gives me the best combination of image sharpness and depth-of-field. For other types of subjects I occasionally shoot as high as f/8 but almost never go beyond that point.
As with most things photographic diffraction is one of those trade-offs that can happen. Being aware of its effects allows us to make an informed decision in terms of our choice of camera settings and balancing those with our image objectives.
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