Advanced Aperture Concepts
It's all about details and changing the aperture goes far beyond just affecting the DOF. The following are some of the little know effects :
- Change in sharpness
- Apertures narrower than f/16 can lead to a reduction in sharpness.
- Wide apertures and poor lens quality combine to cause lens aberrations, color fringing and vignetting.
- The edges of your images will start to look wired!
- Starburst effect happens at narrow apertures and appear around bright points of light.
- Not necessarily a bad thing as it bring some artistic flare to your image.
- At wider apertures the background becomes blurry which is named Bokeh.
- Looked at closer enough, the bokeh has a shape, the same shape as the aperture opening.
What happens when you change the Aperture?
There are many thing to be aware of when using your camera to take an image. This article is the icing on the cake, the details that take an image from good to great. Not knowing these topics won't prevent you from taking a good image but it might prevent you from getting published or selling your image on a stock website. If you are looking for a simple explanation of aperture then check out Aperture - Part 1 or Aperture - Part 2, for this article gets into the gritty details about what happens to your image when the aperture is changed.
This article will talk about 4 main impacts that creep into an images when the aperture is near the extremes. One impact that is not noted in this article is vignetting. Check out this link for information about vignetting.
Negative aperture impact - Change in Sharpness
One thing that happens when shrinking the aperture is that the image loses some of its sharpness. This is especially apparent when using narrow apertures; usually anything above f/16. An image taken with a very narrow aperture might look fine on the camera screen but once magnified, it is apparent that the image has lost some of its sharpness. The reason for this is that at such a narrow aperture (or small shutter opening), the light gets squeezed through the hole and the light bends slightly leading to a loss of detail or a blurring of detail.
Lets explore this topic with the two images overlapping each other above. As you will notices, the two pictures above are slightly off and don't directly overlap each other. This was done on purpose so that it is easier to compare the same area of a photo without having to move the slider back and forth.
Are you able to spot any place where sharpness is reduced or detail are blurred?
The two easiest places to observe a loss in sharpness is at the sign on the top of the warehouse and at the street sign on the left side of the image. The street sign is almost clear enough to read in one image where the other image you can barley make out the shape of some of the letters.
The exact aperture when sharpness starts to be reduced, depends on the camera and the quality of lens. However, for the most part, loss of sharpness starts to impact an image until f/16 or higher. As a rule of thumb, the absolute maximum aperture one should use is f/22, unless absolutely necessary. If you find yourself needing to use this high of an aperture, you should try adjusting the amount of light that reaches your camera via external devices such as polarizing filters or neutral density filters. Images with this issue often go unnoticed depending on the size of the final print.
Negative aperture impact - Lens Aberrations
Another source of image quality issues derive from lens aberrations, which is a fundamental optical problem all lenses suffer from. The severity of which depend on the lens quality (ever wonder why some lenses are so expensive?)
Lens aberrations are especially noticeable at wide open apertures where the corner of the image has problems like color fringing, vignetting or blurriness. (These issues are due to the way the lens is designed). The image below shows a blown up area of a photo taken with a wide aperture. In this enlarged section, several of the lens aberrations impacts can be noticed. For more information about chromatic aberration check out this article.
If a wide open lens leads to the issues listed above, then surly narrowing the aperture would decrease these issues; and it does up to a certain point. However, to make life even more interesting, the amount of aberrations will again increase if the aperture is narrowed too much.
can this be fixed with software?
You might be saying, "Wait a second. If I make my aperture too narrow, I'll have problems with sharpness thanks to diffraction, but if I make the aperture too wide I'll have problems with aberrations. What the heck!" Have no fear, as there is a sweet spot where neither is present. As a rule of thumb, most lenses are sharpest between f/4 to f/8. These apertures are narrow enough to block the light from the edges but still wide enough where diffraction isn't an issue.
Another thing to keep in mind is that post-editing software, like adobe lightroom, have ways to mask and fix some of these issues. However, the best way is still to select settings that avoid these issues all together. Also, unless you are printing large photographs or using them for professional purposes, the above mentioned issues will not be terribly noticeable.
Positive aperture impact - Starburst Effects
The starburst effect is a by product of narrow apertures, like f/16, and is apparent when a bright point of light is photographed. The bright point of light will have these rays coming from it almost like a sunbeam. The number of rays depend on the number of aperture blades in the lens. Just another funky byproduct of light and the way your lens captures it. This is something that is almost completely unavoidable; the good news is that it can give photos an artistic flare.
The image below shows an enlarged section that highlights the starburst effect. While it might be hard to tell, these are actually two streetlights.
Positive aperture impact - Background Blur
As discussed in Part 1 of the aperture articles, the wider the aperture, the more distant and blurred the background becomes. The quality of the background blur actually has a name - Bokeh. Not only do different apertures change the bokeh but it also changes the shape of the bokeh. The shape of the bokeh takes the same shape as the aperture blades. The shape of the aperture blades changes as the aperture open and closes, thus the bokeh will look different wat differnt aperture settings.
A great example of this shape we are talking about is shown in the below image. Lets take a look at the top most part of the image that is boxed. In the enlarged version of this area you can distinctly see a circle like spot. However, upon further inspection this circle is not completely circular, it has sides. These sides are literally the same shape that the aperture blades are making when this image was taken.
The enlarged image from the left side of the image shows another area where the bokeh is easily noticed.
This is just something to keep an eye on for a more artistic look on some photos.