What is spherical aberration, and how does it affect your photos?
Spherical aberration is a fairly technical term, one that’s not often used and frequently confused with similar concepts such as chromatic aberration. However, it’s very likely that your photos are affected by spherical aberration – because most lenses are, in fact, designed with spherical elements.
If you’ve noticed your images are somewhat blurry, especially toward the edges of the frame, then you might have a spherical aberration problem, and this article can be of help. Below, you’ll find out what spherical aberration is and how it impacts your photographs. You’ll also find some advice to avoid or diminish the problem.
Let’s jump right in.
What is spherical aberration?
Every camera lens gathers light rays from your scene, then makes them converge to form an image.
Check out the two diagrams below. Both show the convergence of light through a lens element – but while the top diagram depicts an ideal lens, the bottom diagram depicts spherical aberration:
As you can see, the light rays converge at various points behind the lens element. More specifically, light passing through the edges of the lens converges sooner than light passing through the center of the lens.
Note that this optical problem happens to all spherical lenses – so if your camera lens includes spherical elements, than spherical aberration will impact your photos.
How does spherical aberration affect images?
The point where light rays converge is called the focal point – yet when there’s spherical aberration, the light rays that form your image will all have different focal points.
What does this mean?
The image will not be equally sharp from corner to corner. While the center of the shot may be sharp, light rays from the edges of the scene will converge at a shorter distance, and will create a subtle blurring effect.
If you mainly shoot subjects with a bokeh background, spherical aberration may not be a major issue, though it can still cause a halo, diminish the clarity of your image, or create uneven chromatic aberration.
On the other hand, if you’re doing landscape photography or you try to make a flat lay still life, then you definitely need to deal with the blurry edges caused by spherical aberration, which I discuss in the next section:
How to avoid or diminish spherical aberration
As a photographer, there are a few ways you can minimize spherical aberration in your photos. Let’s take a look.
1. Invest in good lenses
The material, the coating, and the quality of the glass can both increase and reduce spherical aberration. That’s why high-end lenses generally feature limited spherical aberration (as well as minimal chromatic aberration and lens distortion).
In fact, most camera manufacturers have a line of aspherical lenses, which use aspherical glass to prevent spherical aberration. Because the elements aren’t spherical, such lenses need fewer glass pieces to correct the image, so they’re smaller and lighter than regular lenses.
Gradient-index lenses are another option, which prevent spherical aberration by using a different refractive index that decreases towards the edges.
Unfortunately, lenses that deliver higher-quality image are also more expensive, which means they aren’t always an option if you’re on a tight budget.
2. Avoid your lens’s maximum aperture
When you close down the diaphragm of your lens – that is, the aperture – you block light rays from the outer edges of the scene. As a result, spherical aberration will be less pronounced.
So if you’re shooting a landscape scene, by stopping down to f/8 and beyond, you can block the most affected light rays and minimize spherical aberration.
Of course, narrowing the aperture also increases the depth of field, and if your goal is to capture shallow depth of field shots, you won’t want to stop down your lens. Fortunately, blurry edges due to spherical aberration probably won’t be much of an issue; after all, if you’re not looking to keep everything in focus, a bit of blur isn’t bad, right?
3. Keep your subject away from the edges of the frame
As I emphasize throughout this article, spherical aberration decreases sharpness toward the edges of the frame, not at the center.
So if your lens is affected by spherical aberration and you need a tack-sharp subject, try to keep the main subject or the most important elements away from the edges. It doesn’t mean that you’ll have to capture central compositions for all of your images – just moving the subject a little bit toward the inside of the frame will help.
Adjusting your composition won’t prevent spherical aberration, but it will prevent the aberration from affecting the most important parts of the image. Using selective focus to blur the background is also worth trying – as the spherical aberration will become a natural part of the shot – although this isn’t always possible depending on your style and the type of photography.
How to fix or diminish spherical aberration in post-processing
Unlike other types of aberrations such as distortion, vignetting, and chromatic aberration, spherical aberration can’t be easily corrected in post-processing. It’s still advisable to use the profile lens correction to improve the quality of your images, but spherical aberration won’t be reduced.
Instead, you can use the following methods to address spherical aberration issues:
1. Use selective sharpening
Most photo editing programs allow you to make selective adjustments. Simply apply a mask to your photo, then add extra sharpening to the edges of the frame.
2. Crop your images
It’s not an ideal solution, I know – but because spherical aberration affects the edges of the images, you can always crop to eliminate the problem. Consider composing a bit too wide when you take the picture. That way, after the crop, you get your desired composition.
Keep in mind that you’ll be losing pixels, so you won’t be able to print as large as you would have without the crop. Only use this method if you shoot with a high-resolution camera, you’re only planning on displaying your image digitally, or you only print in small formats.
A guide to spherical aberration: final words
As you can see, spherical aberration is something most photographers have experienced at some point – though whether it’s a problem for your photography depends on the type of shooting you do.
If spherical aberration is an issue, you can use one of the above methods to diminish it. Unfortunately, the only way to avoid it entirely is by using lenses that come with a high price tag.
Now over to you:
Are you bothered by spherical aberration? If so, how do you plan to reduce it in your photos? Share your thoughts in the comments below!