Everything Else Aperture Does to Your Photos

Most photographers are aware that aperture affects your exposure and depth of field in photography. (If that’s news to you, I recommend reading our introduction to aperture before this article.) But what else does it affect? There are lots of other variables that also change with your choice of aperture. In this article, I will go through all the other ways that aperture impacts your images, from sharpness to sunstars and more.

Before diving into too many specifics, here’s a quick list of everything aperture affects in photography:

• Exposure and depth of field, as everyone knows!
• Sharpness loss due to diffraction
• Sharpness loss (and other things) due to lens aberrations
• Starburst effects on bright lights
• Visibility of camera sensor dust specks
• The quality of background highlights (bokeh)
• Focus shift on some lenses
• Ability to focus in low light

1. The Negative Effect of Diffraction

Using a narrow aperture gives you more depth of field. So, if you’re a landscape photographer who wants everything as sharp as possible, you should use your lens’s smallest aperture, like f/22 or f/32 to get everything in focus, right?

No!

It’s true that you’ll get more depth of field at a narrower aperture. Here, for example, is f/4 versus f/32:

But when we zoom in, you can see that the second photo is less sharp in the small details. Here are the images when cropped:

Here, you’re seeing an effect called diffraction.

Diffraction is not a lens problem; it’s a matter of physics. When you use a very small aperture like f/32, the light that goes through your lens must go through such a narrow opening that it starts to interfere with itself. The result is that the images grow blurrier in the details, even while depth of field increases.

At what aperture does diffraction start to become an issue? It depends upon a number of factors, including the size of your camera sensor and the size of your final print. Personally, on my full-frame camera, I see a little loss of sharpness because of diffraction at f/8, but it’s not enough to bother me. I use smaller apertures like f/11 and f/16 all the time in landscape photography when I need more depth of field (see “Landscape Photographers – Are You Stopping Down Enough?“). However, I try to avoid f/22 or smaller, since I lose too much detail at that point.

Diffraction isn’t necessarily a huge problem, but it exists. Don’t be afraid to take pictures at f/11 or f/16 just because you lose a little bit of sharpness. In many cases, the added depth of field is worth the tradeoff.

2. Lens Aberrations

For some reason, everyone wants to take sharp photos! One of the ways to do so is to minimize the visibility of lens aberrations. So, what are lens aberrations? Quite simply, they are image quality problems caused by your lens.

Although many image quality problems in photography are because of user error – things like missed focus or poor exposure – lens aberrations are entirely due to your equipment. They are fundamental, optical problems that you’ll notice with any lens if you look too closely, although some lenses are better than others. For example, consider the image below:

What’s going on here? In this crop, most of the lights look smeared rather than perfectly round. On top of that, the crop just isn’t very sharp. Those are lens aberration at fault!

It’s likely that your lenses are less sharp at their maximum aperture values. Lenses also have darker corners at wider apertures, and they have blurrier corners compared to the center. These effects are all due to lens aberrations as well.

Aberrations can appear in several different forms. This article would be way too long if I explained every aberration, so I will link to our articles explaining some of them in more detail: vignetting, spherical aberration, field curvature, coma, distortion, and color fringing. In general, it’s more important to know why aberrations occur, including how your aperture setting can reduce them.

It starts with a simple fact: Designing lenses is difficult! When the manufacturer fixes one optical problem, another tends to appear. It’s no surprise that modern lens designs are extremely complex.

Unfortunately, even today’s lenses aren’t perfect. They tend to work fine in the center of an image, but everything gets worse near the edges. It’s pretty easy to design a lens that’s sharp in the center, but very hard to design one that’s sharp in the corners.

Here’s a diagram that explains what I mean:

And that brings us to aperture.

Many people don’t realize a simple fact about aperture: it literally blocks the light transmitted by the edges of your lens. (This doesn’t lead to black corners in your photos, because the central regions of a lens still transmit light to the edges of your camera sensor.)

As your aperture closes, more and more light from the sides of your lens will be blocked, never making it to your camera sensor. Only the light from the center area will pass through and form your photo! As I mentioned, this central area is far easier for lens manufacturers to design. The end result is that your photos will have fewer aberrations of most kinds when you use narrower apertures.

How does this look in practice? See the photos below (heavy crops from the top-left corner):

What you’re seeing above may look like an increase in sharpness, but it’s really a decrease in aberrations! The end result? At f/5.6 on this particular lens, my photo is much sharper than at f/1.4.

Now, how does this balance out with diffraction, which harms sharpness increasingly more as your aperture gets narrower and narrower? In practice, the answer is that most lenses end up sharpest around f/4, f/5.6, or f/8. These “medium” apertures are small enough to reduce a lot of aberrations, but they aren’t so small that diffraction is a significant problem. However, you’ll want to test this on your own equipment.

Of course, you can still take good photos at large apertures like f/1.4 or f/2. Portrait photographers sometimes pay thousands of dollars to get a lens exactly for that purpose! And modern lenses are sharper than ever at wide apertures. So, don’t lock your lens to f/5.6 just because it gives you a tad more corner sharpness up close. It’s better to choose an aperture that gives you the right artistic look to the image.

3. Starburst and Sunstar Effects

Starbursts, also called sunstars, are interesting elements that you’ll find in certain photographs. When the sun is in my frame, I often try to capture them in my landscape photos. Here’s an example:

How does this work? When you photograph a small, bright point of light (such as the sun, especially when it’s blocked), the aperture blades in your lens will form beams of light. For every aperture blade in your lens, you’ll get one sunbeam (though it looks like two since it crosses the sun all the way). The narrower your aperture, the more distinct the sunstar. When the sun is in my photo, I typically set f/16 purely to capture this effect.

The sunstar effect looks different from lens to lens. It all depends upon your aperture blades. If your lens has six aperture blades, you’ll get six sunbeams. If your lens has eight aperture blades, you’ll get eight sunbeams. And, if your lens has nine aperture blades, you’ll get eighteen sunbeams.

Wait, what? That’s no typo. It sounds strange, but the reason is actually quite simple. In lenses with an even number of aperture blades (and a fully symmetrical aperture design), half of the sunbeams will overlap the other half. This doesn’t happen if your lens has an odd number of aperture blades.

Here’s a diagram to show what I mean:

Most Nikon and Sony lenses have seven or nine aperture blades, resulting in 14 and 18 sunbeams respectively. Most Canon lenses have eight aperture blades, resulting in eight sunbeams. I took the photo above using the Nikon 20mm f/1.8G lens, which has 7 aperture blades. That’s why the image has 14 sunbeams.

It’s not just the number of blades that matters, though — their shape is also important. Some aperture blades are rounded (which usually results in a more pleasant out-of-focus background blur), and others are straight. If your goal is to capture good sunstars, straight aperture blades can potentially produce more defined rays of light.

Again, some lenses are better than others in this regard. For the best results, find a lens that’s known to have good sunstars, and then set it to a small aperture like f/16. That’s going to give you the strongest definition in your sunstars.

Finally, there’s one last related effect that I wanted to mention briefly. When you shoot into the sun, you might end up with flare in your photographs, as shown below. Depending upon your choice of aperture, the size and shape of this lens flare will change, usually becoming more defined as you stop down to narrower apertures. This isn’t a big deal, but it still exists.

4. Small Apertures and Unwanted Elements

When you shoot through things such as fences, dirty windows, plants, and even water droplets on your lens, you may be disappointed by photos taken with a small aperture.

Small apertures like f/11 and f/16 give you such a large depth of field that you may accidentally include elements that you don’t want to be in focus! For example, if you’re shooting at a waterfall or by the ocean, an aperture of f/16 could render a tiny water droplet on your lens into a distinct blob:

In cases like that, it’s better just to use a wider aperture – something like f/5.6, perhaps – in order to capture the water droplet so out-of-focus that it doesn’t even appear in your image. In this particular case, you could simply wipe the droplet off, but that’s not possible if you’re shooting through something like a dirty window.

Another example of shooting through things is when a piece of dust lands on your camera sensor. Unfortunately, as you change lenses, this can be very common. Dust specks on your camera sensor will show up clearly at small apertures like f/16, even if they’re invisible at something larger, like f/4.

Luckily, they are very easy to remove in post-production software like Photoshop or Lightroom, though it can be annoying if you have to remove dozens of them from a single photo. That’s why you should always keep your camera sensor clean.

5. Changes to Your Bokeh

What is bokeh? It’s simply the quality of your background blur. If you take a lot of portraits, macro photos, or wildlife photos, you’ll end up with out-of-focus backgrounds in most of your images. Naturally, you want them to look as good as possible! This is another area where aperture plays a role. Simply put, different aperture settings will change the shape of your background blur.

Why is that? It’s because the background blur of your photographs always takes on the shape of your aperture blades. So, if your aperture blades are shaped like a heart, you’ll end up with heart-shaped background blur. Most of the time, that would qualify as distracting bokeh, although it’s kind of cute in this photo of two fake tortoises:

On most lenses, aperture blades change shape a bit as they open and close. Large aperture settings (such as f/1.8) often have rounder background blur compared to smaller aperture settings. You’ll also get more background blur at large apertures, since your depth of field is thinner.

If bokeh is something that matters to you, you’ll want to test this on your particular lenses. Take some out-of-focus photos of a busy scene, each using a different aperture setting, and see which one looks the best. Most of the time, it will be the lens’s widest aperture, but not always.

6. Focus Shift Issues

With certain lenses – even if you’re in manual focus, and you don’t move your focus ring – your point of focus may shift slightly as you use different apertures.

Obviously, this isn’t ideal. How do you tell if your lens has problematic focus shift? It’s pretty easy. Here are the steps:

1. Put your camera on a tripod, and set your lens to manual focus.
2. Find an object with small details that extends backwards, and focus at the center of it. A table with a tablecloth works well.
3. Double check: Make sure that part of the subject is sharp with pixel-level details, and part of the subject is clearly out of focus.
4. Take a photo at your lens’s widest aperture, and then at progressively smaller apertures. Be sure not to move your focus ring, and double check that you are using manual focus.
5. On your computer, zoom into 100% on these photos and see if the sharpest point of focus moves continuously farther back as you stop down. The more it moves, the worse your focus shift issue is.

You’re done!

If your lens has extreme levels of focus shift, you’ll want to compensate for it. All you need to do is set your aperture first, and focus second. This is good practice anyway, but it is especially important if your lens has a lot of focus shift.

7. Ease of Focusing

Your camera’s autofocus system won’t work well if it doesn’t receive enough light.

If you shoot with a cheaper lens, especially a zoom, the lens’s maximum aperture probably will not be wide enough to gather a lot of light. For example, a lens with a maximum aperture of f/5.6 or f/6.3 will have a harder time focusing than a lens with a maximum aperture of f/2.8.

This is part of why a lot of pro photographers prefer more expensive lenses like a 70-200mm f/2.8. It isn’t just about getting a sharper lens or even shooting at f/2.8 necessarily – it’s also the ability to focus at f/2.8 to keep shooting in very low light.

You’ll also enjoy a brighter viewfinder (on a DSLR) or a less noisy viewfinder (on a mirrorless camera) if your lens has a large maximum aperture. If you take a lot of photos in low light, this can make it much easier to focus and compose your images.

Conclusion

Aperture is about more than just exposure and depth of field. Of course, those are the two biggest effects that you’ll notice as you change your aperture, but I hope this article gave you a sense of the others.

If you have any questions or think of any effects that I missed, let me know in the comments below!