One of the most common image quality challenges in photography is to get sharp corners. Landscape photographers (among others) often pay big bucks for lenses with high resolving power at the edges of the frame. But a new lens may not solve your problem – because blurry corners usually aren’t due to the lens.
I feel obligated to start this article by saying that corner sharpness is overrated. Many, perhaps even most, photographers don’t need to worry about it very often at all. For instance, I can’t think of a single time that I’ve needed pin-sharp corners for any of my macro photos. I think that most portrait and wildlife photographers would be in a similar boat.
Still, in some genres of photography, sharp corners are important. Almost every landscape, architectural, and Milky Way photographer I know is someone who wants crisp details throughout the frame.
It’s a fact that most lenses are blurrier in the corners. Here’s a diagram from a common lens, the Nikon 24-70mm f/4 S at its sharpest focal length of 35mm:
Clearly, the corner sharpness numbers aren’t as high as those in the midframe or center. This is the case with almost every lens on the market today. So, it’s tempting to blame your lens if you’ve taken a photo with blurry corners. That’s especially true if everywhere else in the photo looks good, and the corners are the only blurry regions. Surely the glass is to blame at that point… Or is it?
Looking closer at the diagram above, you’ll notice that the corner sharpness actually reaches quite a high level. In fact, at its sharpest aperture of f/5.6, the corners on this lens are just as sharp as the center is at f/11! Have you ever made a large print from a photo taken at f/11? I certainly have, and even at pretty huge print sizes, photos at f/11 can look very sharp. Plus, this is just a kit zoom (albeit a good one). Almost any modern prime has even more corner sharpness than this.
In other words, with nearly any lens today, you should be getting at least enough detail in the corners of your images to make sharp, wall-sized prints all the way out to the extremes of your frame. So, what if you aren’t? The truth is that there are many reasons why the corners of an image could be blurrier than the center, aside from a low-quality lens. In this article, I’ll discuss four of the most important factors and how to make sure they don’t lead to blurry corners in your images.
Insufficient Depth of Field
In most landscapes, the closest thing in your photo will be along the lowest edge of the image. In fact, it tends to be in one of the two foreground corners.
You might think that the bottom center of the image is usually where the closest object would be. But from the standpoint of depth of field, that isn’t true. In real-world landscapes, there are usually slight slopes and bumps to the foreground in front of you. The result is that the single closest object (to the plane of your camera sensor, that is) will fall in the corner much more often.
The photos below are examples of typical landscapes at both wide-angle and telephoto perspectives. In both examples, the nearest object is in a corner rather than the bottom center. In this image, it’s the patch of sand at the lower left:
And in this shot, it’s the grove of trees at the lower right:
If you think that these examples are atypical, I encourage you to look through your own landscape photos with a close eye. I think you’ll find that the closest objects appear in the corners more often than anywhere else.
Personally, after scanning my landscape photography portfolio of 45 images, I confirmed that this held true. In six of my photos, the closest object was at the bottom center. Meanwhile, in a whopping seventeen images, it was at one of the bottom corners. (In eleven photos, everything was at infinity, and in the remaining eleven, the entire bottom edge of the frame was equally far from my lens.)
This all leads to me saying: You might not have a corner sharpness problem, but instead a corner depth of field problem! If your corners are closer than anything else in the photo, they’ll be disproportionately affected by a shallow depth of field – no matter how good the lens is.
Depth of field is already one of the biggest issues a lot of landscape photographers face in getting sharp shots. It seems that many photographers take pictures at wider apertures like f/4 or f/5.6 because those are the sharpest test-chart apertures, even when the real world scene in front of them demands f/11 or f/16 for proper depth of field. Yes, there’s more diffraction at those apertures, but there’s much less blur from out-of-focus areas. It’s worth the tradeoff, especially if you want your corners to look sharp.
That’s all fine and good when you have a close foreground, you might say. But what if everything is in the distance, and your corners are still blurry? It could be a bad lens, but it still might be that you lack depth of field. This is particularly true if you’re shooting with a telephoto lens.
Telephotos have less depth of field than wide angles, and “infinity focus” with a telephoto can be quite distant. You may have some objects in the bottom corners of your frame that are further out-of-focus than you thought.
In the photo below, I had to shoot at f/16 because the fence at the bottom – distant though it was – still was close enough to be out of focus at f/5.6 or f/8. Even at f/16, it’s barely within my depth of field, and the corners are still slightly weaker in sharpness than the rest of the frame.
It’s also possible that your lens could have significant field curvature – enough to harm corner sharpness, even when the lens is technically sharp enough to resolve those details. Field curvature is also a depth of field issue, although this one is the fault of your lens. Nevertheless, stopping down sufficiently (or focus stacking) is enough to fix it.
In any case, there’s an easy way to determine if your corner sharpness woes are the result of depth of field or not. Simply re-focus your lens directly in the corner of the image. If the corner suddenly looks a lot sharper, congrats! You don’t need to buy a new lens after all. You just need to use a technique that ensures your depth of field intersects with the nearby corners of your frame.
Incorrect Focusing Distance
Along similar lines as an insufficient depth of field, you may also be placing that depth of field (i.e., focusing) in the wrong spot. Take the example diagram below:
Which flower would you focus on if you want to maximize depth of field from the front to the back of the frame? (In other words, the focusing distance that equalizes the sharpness of the closest flower and the distant mountain.)
If you aren’t already familiar with the topic I’m about to mention, it may seem logical to focus on one of the middle flowers, or maybe one of the slightly closer flowers, like the one that’s 4 feet away or so. You’re probably also thinking that it’s a bit of a guess and depends upon how the scene looks at the time.
In fact, there’s no guesswork needed; the diagram above has all the information you need in order to determine the mathematically best place to focus. To be specific, the focusing distance that maximizes your sharpness from front to back is the flower at 2 feet away, because it’s twice as far away as the nearest object in your photo. This “twice as far” point is very powerful, since it equalizes foreground and background sharpness no matter what focal length you use. I’ve already explained it in detail in my articles on hyperfocal distance and the double the distance focusing method, so I won’t go into it again here.
Suffice to say, many landscape photographers focus too far away most of the time if they’re after the optimal depth of field – and, therefore, optimal corner sharpness in the foreground. Even if you’ve chosen an aperture that gives you lots of depth of field, a bad choice in your focusing distance can place that DoF incorrectly and come back to bite you.
And yes, lens sharpness can still play a role in making things worse. Even though almost every modern lens is sharp enough throughout the frame, the corners are the blurriest parts of a lens. If you combine that blur, slight or not, with the additional softness from out-of-focus areas, it’s no wonder that so many photographers are getting blurry corners!
As before, focus stacking is still a viable option to fix the focusing distance problem, but I find that it’s only needed in the most extreme of cases. Usually, the proper focusing distance combined with a narrower aperture will clear up the corners all on its own, while still leaving the rest of the frame very sharp.
Uneven Effects of Camera Shake
Another cause of disproportionately blurry corners is camera shake. If you’re shooting handheld, this is especially true, but tripod-based photography can run into shake issues as well. In windy conditions (or something similar like putting your tripod legs in a stream), you may think that your setup is stable, and even see proof of that with sharp results in the center of your image. But camera shake almost always affects some parts of a photo more than others.
To be specific, if there is any rotational shake happening in your camera system, the center of the image can look sharp even when the edges look like mush. Almost all the blurry corners I see in handheld photos – and some in tripod-based photos – have camera shake as the root cause.
Try it: Set your shutter speed close to the limit of what you can capture sharply handheld, like 1/40 second for a 50mm lens, or 1/3 second if you have good image stabilization. Then shoot a handheld picture of a brick wall focused in the center. (I know that brick wall photography is taboo in some corners of the photography world, so if anyone yells at you, say that Spencer told you to do it.) I bet you’ll get pretty good sharpness in the center when you zoom in, while the corners will look much worse. Now repeat the same test at a higher ISO and a shutter speed like 1/250 second. The center might look about the same, but the corners will look sharper!
I did that here, photographing some nice aspen trees instead of a brick wall:
The photo above was shot handheld as a test of the Nikon Z7’s in-body image stabilization system. I’d say it worked pretty well, with the center of the image looking very sharp in a 100% crop:
What about the corner? Much worse:
The blur in the image above isn’t due to a bad lens, but almost entirely due to handheld camera shake! Actually, this is why I find most claims from camera companies about image stabilization to be overly optimistic for real-world shooting. Yes, I’ve taken photos at 70mm handheld at 1 second with modern IBIS. And those photos are sharp, too – in the center. But the edges and corners rarely hold up well. For photographing something like portraits or flowers, the 6, 7, or 8-stop image stabilization claims from today’s companies may be accurate, but they fall short when you need sharp corners.
Uneven Motion Blur
The fourth and final reason for blurry corners that I’ll cover in this article is motion blur in the scene itself. Even if everything in the scene is moving at a similar rate (like grass blowing in the wind or ocean waves rolling onto shore), the corners of an image usually bear the brunt of the blur in an image.
Why is that? For the simple reason that the objects in your corners are so close to your camera. They’re magnified – which means their motion blur is magnified, too.
The simplest example of that is an ocean wave. Take a look at the photo below:
In this image, I obviously don’t care about corner sharpness, and even prefer that the water in the foreground has some nice motion blur. But you’ll notice that the waves in the distance, especially close to the horizon, don’t look very blurry. They were, of course, moving at the same speed as the wave that just crashed ashore. But because the foreground wave is so much closer, its motion blur is much more magnified!
This same thing will happen when there are rustling leaves or blades of grass in your frame. They might be perfectly sharp in distant regions of the photo, yet full of motion blur in the nearby corners. Once again, the lens isn’t to blame, and better technique (like a faster shutter speed or better timing for a lull in the breeze) is needed.
Lastly, something similar can happen in astrophotography depending on the direction you point your lens. In the photo below, it’s pretty obvious that the stars at the corner are blurrier than those in the center, and not because there’s a fault with the lens:
I took the photo above with a 120 second exposure to exaggerate the effect, but it still appears at more typical Milky Way shutter speeds like 20-30 seconds. This star movement won’t always affect the corners more than the center, but in many cases, it will.
If you’re the type of photographer who hates blurry corners, I hope this article gave you some ideas of how to avoid them. I also hope it helped reduce your gear acquisition syndrome (GAS) a bit.
Yes, some lenses have sharper corners than others. And if you’re shooting with decades-old glass, it might be time for an upgrade. But in the vast majority of cases, unsharp corners have a root cause other than the lens. It may be your depth of field, focusing distance, camera shake, or motion blur. If you can minimize all these sources of blur, you’ll find that you can take pin-sharp photos throughout the frame even with an inexpensive lens.