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.
Table of Contents
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.
Great article. Back to the basics, but sometimes we forget about them (I got bitten several times in landscape photography by that “I’ll take f/5.6, that’s the sharpest aperture”). And obviously focusing too far away to make matters worse…
It’s not so simple in my opinion. Edge sharpness has mostly to do with the quality and construction of the lens. You can blame an incorrect aperture value in a given focal length for edge (or better, front or back) blur only to a point. I have my examples: with a (Canon) kit lens, 18-55 f4-5.6 IS STM for APSC cameras you need something more than f/7.1 for acceptable edge sharpness (independently of DoF, that applies even if you are shooting a flat wall). Let’s say (for comparison) you take a photo of a seascape with the nearest object at 20 meters away using both the kit lens at 55mm and the Zeiss Otus 55mm (i have both so i can tell). At f/5.6 the edge sharpness of kit lens will never even approach that of Zeiss even without 100% zoom! Why? Not only because of the gap on built quality and cost but even more so the Zeiss is a full frame lens and on APSC camera you cant see the edges of the lens. So the comparison is uneven.
My second point has to do with the crop factor. You can’t just say “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.” It’s definitely not wrong but misleading because not everyone uses a full frame camera. The f4 on a MFT camera is equivalent to f8 on a full frame and an f8 on APSC is equivalent to f/11 on FF. The same applies to the focal length (if you use a FF lens on smaller sensors) and the ISO.
Finally, in my opinion you can avoid the loss of edge sharpness in two ways: 1) with very expensive glass and camera. 2) using what you have but make the correct settings and using the scene cleverly. If you are with a kit lens or a very cheap one you will never gonna have the center AND the edges tac sharp. Stopping (way) down for edge sharpness will rob you of image quality in general and remaining in the f5.6-f8 (APSC) (f8-f11 on FF) will cost you at the edges… Personally, if i can exclude some very near foreground elements i prefer to shoot a little bit higher and keep the diffraction at bay with a moderate aperture value, even with the Zeiss…
I agree with you on pretty much every count, and of course there are still differences from lens to lens, big or small. Also, yes, the aperture values I suggest are for full-frame cameras. Crop-sensor users need to divide by their crop factor to get the equivalents.
However, keep in mind that this article isn’t about getting corners that are “not optimal” but are actually blurry. Aside from lenses that were broken or had a lousy UV filter attached, I’ve never seen a lens so bad that it gives hopelessly blurry corners regardless of the photographer’s technique. Ok, maybe the Tamron 18-400mm :)
….since it equalizes foreground and background sharpness “no matter what focal length you use”….
Basically, If I’m shooting with a 500mm lens, and adhere to the twice the distance rule, I could get front to back in the DOF? Hmm… I’m not so optimistic it works with ANY focal length. I’m willing to try it on a big open field.
Not quite, the mathematical principle is that focusing at the double-the-distance point *equalizes* foreground and infinity sharpness. This fact is what’s true with any focal length (and actually, any aperture setting).
However, if your lens is too long, aperture too wide, or foreground too close, it’s still no guarantee that the foreground and background will actually be sharp. They could just both be equally blurry!
Got it! I was missing the term ‘Equalizes’. Good or bad. Soft or sharp. Thats good news too, because I was thinking of taking a picture of a nice scenic view to be printed on a fabric and use it as a backdrop in studio, but I wanted it to be equally out of focus / soft. / Bokeh, Just like the person being photographed would a actually be there. I have to try this.
When we know our wanted near and far DoF limits then the lens focus distance must be set to the harmonic mean of the two limits:
focus distance = 2 × Near × Far / (Near + Far)
For the limiting case of Far=infinity, the focus distance is simply 2 × Near.
The f‑stop is irrelevant to the required focus distance; it affects only the level of diffraction and the size of the CoC. The CoC will be the same size at the Near limit as it is at the Far limit, which is exactly what we want.
I hadn’t heard of this harmonic mean method previously. How does one apply it in the field, without having to do mathematical calculations? Is the method superior to focusing at twice the distance of the nearest subject? If so, why? Thanks.
Hi Gary, When the Far distance is infinity:
Far / (Near + Far) = 1
focus distance = 2 × Near × 1
which is twice the distance of the nearest subject.
However, when the Far distance isn’t a great deal further away than the Near distance, it’s best to calculate the harmonic mean, which will be significantly nearer to the camera than twice the distance of the nearest subject.
The harmonic mean distance is the centre value found from depth of field scales on manual focus lenses:
See the subheading a bit further down that article:
Focus and f-number from DOF limits
Very good article based on minute observations and analysis. Helpful to improve understanding.👍👍
Happy to hear it, thank you!
I am not a technical guy, but I love landscape, and do my best for every picture I take. And you know what? When I see the darkness in any corner, I just smile, because I look to the whole context, not just the corners.
You make a good point. Nothing wrong with having zero detail in the corners, or anywhere else in the frame, if the subject doesn’t demand it. That’s the case in the photo in this article with the ocean wave crashing on shore – nothing I want sharp in any of the corners. It’s yet another reason why corner sharpness is overrated.
Plus you can achieve sharp corners and this is a generalized helpful article for beginners and advanced photographers as well. However for any experienced professional, it’s pretty easy to achieve. You can do a few things. First buy the best of the best lenses and hopefully a decent/great camera and sensor can help a tiny bit. Most importantly though, you need to weigh the risks/rewards for any situation. Do you want everyone/everything in focus or why do you need sharp corners. If it’s landscapes/architecture and or reproduction type work there are options for this, such as the D850/Z7 with newer type, extremely sharp lenses like the Nikon 28mm f/1.4E lens, or Zeiss Otus line, or any modern super-tele such as my 500mm f/4E VR FL or 400mm f/2.8G VR/FL lenses I had previously. Some lenses, sensors and techniques or practice can easily help improve your corners. However like it’s written you most likely DO NOT want your fat corners sharp or don’t care. I certainly don’t always care, but I still buy the best and most sharp lenses money can buy, I skipped the Z9 for now for glass and lighting instead. Hard choice, but right choice for most!
Another issue that I suspect causes this is that the lens collects light in a spherical manner and then project the image onto a FLAT sensor. Another way of stating this is that the corners of the sensor are further from the centre of the lens than the centre of the sensor is, and so require different focussing.
This effect would be more pronounced with a wide-angle lens than with a telephoto lens because the wide angle lens collects light more spherically; or to look at it another way: when the lens is closer to the sensor (wide angle) the proportional difference between the distance from the centre of the lens to the centre of the sensor, compared to the distance to the corners of the sensor, is greater. A solution might be to have a slightly concave sensor; although the ideal amount of concavity would depend on the focal length, so could only be completely right for one focal length.
Regarding focussing — perhaps a technique for focussing (aperture aside) is to bring the focus back until the most distant object that you want to be focussed goes out of focus, and then take it forward a little to just bring that object back into focus.
See What is Field Curvature? by Nasim Mansurov:
See also Kepler’s image sensor array. The array is curved to account for Petzval field curvature.
Yes, that’s it.
Great article! I never experienced soft corners (caused by the lens) with any lens in 40 years
So, either it’s my eyes or bad reviewers.
Dont rely on reviews and always try a lens before purchasing it. A memorable image starts with great content.
Thanks for adding your experience, Mike! The content of the image is always the key.
In regards to astrophotography, one thing many photographers don’t realize is that the apparent motion during a long exposure, of celestial objects, differs depending on their latitude on earth and to what part of the sky the camera is pointed. A long exposure will reveal more motion toward the celestial equator than toward the poles. A wide angle lens that covers a greater portion of the sky will exaggerate this effect in certain parts of the image more than in other areas, and not always in the corners. An image taken with a longer focal length lens will usually have star trails of similar lengths due to the narrower FOV.
Absolutely! In the astrophoto in this article, I pointed directly toward the North Star, hence the sharp results in the center and blurry corners. Facing in the direction of the core of the Milky Way (at least with an ultra-wide lens), I still find that the corner stars have longer trails than the center, but it’s more subtle. In other directions, the strongest star trails may not be in the corners at all.
Hai Spencer, doing a lot of architecture myself i am always keen to have the sharpest possible corners…
What i noticed… three things…
Using a 24mm a lot on a nikon Body ; many designs of the 24 mm lens have a curved sharpness profile at far distances.( corners are sharp on shorter distances than the centre) This depends on the aperture used. Closing the aperture to 8 or 11 will cure/mask that because of the larger depth of field.
The IMATESTs as you show are often measured at very short distances and do not tell you much how the lens will perform on near infinity distances..
I would like to thank Nikon for implementing the so called ‘Splitscreen’ in their pro bodies. Now you can see in liveview the sides and the central area at the same time and at pixellevel . In my case with a 24mm 1.8G lens it is often so that i have to manual focus to get sides and central area sharp…
Peter, you’re absolutely right. What you’re seeing is field curvature. That lens does have some. It sounds like you’ve already been implementing the right solution: focusing at a distance that equalizes sharpness in the center and the edges, and then stopping down.
Our Imatest results are taken at moderately close focusing distances, and those don’t always tell the full story of the lens. I make sure to check sharpness with real-world sample photos at infinity and at the minimum focus distance with each lens I test. If my impressions from those results don’t line up with the test chart, I make sure to explain why in the review.
Spencer—Thanks for the insights. And I’ve learned about the faults of older zoom lenses; the ones I have are plastic-bodied and just made with alot of slop in their action, which is not hard to see when you blow an image up. Can you tell us where the image on the title page is from?
Older zoom lenses are the one category of lenses (other than superzooms) that can be blurry enough that I’d consider replacing them, if sharp corners are your goal. Even so, with good focusing technique and a narrower aperture, you can get some nice results.
If you’re talking about the photo of the rainbow, I took that near the town of Doolin, Ireland facing toward the Cliffs of Moher.