A decentered lens contains one or more optical lens elements that are either moved or tilted from the principal axis of the lens. Such shifting or tilting of lens elements can potentially lead to blurring / softness of parts of the image due to divergence of light rays. While very slight decentering of optical elements can be observed in many lenses, especially on superzooms, severe decentering and tilting can render the whole image blurry, with the lens unable to achieve good sharpness across the frame.
When purchasing a lens for my own personal use, I always take the time to first test the lens before I decide to keep it. As explained in my things to do after buying a new lens article, it is always a good idea to test lenses you buy, even if they are brand new. Unfortunately, despite efforts by manufacturers to make every lens superb out of the box, many things could go wrong by the time you receive yours. In some cases it is the fault of the manufacturer (poor quality assurance / low standards) and in some cases it is the fault of the delivery guy that carelessly throws the box containing your expensive lens. Other times, it is our fault or just bad luck, with lenses getting bumped and dropped. Sadly, these issues can negatively affect the performance of lenses and create all kinds of problems. Decentering of lens elements is one of them and it is a fairly common one.
1) Lens Decentering
When testing lenses for my in-depth lens reviews, I come across lenses with all kinds of optical problems. One of the most common issues that I see is when sharpness is uneven across the frame. In some cases it is just one corner of the frame that stands out and in other cases, more than half of the image is soft. Although such optical aberrations as field curvature can cause softness outside of the focused area, a decentered lens exhibits different sharpness patterns. When a lens suffers from field curvature, if one were to focus in the center of the frame, areas outside of the focused area would appear evenly soft. In the case of simple curvature of field, one would observe sharpness dropping away from the center of the frame, with the corners being the softest. In more complex cases of “wavy” field curvature, sharpness could drop towards the mid-frame, then pick up again in the corners. In both cases, sharp and soft areas would appear even across the frame – if one corner is soft, the other three would also appear similarly soft.
A decentered lens, however, could show some corners to be visibly sharper than others. Depending on the angle of tilt, sharpness could vary greatly from one side of the frame to another. Having tested hundreds of different lenses, I have seen all kinds of lens decentering. Some decentering can be compensated by moving the camera away from the center (up, down, left or right), while in other cases nothing can be done to achieve even sharpness throughout the frame.
Here is an illustration of a “perfect” lens (not that one exists):
As you can see, light rays that enter a perfect lens are bent normally, per original lens design, converging correctly. Now let’s take a look at a lens with a severely decentered lens element:
When a lens element is decentered, its axis is no longer aligned with the principal axis of the lens. This results in light rays getting bent differently, which can drastically reduce the sharpness of the image.
And here is an illustration of a lens with a tilted element:
For illustration purposes, I over-exaggerated both the decentering and the tilt of lens elements (the darker ones). As you can see, the light rays here are not perfectly converged and deviate quite a bit, causing severe blur in images. It is important to note that such severe tilting of an element is highly unusual and most likely would lead to unusable, soft images. In reality, most decentering and tilting issues are very minor, with lens elements being decentered / tilted by less than a millimeter from edge to edge. At the same time, even a millimeter can cause very noticeable blur in images, so you can imagine how complex lens assembly and quality control are.
Cheaper zoom lenses (especially the superzoom types, as noted in the beginning of this article) are more prone to decentering, because they have plastic barrels that often have some “play” when zoomed in / extended. Optical elements are secured by plastic components, which can shift or even break overtime. You can imagine that such lenses can be extremely difficult to test using flat charts. It does not mean that decentering never occurs on prime lenses though – the example presented below is from a high quality prime lens.
2) Test Chart Sample
How does decentering look on a test chart? Take a look at the below image, which I captured with a brand new Fuji XF 23mm f/1.4 R lens at f/1.4:
Even without seeing the high resolution version of the image, it is pretty clear that something odd is happening here – take a look at the postal stamps on the left side of the frame and compare them to the ones on the right – they appear noticeably softer. If you open up the whole image, you will see that the sharpness of the lens quickly degrades right off the center of the frame to the left. The borders of the squares are very soft. Now take a look at the right side of the frame. You will notice that the right side does not suffer from sharpness loss as badly as the left side. This is a pretty bad case of lens decentering, because I could not achieve even sharpness across the frame even after moving 3 feet from the dead center of the chart. The lens is defective and had to be exchanged.
3) Reducing Lens Decentering
If an element is only slightly decentered, usually stopping down to a smaller aperture will take care of the problem. In the case of the Fuji 23mm f/1.4 lens above, softness was reduced substantially by f/4 and was not visible to the eye at f/5.6. So if you strictly shoot landscapes and architecture at small apertures, this might not be as big of a problem for you. However, for a 35mm equivalent prime that is expected to perform well in low light situations, stopping down is not really an option. Although I would rarely photograph flat subjects, judging by how bad sharpness dropped just a little bit away from the center, I would not want to have a partially sharp face.
If a lens suffers from severe shift of optical elements, it could lose its ability to focus at infinity. If a lens element is tilted a bit too much (and by “too much” I mean as little as one millimeter!), even stopping down aggressively would not necessarily improve sharpness. There is no post-processing fix for decentering either.
So the only real option is to either return and exchange the lens for a different sample (if you are within the return grace period), or to get it serviced by a technician.
4) Testing for Decentering
If you have a “suspect” lens that shows uneven sharpness as explained above, there are different ways you could test it. The ideal case scenario would be to use a proper setup with a large test chart with the camera perfectly parallel to the chart, but that is a very costly solution and not something I would personally recommend. The easiest, cheapest and simplest method of testing decentering is shooting very fine detail at infinity. Here is what you need to do:
- Make sure that both the front and the rear lens elements are clean
- Find a bunch of trees / bushes / grass, i.e. anything with very fine detail that is located relatively far away for you to be able to focus at infinity. An overlook with no close objects is ideal for this. If you live in a high rise apartment complex, focusing on distant trees and architecture would work great as well.
- Frame your camera so that the details cover the entire frame. If you cannot fit everything, at least cover from bottom left to bottom right of the frame
- Set your lens to maximum aperture (for example f/1.4)
- Focus in the center of the area that contains the fine detail using live view (to get good focus)
- Take a picture
- If the fine detail does not cover the entire frame and you only did the bottom two corners, then reframe your shot so that it now covers the two top corners and take another picture
- Analyze images at 100% on your computer. Focused at infinity, the area that you focused on is going to be sharp. Now look at the fine details from left to the right side of the frame. The extreme corners should be very similar on both sides (whether looking at top or bottom) – similarly sharp or similarly blurry. If one side is sharp and the other is noticeably blurry, you have a decentered lens
What if you do have a decentered lens? Should you send it to the manufacturer for repair or return it? It depends on how bad the decentering really is. As I have noted above, I find that many lenses have decentering issues – some are worse than others. If the problem is minor and you can only see slightly more blur, don’t worry about it. But if you see a drastic difference, then repeat the above test at apertures that you actually use. If you are a landscape photographer, stop down to f/5.6 and see if the blur is noticeable or not. If it is not, don’t worry about it. If you can still see a huge difference, then it is probably a good idea to return the lens or get it serviced by an authorized service center. Don’t try to open the lens and do this on your own, since it would void the warranty and you could actually make it much worse.
5) How bad can it be?
If you are wondering what lens decentering could potentially do to your images, take a look at the below image that I captured of the Delicate Arch in Utah, with the Nikkor 24-70mm f/2.8G that I dropped on the carpeted floor a few weeks earlier (it was mounted to a pro-level body):
See how blurry the image is on the right side of the frame at f/5.6? The lens looked fine from the outside, but the drop did tilt one of the lens elements. I had no idea how bad it was until I came home and looked at images. It cost me $650 to get Nikon to repair it and get it properly aligned back to its specifications (Nikon had to replace the mount too).
Thanks, Nasim! Another great article which is very clearly written and very useful. The abundance of such helpful articles, not self-serving promotion, has made Photography Life far and away the best photography site on the internet for me. Great job!
Thank you Anthony, appreciate the feedback!
Thanks Nasim!
I bought a Tamron SP 23-70mm f/2.8 USD Di lens on my 21st birthday, and I am wondering if it is decentered. Here is a picture taken with it at 58mm “orbost”-by-christopher-bayliss from near the center through to the left it slowly gets blury. Do you think it could be decentered?
Your opinion would be appreciated. :-)
I have more pictures taken at 58mm that have the same results but worse.
Christopher, it is hard to tell from looking at a small image. Take a look at the corners on both sides – if both sides look similarly blurry, then you should be good. If one side looks sharp and the other one is blurry, you might have a decentered lens.
OK, thank you for the help! I have looked closer at my images, I think there definitely is a lens problem. I’ll be taking my lens back to my friendly camera store to have it checked out.
Thank you again for your effort to write these articles Nasim!
I think that your lens is decentered. The trees and the horizon on the left side are more blurry than the trees and the horizon on the right side. If you have used an aperture like 8 or 11 than the lens is probably decentered. But If you want to be sure, you have to look to the picture with 100% magnification.
Thank you, I had a look at more of my images at 100% and there is definitely an issue with my lens. I have a shot of a Shell Service Station that I took at 46mm f/5 and the station is clear and so are the trees in the background, but some trees on the left between the background and the servo are blurry. I have a lot of images that are like that.
Great article, very useful to help us understanding why our images might look soft on some lenses even though focus and shutter speed were correct, thank you so much Nasim!
http://www.santosalves.com
Thank you for your feedback Ricardo!
Thanks Nasim for bringing this “issue” to our attention and pointing out the real world photographic results that can result from a decentered lens. For any of you that are interested more detailed discussions of this sort of lens problem along with general discussions of lens performance and testing, you may also be interested in articles posted on LensRental.com. I have no financial or other interests with this site, but find the discussions and comments are all quite informative and well worth a look.
You are most welcome Jim! LensRentals.com is a great site and Roger Cicala is a good friend!
Thanks! Photography blogs rarely do articles on non-perfect-state lenses, but now I know where to look when I find myself in a similar situation like you in Arches National Park ;-)
You are most welcome Casper!
The diagram of the decentered lens shows a tilted lens element and not a decentered element; they are not technically the same thing although they may show a similar effect. A decentered lens has one or more lens elements where the axis of the element does not align with the axis of the lens.
Stephen, yes, I am aware of that – I initially drew a decentered lens element, but it was tough to make much sense out of it with light rays, because one has to draw two axes. A better representation would be a 3D drawing, but I am not that good at drawing :)
Usually, tilting and decentering is expressed together, because both result in uneven sharpness across the frame. It is much easier to visualize tilting than decentering. And worst of all, sometimes a lens element can be both decentered and tilted!
Stephen, please see the article again – I updated it by adding separate examples of tilting and decentering. Hopefully I did an OK job with the diagrams, although I must admit, I really suck at those!
Very useful. Great artical!!
Thanks Corlia!
What is interesting is that in the film only era, rarely was decentered elements discussed or recognized as a problem, even in professional circles. It’s wasn’t that quality control was any better in those days and from my own extensive testing of high quality optics back then, it wasn’t. It’s simply with the advent of digital photography, instant feedback and ability to quilt enlarge an image on a monitor, that allows us to test for such optical anomolies.
It should be pointed out that the lenses that often exhibit decentering are ultra wide angle lenses and ultra wide angle zooms. These lenses contain a significantly large number of very small elements that have to be critically aligned as compared to say a telephoto lens who’s elements are significantly larger and generally contain fewer of them.
Many years ago I owned one of the very early very expensive Pentax 15mm f3.5 rectilinear wide angle lenses which exhibited extremely poor decentering. Even a trip back to Japan and their main optical facility for months, couldn’t completely rectify the situation. Although an all metal lens in those days with unlimited range to adjust from what was explained to me by a major optical tech in Japan, it was simply too complicated an optical design in those days to be able to always achieve perfect element alignment. Since it was built in the film only era, few noticed until lens was used on DSLR’S decades later.
Thanks a lot :)
Nasim,
How much do we need to “baby” our lenses in order not to encounter with this problem? For instance, if you are taking several lenses outside for shooting, do you keep each of your lenses in a separate lens bag/case which you will put altogether in your camera bag?
Interesting article Nasim , thanks.
I agree with 1) Anthony re the non self-serving attitude to your great PL site.
I too have dropped a lens on a pro body on carpet, but in that case had a severe problem with Nikon repair in 2006.
They stated each of 4 times lens was returned for very same fault that “resolution to Nikon standards”. Rubbish. They later stated that I must have dropped it again, rubbish. Eventually Nikon replaced a split metal alloy locater ring from inside the lens which they then included with the returned lens. Why did they not do that first repair? This took over a year. Calumet were excellent and lent me their copy at one stage.
Repair centres may have now improved, but that incident made me angry and my clients ceased to give me the same quantity of work.
If you wish to buy a new lens, locate another copy of the same (either rental or colleague) and see which copy you prefer. Hopefully they will be identical.
Hi Nasim,
Could decentering be the cause of teleconverters degradation in quality, where the rear element of the lens and teleconverter are misaligned? Or is this a different scenario?
Leon
Leon, if the mounts are not 100% secure and tight, which would cause the teleconverter to “hang”, absolutely!
Thanks Nasim.
I look forward to reading more great articles written by the talented group of people on here.
I’m bummed, Nasim. A couple of weeks ago, I dropped that very same lens, a Nikon 24 – 70 mm f/2.8. It fell out of my camera bag. I took a lot of images this past weekend, and a number which I shot from a distance at f/8 do not have the entire frame in focus. I guess I’ll have to send that lens into Nikon tomorrow, hoping that I get it back in time before I leave next month out of town.
I just bought that lens in April. I think I have noticed some odd focusing effects before I dropped it….will have to look at some images before I dropped it….but guess that won’t matter to Nikon.
But, thanks for the article!
Hi Nasim,
Thank you so much for your wonderful website…so informative, accessible and well written…
I rented a Nikon 2.8 25-70mm lens recently for a wedding and got home only find some of the most important shots ruined by what I know now must be a decentered lens. A real drag, as I now must face the music and explain everything to the newlyweds. I should have trusted my instincts, as I knew something wasn’t right when I first used it-but assumed it was my inexperience, and continued. Live and learn and thanks to you-I can!
Looking forward to more great articles,
Ann
Great article Nasim! Btw I love reading all of your articles. I’m glad to know I’ve been testing my lenses correctly all this time ;). One additional test I do is to manual focus the lens on the left border, center, and right border, and compare them on the computer. I found a decentered lens that way once.
Btw you shouldn’t have repaired your 24-70, you could have sold it as the world’s first tilt-shift zoom lens haha!
What do you mean by focusing on the borders?
Hi Nasim,
Great article!!
I own a Canon 70-200mm f/2.8 IS II . I bought it three years ago. I am photojournalist (i just say this just to mention that i have
used this lens a lot).
I have noticed that when i am full zoomed (200mm), at f/2.8 and my focus is at infinity, the image that i take is not good quality. That happens sometimes. When i am at f/4 or f/5.6 there is not problem.
I send you this message after reading your post “What is a Decentered Lens?” and i understood that you have a lot of experience and maybe you can guess which the problem can be, a decentered lens or something else (like IS not has been moved off position, front/back focus)? Maybe you have faced it before. I understand that you can not figure out a lens problem from distance so it is ok if you can not give me a safe answer.
I attach you some pictures taken with that lens, Image stabilizer was off and in some pictures on, handheld camera, iso 200, speed 1/6400, f/2.8, focus infinity, Focus type: one-shot, camera EOS 7D. All images are cropped.
-Image 1, the Image stabilizer is OFF: The satellite dish and “Nova” looks sharp and clear
-Image 2, the Image stabilizer is OFF: A click right after the Image 1, satellite dish and “Nova”
are not clear.
-Image 3 & 4, the Image stabilizer is ON: The satellite dish are clear but not like the image 1
-Image 5, the Image stabilizer is OFF: Image looks clear but not so much i think
-Image 6, the Image stabilizer is OFF: Image looks messed up like the second one of the satellite dish
-Image 7 & 8, the Image stabilizer is ON: Image number 7 looks bad and 8 not so bad.
Last but not least i have to say that before a couple of years that lens it slipped from my bag and fell to the pavement (it wanst attached to my camera). It fell from a high about 16inches, maybe less. It landed horizontally, maybe the side that gets attached to the camera landed seconds faster than the rest of the lens “body”
Thank you for your time.
Kind Regards,
Mene
This is a major issue with epson projectors optics, the focus uniformity and the bad control quality, the lens calibration i think is human based performed, and some guys are doing a bad job with the calibration. I have the epson 3010 and is impossible to focus the entire image at once, the focus degradation comes to left to right…left is soft….going to the center with a better focus and finally to the right side with full sharp focus. Why this? now i know why.
Great article as all on your site are. Perfect information, easy to understand and a nice layout.
Thanks for this in depth and easy to understand article, I just wanted to comment on the part where you said there’s no post process fix for the loss of detail raught by a decentered lens. I immediately thought of using a graduated filter to add sharpness to one side of the image, assuming the effect is predictable in location, couldn’t that work?
How did you move all the people out of the way to get that great shot of Delicate Arch? ;-)
Hi Nasim
A really well explained article and very informative.
I have a canon 600D and 4 EF-S lenses. All lenses work well except my 15- 85 mm zoom. When testing this lens at f8 and f11 the corners at the bottom of the image are super sharp but the corners at the top of the image are very soft. Could this be a decentered lens problem?
Thanks and best regards
Stan Pearson
Very informative article, thank you! I bought a 24mm f/1.8 G in 2015 – upper top center of horizontal photos/ center focus point/ self-timer, is much less sharp while the the bottom of images reveals blades of grass, etc. I tried adjusting for front focusing – as long as I use small pastures and more distance, no problems; the same D800e with a used 85mm f/1.4 G has no problem focusing at f/5.6 – so it seems to be a lens problem and not my camera. Time to put your information to work…
Great article thanks.