There are many things that can be wrong with a photographic lens. Optics require tight manufacturing tolerances in order to produce high resolution images. Copy to copy variation is a serious issue, even (or especially?) nowadays. While most lenses leave the factory in good condition, a non-negligible percentage reaches the market with unacceptable optical flaws. One of the most common issues is decentering.
Since this happend to me a few times already, I decided to write a short article about it. I will first explain what decentering means and then continue with a simple procedure on how to test a lens.
What’s the problem?
An ideal lens will focus everything from a certain distance in a plane perfectly coplanar to the image sensor as shown below:
In reality however, most lenses suffer from a certain degree of field curvature, meaning the focal plane has the shape of an arc when viewed in a two-dimensional space. As a result, the edges and corners will always be slightly less sharp than the center, especially at large apertures.
As a consequence, it is impossible to bring everything in the whole frame into focus at the same time. At least this holds when focusing at infinity, i.e. the objects in the scene must be very far away. However, it should still be possible to bring all four corners into focus at the same time. Something to keep in mind, but not what I actually want to talk about in this article.
Now if a lens is decentered, the focal plane will be tilted in one direction (yes, it would also be an arc, but I used a line to better visualize the problem):
So how does decentering show in the photo? It is best visible when taking photos of objects that are far away and stretch accross the whole frame (e.g. landscapes) or of large objects that are aligned parallel to your camera (e.g. a wall of a building). In those situations, one side of the image will typically be sharper than the other.
If you think “but I’m not a pixel peeper” or “this is not relevant in real world use”, then you are wrong. Decentering issues are a serious optical flaw that cannot be corrected in post and will be visible in your photographs. In extreme cases, it can happen that one half of the image is perfectly sharp while the other half is blurry (out of focus).
How to test
I find that the simplest way to test a lens for decentering issues is to compare edge sharpness when the focus is set to infinity. It is important to focus at a far away object. I’ve had decentered lenses that did not show any issues at close focus distances (e.g. when photographing a brick wall), probably because it is very hard to perfectly align the image plane to the subject.
The effect of focus tilt is best visible at max. aperture and often goes away or at least becomes less noticable when you stop down (e.g. to F11).
The procedure is really simple and only takes a few minutes:
- Choose a scene with as few foreground objects as possible. Ideally you should fill the middle of the frame edge to edge with distant objects such as a forest line, a mountain range or a city scape (see red box in photo above).
- Choose the max. available aperture of the lens.
- Use a tripod or a fast shutter speed (~1/1000s).
- Set the focus point to the middle of the frame at a far away object (green box).
- Take a photo.
- [optional] For easier comparison: Rotate the camera by 180° and take another photo without changing the focus. You can for instance use manual focus mode to ensure that the camera does not refocus when taking this 2nd photo.
- [optional] To also test for decentering along the vertical axis: Bring the camera in portrait orientation (rotate 90°) and take another shot.
Now open the files on your computer and look at the edge sharpness at 100% magnification. Compare the left side with the right side of the image (yellow dashed boxes). If you can clearly see a difference in sharpness, then the lens is most likely decentered.
Note that it is totally normal for a fast lens to show a sharpness falloff and even some blurredness towards the edges due to field curvature or optical aberrations (such as CA). However, the image quality must be equally good/bad on both sides.
If both edges turn out blurry, then your lens probably suffers from severe field curvature (it is considered normal). In this case, you could move the focus point from the center to the edge (either left or right) and redo the test.
Hint: If you have trouble to compare the left side with the right side because the scene is totally different or you are not certain that the distance is roughtly the same, you could do the suggested optional step in the list above (rotate camera by 180°).
Remark: Sometimes, decentering shows better when you stop down by one or two F-stops, especially with very fast lenses (e.g. F1.4). Therefore, I usually repeat the procedure above with at least one other aperture setting.
Alternatively, you can also align the horizon diagonally as shown below. Decentering effects are even more pronounced when you compare the extreme corners (but also optical aberrations will be much higher, which potentially complicates the diagnosis).
Side note: If you find that all lenses that you own are decentered or you have tested 3 different copies of a certain lens and all showed the same problem, then you should consider that the sensor in your camera is misaligned. It is much less likely than a faulty lens, but still a possibility.
Examples of a decentered lenses
The following examples demonstrate how decentering looks. The first candidate is the Olympus M.Zuiko Premium 25mm F1.8 lens (~320€). As explained in the procedure above, I took the following photo at the max. aperture (F1.8):
The red boxes mark the regions of interest. Let’s first have a look at the center:
No surprises here, center sharpness is excellent. Now let’s compare the crops from the left and the right side (click to enlarge):
The right side is clearly sharper than the left side.
You can download the full size jpeg here (right click, open in new tab).
The 2nd example that I am going to show here was taken with a Tokina Firin 20mm F2 lens (~650€).
Here are the crops:
You can view the full size jpeg here (right click, open in new tab).
Both lenses, the Olympus 25mm and the Tokina 20mm are high quality prime lenses and should therefore perform accordingly. Such a high degree of blurredness is not acceptable in this price range.
As already mentions, decentering is quite common and for certain lenses it even seems to be in the two-digit percentage range. The resulting tilted focal plane is especially problematic and best visible with fast wide angle and normal lenses.
The manufacturers really need to up their game in terms of quality assurance. They try to save cost by omitting proper quality control or go for a production with less tight tolerances. The sad truth is: even if 10% of all lenses are returned to the manufacturer, it is probably still cheaper for them than strict QC. Therefore, I strongly recommend you to be picky and always send your lens back if it is not perfectly fine! That’s the only way they will eventually learn and improve QA/QC.
Test your lenses, it is really not that hard 😉