WHAT IS A DECENTRATED LENS?
A decentralized lens contains one or more elements of an optical lens that are either shifted or tilted from its main axis. Such a shift or tilt of the lens elements can potentially lead to blurring / softness of parts of the image due to the divergence of light rays. While in many lenses you can observe very slight decentration of the optical elements, especially on super zoom, strong decentration and tilt can make the whole image blurry, because the lens cannot achieve good sharpness over the entire frame.
It is always useful to check the lens before buying, even if it is brand new. Unfortunately, despite the efforts of manufacturers to manufacture each lens, many things may go wrong once you get your copy. In some cases, this is the manufacturer’s fault (poor quality guarantee / low standards), and in some cases it’s the fault of some guy who casually throws a box with a lens during transportation. Unfortunately, these conditions can negatively affect the operation of the lens and create all kinds of problems. Decentration of lens elements is one such problem, as photographer Nasim Mansurov talks about in a blog post.
Lens decentration
One of the most common problems Nazim faces when testing lenses is heterogeneous sharpness across the frame. In some cases, only one corner of the frame is highlighted, and in other cases, more than half of the image is soft. Although optical aberrations such as field curvature can cause softness outside the focused area, the decentralized lens exhibits different sharpening patterns.
When the lens suffers from a curvature of the field, if you focus in the center of the frame, the areas outside the focus area look uniformly soft. In the case of a simple curvature of the field, you can observe the sharpness falling to the side from the center of the frame, and the corners will be the softest. In more complex cases, the sharpness may decrease towards the middle of the frame, and then increase again in the corners. Sharp and soft areas will appear even across the frame – if one corner is soft, the other three will also look equally soft.
However, a decentralized lens may indicate that some angles are noticeably sharper than others. Depending on the tilt angle, sharpness can vary greatly from one side of the frame to the other. In some cases, the decentration can be compensated by moving the camera from the center (up, down, left or right), while in others nothing helps to achieve clarity throughout the frame.
What is a decentralized lens?
Here is an illustration of a “perfect” lens (nonexistent)
The light rays that fall into the perfect lens bend normally, in accordance with the original design of the lens, converging correctly. Now let’s take a look at a lens with a strongly decentralized element.
When the lens element is decentered, its axis no longer coincides with the main axis of the lens. This leads to the fact that the rays are bent in different ways, which can significantly reduce the sharpness of the image.
What is a decentralized lens?
Tilt lens
For illustration purposes, Nazim exaggerated both the decentration and the tilt of the lens elements (darker). The light rays here do not converge perfectly and deviate slightly, causing strong blurring in the images. It is important to note that such a strong tilt of the element is very unusual and, most likely, will lead to literally unsuitable soft images. In fact, most of the problems with decentering and tilting are very minor: the lens elements are decentered / tilted less than a millimeter from edge to edge. Even a millimeter can cause a very noticeable blur.
What is a decentralized lens?
Budget zoom lenses (especially zoom lens types) are more prone to decentralization because they have plastic frames that often start to “play” when zoomed in. The optical elements are fixed with plastic components that can move or even break with time. This does not mean that decentration never happens on fixed focal length lenses – the example below describes just such a case.
Sample test chart
What does decentralization look like on a test chart? Take a look at the image below that was taken with the Fuji XF 23mm f / 1.4 R lens at f / 1.4
What is a decentralized lens?
X-T1 + XF23 mm F1.4 R @ ISO 200, 1/80, f / 1.4
Even without seeing the high-resolution version of the image, it is clear that something strange is happening here – take a look at the postage stamps on the left side of the frame and compare them with the right ones. They look noticeably softer. If you open the whole image, you will see that the sharpness of the lens quickly drops right from the center of the frame to the left. The borders of the squares are very soft.
