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I just saw the following claim on an amazon review and can't make much sense of it.
Chromatic aberrations after 10 meters become significant. After 15-20 meters the picture becomes unusable.
Is there any truth to this? Do chromatic aberrations get worse when the object is further away from the camera? And if yes, why?
The statement seems to be backwards in regards to the physics.
CA is a form of spherical aberration where different wavelengths focus at different distances behind the lens. In this respect subject/focus distance doesn't really matter.
However, spherical aberrations also include the effect that marginal rays require more bending than more centered/paraxial rays, and they will focus at a shorter distance behind the lens if not corrected.
Typical lens diameters and subject distances result in all rays reaching the lens to be essentially parallel.
But it is possible for the subject distance to be short enough, and/or the objective element to be large enough, that the rays from the subject/source reaching the marginal areas of the lens are at a significantly greater/different angle than those reaching the center of the lens. And that changes the amount of bending those rays require to be in focus, which doesn't happen because the correction (or lack of) isn't variable; and therefore the appearance of spherical aberrations increases at shorter subject/focus distances (and larger apertures).
Edit to add: And then there is the change in focal length lenses exhibit when focused at distances shorter than infinity.
If the lens is an external/front focusing lens its' *physical focal length increases in order to focus closer... this is the same effect as adding a bellows or extension tubes. The recorded FOV is smaller, and the subject magnification greater; and therefore any aberrations will be more apparent in the resulting image (larger). Again, this is opposite of the statement quoted.
If the lens is an internal/rear focusing design its' physical FL does not increase as it is focused closer. So the recorded FOV and magnification remains nearer the same... this effect is called "focus breathing" as compared to the other unit focus design; and it will minimize any effect on the appearance of aberrations.
(*The physical increase in FL may be internal. Many lenses use a combination of front/rear focusing type designs; particularly zoom lenses)
There are two different variables in play here: focusing distance and object distance. Chromatic aberration correction is with some priority employed to stop colored fringes to appear when focusing on infinity: when everything is in the "default" out-of-focus situation, colorful fringes are really undesirable. A similar reverse "default" situation is the bokeh of "infinite" objects when focusing at a finite distance. A 35mm prime with fuzzy rainbow bokeh would be undesirable in most cases. But there are lots of other combinations as well.
When the review states "Chromatic aberrations after 10 meters become significant. After 15-20 meters the picture becomes unusable." it is not clear whether the stated distance is supposed to be the object distance or the focusing distance or both (namely describing only the behavior of image elements strictly in-focus).
The job of the lens is to project an image of the outside world on the surface of film or digital sensor.
To do this job, light rays from the subject traverse the lens and the shape of the lens surfaces and the density of the lens alter their travel path. In other words, the lens acts a light wave guide. The focal length is a measure of the distance, lens to image plane. A 100mm lens forms an image of a far distant object 100mm downstream of the lens. However, the focal length is actually a variable depending on object distance. Nearby objects come to a focus further than 100mm d downstream.
Additionally, the color of the object also influences the lens-to-image distance. Blue comes to a focus closer to the lens while red focuses last, the other colors at in-between distances. We are talking, chromatic aberration. To correct, the optician constructs the lens using multiple glass elements, each with different power and density. This method brings the red and violet rays to a focus at nearly the same place. This color corrected lens is called an achromatic, Latin for without color error. We wish this were true but alas, some residual aberration remains.
If the lens is poorly constructed, the resulting image consists of a series of image, each a different color, juxtaposed, Because of projection distance of warmer colors is elongated, the yellow and red image are somewhat larger than the blue and violet images. What happens is, the final image is befringed with a halo or rainbow of colors. Object distance plays its part. The separation of the various images is more pronounced when the lens is working in close.
Addendum Yes subject distance impacts chromatic aberration.
To correct for chromatic aberration, two lenses are combined. The first is a converging lens made from crown glass. This lens produces images at different distances downstream from the lens. First violet comes to a focus, then the other colors based on their wavelength. Red and infrared come to focus further downstream. The second lens is a diverging lens made of flint glass. The behavior of the two are opposite as to color. This companion, called an achromatic Put together, this compound converging lens well corrected for color imaging. However, residual color errors has the effect of producing different sized images for each color.
This color error is sometimes called chromatic difference of magnification. The magnitude of this error is focal length dependent. As you image closer and closer to a subject, the back-focus distance elongated. Technically the focal length remains unchanged as you close focus. This is because focal length is defined as a measurement taken when the camera is imaging an object at infinity.
I believe the lens had a defect, maybe a production fault, maybe a damage from mis-handling.
As user98068 said, there are two different variables:
For a 35mm lens, 10m vs. 20m object distance is nearly the same, the light rays follow nearly the same path (even wide open at f/2, it's easy to have both distances within Depth of Field). So, with a given focus setting, I can hardly imagine any difference in chromatic aberration (CA) between the different object distances.
But with a poor lens design or some mechanical problems inside the lens, it can be possible that at some focus settings, the CA compensation degrades or fails. It's hard to believe that by design this happens at distance settings so close to infinity. To me, it seems much more plausible that the lens had a defect, causing some elements not to move as designed, close to the "infinity" focus setting.
