Why is low chromatic aberration considered important in the digital era?

Question

I have read many lens reviews. It seems that low chromatic aberration is considered an important property of lenses.

However, film is a dead dinosaur today. Practically everyone is using digital cameras, and in fact many lenses (e.g. Canon EF-S) fit only digital cameras.

With digital cameras, storing the photograph in RAW format allows one to do lens corrections for the data the sensor captured. It also doesn't seem impossible for the camera manufacturer to include these lens corrections in the JPG creation pipeline. Of course, this would require a lens database in the camera, preferably one that can be updated, so that old DSLRs could support corrections for new lenses.

So, why is the chromatic aberration of lenses considered an important property? It seems to me that features that are harder to fix such as poor sharpness would be far more important.

Answer 1

ideal image - no abberration:

simulated chromatic aberration:

correcting the chromatic aberration by adjusting the RGB channel positions (in a more realistic example this would also involve stretching the images, losing even more quality)

conclusion: chromatic aberration is not just misaligned color channels - this would be easy. each individual channel is blurred and this defect cannot be fully corrected in digital.

Answer 2

You can correct chromatic aberration computationally by realigning red/green/blue layers. However, like correcting geometric distortion, those corrections usually are not by whole multiples of pixels and thus have to distribute the light on one source pixel to at least two target pixels. This causes a loss of sharpness. If you try countering this by resharpening afterwards, you amplify noise and are prone to halos.

So far this does not sound worse what distortion correction already does and you can basically combine the corrective actions of distortion correction and chromatic aberration correction before resampling to a rectangular grid in order to get less cumulative blurring than if you resampled independently.

So far, so bad.

The next problem is that chromatic aberration comes in two flavors. What I talked about just now only deals with lateral chromatic aberration which tends to be stronger the more you move from the center. There is also longitudinal chromatic aberration with its main consequence being purple fringing: if you photograph a tree's branches against a backdrop of a blue or clouded sky, significant amounts of ultraviolet and near-ultraviolet light are detected by the blue sensors. Longitudinal chromatic aberration means that this light typically is bent stronger than other light, putting its focusing plane before the sensor. This leads to unsharp purple halos around branches to both sides assuming that the branches are in-focus. If they are out-of-focus, the bluish components may actually be in-focus, giving slight red fringing (you rarely see that since it requires the focus to be too short in the first place). How much of those purplish unsharpness appears depends on the distribution of wavelengths hitting the blue sensor. Indoors LED and fluorescent lights will be harmless in comparison, incandescent light usually at least is colder (regarding color temperature rather than painter terminology) than sunlight.

Which brings us back to lateral aberration: it is not just the blue sensor which is receptive to several different wavelengths: all sensors detect a whole range of wavelengths with different sensitivities, and chromatic aberration hits all of those wavelengths differently, causing the signal of each sensor to be not just moved but also spread out according to the distribution of wavelengths hitting it. What distribution would that be? Different white balance settings take a guess at wavelength distributions but that guess is focused on getting the balance between three primary colors right.

Guessing the right amount of unsharpening means getting the balance of more than just three primary colors right, and that balance changes a lot more across the scene than basic white balance would.

So while you can statistically more or less guarantee that your fringing mostly averages out to average to gray in all directions, a sharp black-and-white edge, when looked at closely, will still kind of resolve into a small rainbow due to different (and only statistically predictable) amounts of unsharpness.

Lens corrections of chromatic aberrations don't work just with a plane of info but are three-dimensional constructions that can be calculated to bring a continuum of wavelengths mostly to the same spot on the same focusing plane. This kind of correction is not possible to do with the 3-band reduced data from a single focusing plane because it just does not contain the same amount of information.

Answer 3

There are a lot of things, including chromatic aberration, white balance, color temperature, barrel/pincusion distortion, coma, ....... that can be corrected in software. But it's still, and will always be, better to get it correct in camera, so that you don't have to do so many corrections in post processing. Even though these corrections can be done in post, every correction introduces a bit of resolution/detail loss, and may in fact introduce additional artifacts or unwanted modifications. So, delivering the best image you can to the sensor is always going to be the best option overall.

Answer 4

The job of the camera lens is to project a miniature image of the outside world on the flat surface of film or digital image sensor. Opticians strive to make lenses that yield a faithful image, but alas, they are plagued by seven major image defects we call aberrations. You should study up on these seven 1. Spherical 2. Coma 3. Astigmatism 4. Curvature of field 5. Distortion 6. Diffraction / Interference. 7. Chromatic aberration.

Chromatic aberration – Light rays traverse the lens. The curve (figure) of the lens and the density of glass affect their path of travel. They are caused to swerve (refraction -- Latin to bend inwards). We can trace these ray paths, and when we do, the trace reveals a cone of light reaching an apex at some distance downstream from the lens. The measured distance, lens to apex, is the focal length of the lens. The longer the focal length, the larger the image (magnification). Carefully examining a ray trace reveals violet light coming to a focus first, followed by green, then orange and lastly red.

Thus each color has an independent focal length, and each projects a different image size. The red image is the largest and the violet image the smallest. When we view this image we see color fringing caused by the overlapping of different image sizes.

First to attempt to mitigate chromatic aberration was John Dollond, who in 1757, demonstrated a compound lens made by sandwiching a strong positive lens made of crown glass with a weak negative lens made of flint glass. This combining significantly reduces chromatic aberration. This design is called an achromatic lens (Latin -- without color error).

Chromatic aberration is a plague regardless. It degrades every optical system. It can be mitigated, but to date, never eliminated.

Answer 5

If instead of the three (RGB) sensors in each pixel you had (say) 100 sensors, each sensitive to a range of wavelengths of 1/100 of the visible spectrum, then you would have enough data to correct chromatic aberration digitally. With only three sensors you can't tell (for example) whether a light source is red or orange, so you don't know how much to move it to correct the aberration.