What is chromatic aberation? (CA)

This image is taken by Pentax A 50mm f1.2 at federation square of Melbourne.
Chromatic Aberration

Many a times we come across various Optical terms that seem to fall on deaf ears. Not only they prove to be jargons but also drown us in a state of confusion as it is a human-tendency to start guessing thing which we don’t know.

Chromatic Aberration is one such term that many of you might no be able to explain. A person who uses any form of camera must know this term well as it is closely related to property of a camera-lens.Starting with the basic meaning of Aberration, it is stated that it is the inability of a lens to produce an exact image, particularly at the edges of the photograph. Hence Chromatic Aberration would simply mean the inability of a lens to converge all the wavelengths to a single common point, thus causing a blur. You can also say that Chromatic Aberrations are color deviations of lens systems.This is a common problem in large aperture telephoto lenses that are used to detect astronomical activities and even shoot the closest wild pictures of rare species.In modern times, most of the expensive cameras have special lenses that are protected from such defects. But in most of the normal cameras, the problem of Chromatic Aberration persists.

Every one of us has heard a common term in photography, ‘Purple Fringing’. The extremely small micro lenses of our digital camera that are used to collect more light for each CCD pixel are usually built and fine-tuned in such a way that they can focus the green wavelength perfectly.

The inability to focus the red and blue wavelengths results in purple fringing around the images.Ordinary cameras which have a very small pixel-pitch as in non-DSLR digital cameras have to incorporate a special processing step to remove it.Sometimes extremely sharp digital cameras may face the same problem. The image captured by the lens might have some really small areas. This tiny area is unable to stimulate red, blue and green color pixels and due to absence of some wavelength the captured image has some portion stored with incorrect color.Some of you might be thinking that avoiding colored photography is the simplest solution to this problem. But you are in for a surprise again because Chromatic Aberration affects Black & White Photography also. The phenomenon blurs the image completely. This problem can be overcome by exposing your original image for a longer duration of time to the lens.A very common example to experience is the blooming period for new leaves.

At the time of sunrise, try capturing the image of the filtered sun-rays that pass in between the new leaves and reach the ground. When you see the image, you’ll see the increasing visibility of purple-fringing.Hence, it is a good piece of advice to thoroughly check the lens features for Chromatic Aberration when you go to buy a new camera.

What is MTF? Not WTF

This image is taken by Canon 50mm f1.2 L in Country Victoria. East Gippsland

MTF

What is the first thing you look for in a photograph? Is it the colour quality, brightness or the sharpness that you focus on?

Everyone loves a good sharp image. If the image does well in that perspective, only then people love to admire the colour combination and other features.

The sharpness of a photographic imaging system or any component of this system such as lens, sensor etc is characterized by a parameter called MTF [Modulation Transfer Function].

Learned opticians love to call it the spatial frequency response but we would like to keep it simple and easy to understand.There are thousands of formulae and many complicated MATLAB functions that help us to understand the complete phenomenon but we’ll generalize the understanding part.

The MTF factor needs to be compared when you buy a new digital camera.

As a layman, how do you know whether MTF50 or MTF50P is better as a comparison parameter?

All latest cameras have the ability to sharpen the image to some extent. Sharpening an image surely increases its contrast at the boundaries by reducing the rise distance. In such cases, the edges overshoot thus enhancing the perception of sharpness. But a large overshoot can create ugly halos near the boundary section which become obvious at high magnifications.

Such a mistake can destroy the sharpness and quality of the image completely.Every digital camera will have MTF50 and MTF50P specifications. While MTF50 corresponds to the frequency band where MTF becomes 50% of its low frequency, MTF50P implies to 50% of its peak values. These 2 indicators greatly determine the perceived sharpness.

Along with increase in sharpness, noise also increases.Suppose a digital camera sharpens an image which increases 50% MTF frequency. If the camera does over-sharpening, we may have an image with good MTF50 but very poor image quality; whereas a low sharpening will show the low potential of a camera. Either ways, MTF50 comparisons fail to help in picking the right camera.Moving on to the MTF50P factor, we see that MTF50P is same as MTF for a moderate level of sharpening. The MTF50P falls if we go for over sharpening.

Hence this is better as compared to the MTF50.The MTF factors come into picture only when a high degree of sharpened image needs to be evaluated. Otherwise, Standardized sharpening and Raw MTF50 without standardized sharpening are better comparison attributes for images.

MTF50P is one of the best features to compare while going for over sharpening of images.As discussed earlier, small overshoots across the edges give a good overall sharp image. Suppose you are going for a customized camera, then you need to keep the lens features in mind.While comparing the lens performance for the same camera, uncorrected MTF50 is one of the best parameters to compare. Sometimes people tend to confuse the term of ‘sharpening’ with "Unsharp masking’.

You must be remembering the old films and the way the slides for the film were prepared. The process of ‘unsharp masking’ reduced the overall contrast of the image which is needed for a better print version and increased the contrast near the edges rather than at a distance from the edges.Thus unsharp masking Is a bit tedious and cumbersome too.

In a nutshell, we have discussed as to how MTF factor can be used to compare the features and picture quality of cameras and how does the sharpening process depend so much on MTF.