Chromatic Aberration in Photography

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What is chromatic Aberration?

by Suzanne Britt
Chromatic aberration might sound very abstract, but it is quite easy, really. Imperfections in the camera lenses create the fringes, that are seen as magenta and blue-green fringes. It comes in two varieties: 1. The different colors do not focus on the same sensor plane. 2. The individual colors produce images of different size. In the following article we will look in depth at the phenomena of chromatic aberration and how to avoid or solve it.Chromatic aberration is caused by the refractive index of glass, so let us first look at what refractive index is. When light passes through a medium, for example the glass of the lenses, the angle of the light changes. For example light may hit the lens at a 90 degree angle, but leave the lens at an 80 degree angle. The problem is that the different wavelengths of light have different refractive indexes. Red might leave the lens at 81 degrees, while blue leaves at 79 degrees. This produces what is known as longitudinal chromatic aberration where you get thin magenta fringes. The sensor focuses on the green channel and chromatic aberration causes the blue and red to be slightly out of focus, which creates the combined magenta fringes.

Transverse chromatic aberration arises when light does not reach the lens at 90 degrees, but from a different angle. In this case the different colors focus evenly, but not at the same spot. This causes the red image to be larger than the green and blue, and the blue the smallest of them all.This also produces colored fringes, but now both a magenta and a blue-green one. Chromatic aberration is hard to avoid, since it is in the nature of light, but of course lens manufacturers do their best to eliminate it.

You get different kinds of fringes for each kind of chromatic aberration. Longitudinal aberration shows as magenta fringes around objects and is distributed uniformly throughout the image. Transverse aberration is absent at the center of the image, but grows in intensity towards the corners. Longitudinal chromatic aberration is most pronounced in wide aperture lenses. It can be reduced by using a small aperture. Transverse chromatic aberration is most pronounced in telephoto lenses. However, lenses can be designed in many ways. The so called achromatic lenses are by far the most popular with minimal chromatic aberration. More rare are the superachromatic and apochromatic lenses, that virtually eliminate chromatic aberration. Digital images tend to show more chromatic aberration than film for some reason. This may be because the sensors are more sensitive to ultraviolet and infrared light, which are at the outer edge of the spectrum where aberration is most pronounced.

Software can fix cromatic aberration. Longitudinal chromatic aberration is somewhat corrected by sharpening the red and blue channels; the green channel is used to focus the image and is sharp. Transverse chromatic aberration is satisfactorily corrected by radially enlarging the blue channel image and radially reducing the red channel image.

Purple finging is a different kind of chromatic error. It appears along hard contrast edges when photographing something against a hard back light, or when photographing a light source against a dark background.The purple fringe invades the dark area. Purple fringes are sensor errors, whilst chromatic aberrations are lens errors. Purple fringing is not a simple geometric error like transverse chromatic aberration, but is an overflow of light from the brightly illuminated sensor to its neighbors; hence it is very difficult to correct with software. Also the real color is usually lost. Software can reduce the color of the purple fringe to a grayish tone. At best the local color is not completely eradicated by the purple fringe and can be reconstructed.

 

About the Author:
Sources for the article are this: chromatic aberration I and this: chromatic aberration II

great photography – contrast and white balance

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Aspects of Contrast and White Balance

 

by Karen Hanson
When retouching pictures you should first address white balance and contrast. White balance is usually what you ought to consider first, then contrast.This order is important, because you can not set color contrast properly if the image has a color cast.White balance is concerned with the color of the light within the image and sets white as an ideal. White balance applications attempt to retouch the tint of the illumination to neutral and in order to do that, the program needs some whites or grays in the picture to calculate the correct correction tint from. There are dedicated white cards, but one can also do with a sheet of white paper or a white wall. Gray cards are manufactured for the purpose of adding a neutral gray to the photo.White balance software usually has both a manual and an automatic mode. Manual correction comes as a temperature slider, which is fine for incandescent light, but not for fluorescent light or mixed light. When opening RAW images, one normally has a temperature slider. Some RAW converters also have three color sliders for red, green and blue. Fluorescent and mixed light can be somewhat corrected with color sliders, but unfortunately color sliders usually tone the blacks and whites in an undesirable way. For automatic corrections, the software normally needs neutrals in the image, like a gray card and/or a white card. Some programs can dispense with that, but usually neutrals are needed.

There are three kinds of contrast: hue, saturation and brightness. Very few applications have more than a single slider for contrast, that addresses all three kinds of contrast at once. It is not ideal with a single slider for all three, since the result usually suffers from over saturation and colorfulness. At best the software will have a control for luminance contrast and for color contrast.

The usual way to manipulate contrast is simply by altering the difference between the individual red, green and blue values and the average value (128); like this: R= (R-128) * contrast + 128; and likewise for green and blue. This method is only suitable for images that cover the entire brightness range. What if the image is very pale or very dark? In that case you need to change the algorithm to use the average values of the image’s R, G and B channels, like this: R=(R-RAverage)*contrast + RAverage. And so on for G and B. The algorithms are essentially the same since a full brightness range image will have 128 as an average value.

If the darkest and brightest areas are not black and white a different situation arises. If that is the case, one should also be able to expand the brightness range to reach black and white. This is essentially what levels adjustment does. If one’s software does not offer the option to expand brightness range, one can do it with Photoshop’s levels adjustment like this: First convert the image to Lab mode, select the L channel only and run auto levels on that. Then convert back to RGB mode.

Read more here concerning white balance and photo retouching

Great photography tips

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The Serious Tips That Will Help You Become a Cool Photographer

 

by Sanford Screen
I am just letting you know right this moment that the sole method to generally be truly wonderful in any arena out there is to start committing to the hard work. If you cannot sit back and put in the work every day of your life then there is just absolutely no way in the world that you will be capable of getting the long term outcomes you want. This is very true in relation to digital photography. You must follow and practice specific things to become truly great. For anyone who is among the many handful that are really prepared to do the work then believe me there is merely no way that you are not going to be able to see great successes in a very short while from now.The biggest thing you need to do is actually concentrate on the items that really make a difference. You will need to start paying attention on a regular basis because that’s the key to actually being willing to capture all those specific moments on camera. It is important to begin working on your ability to be attentive to your environment. The only way that I’ve found to help me obtain the long run results I would like is by really taking the time to eradicate the many disturbances that are around. Consequently it will definitely be in your interest to eliminate that tv now.The next thing you’re going to desire to look closely at is the digital camera you are genuinely working with. So trust me, you really need to truly take out the time to get the most desirable digicam that you can afford currently! Though it might cost you a chunk of income to get, it will certainly be worthwhile in the end. You can expect to learn to observe a terrific advancement in the photos which you take once you get a very good digicam.

The following thing you have to do is start to try things out a lot. It is important to go out and start to experiment by taking a selection of different photographs. Try to capture a variety of distinct photographs, landscape, portrait and perhaps black and white. It is the best method to find your specific niche market and sort of digital photography that you might love to do everyday. If you obtain the specialized niche that you’re going to wish to work at for the rest of your daily life, you must get started with practicing it every single day.

Trust me, if you’re able to abide by these rules then there’s just no chance that you are not going to be capable of become a terrific digital photographer very quickly. Remember that there isn’t any magic technique, just effort and hard work.

 

About the Author:
There’s no question that the individuals who perform the most comprehensive study are the very types that discover the best camera at the end. So get out there and read everything.