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Chromatic Aberration

dudler

Time for an update: I still use film, though. Not vast quantities, but I have a darkroom, and I'm not afraid to use it.

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Chromatic Aberration

25 Mar 2020 8:30AM   Views : 260 Unique : 118

OK, physics time. C is for chromatic aberration...

You know that classic Pink Floyd album cover, with the prism splitting light into a rainbow?

Well, you can think of a lens as being a series of prisms, and so they bend the light, and (hereís the thing) bend different colours to different degrees. Blue light comes to a focus closer to the lens than red light.

With early cameras and plates that werenít colour-sensitive, this was a problem, because focussing by eye uses predominantly green light, while early emulsions were sensitive to blue and ultra-violet light.

The solution was to use a combination of lenses, made of different types of glass. Different glasses bend light by different amounts, and also separate the colours more or less. While the maths is utterly beyond me, I get the idea that you can make problems cancel each other out by careful choice of the characteristics of the glass, and the curves of the elements.

A lens corrected so that green and blue-violet light are in focus in the same plane is called an achromat. As photographic emulsions that are sensitive to green and then red light were developed, even more correction was required, and this gave apochromatic lenses.

But nothing works perfectly, and so you may find that there are still small problems under difficult conditions. Usually, this shows as green and purple fringing of fine detail against the light. Now, my Ilford Manual tells me that this is, strictly, not quite the same thing, being the result of light of different colours forming images that are different sizes, rather than at different distances from the lens. Itís called lateral colour. The image below was shot with a Minolta 85mm f/1.4 lens, at full aperture, and is from near the corner of the frame. Not at all a bad lens - but not perfect...

11864_1585124893.jpg



The root lies in the same thing, though Ė the fact that all light is not equal in the lenses of your camera.

Comments


mistere Plus
6 6 3 England
25 Mar 2020 9:48AM
Dark Side of the Moon. Smile Know it well..
Another good idea for passing the time and doing something different. Play about with a prism or just a glass of water, bending and refracting light can be fun, and gives some interesting and artistic results.
Dave.

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cats_123 Plus
16 4.9k 30 Northern Ireland
25 Mar 2020 9:52AM
Thanks for my daily lesson...I didn't learn well at school, but now have the time to appreciate all the wonders of the world. Keep safe😁
Irishkate Plus
9 41 118 United Kingdom
25 Mar 2020 12:29PM
Now we know why lenses are so expensive. Interesting read.
I don't see so much of the purple/green aberration as I used to
especially around buildings/windows where strong contrast of light is.
Thanks for the education.
I watched a School programme yesterday on BBC3 - about Chemistry & Physics
because we only did general science at school. I was asked to join the chemistry class
but declined foolishly but regret not knowing all that stuff!
KateGrin
dudler Plus
16 1.1k 1605 England
25 Mar 2020 2:33PM
Thank you all!

I'm doing htis partly to keep busy and give myself some structure, partly as a challenge (I'm not a sticker-at most htings), and partly to push myself to find out a bit more about things...

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