quinta-feira, 25 de fevereiro de 2010

About confusion circle


Before someone asks, let me try to explain what “confusion circle” is.
I’m not an expert on the subject. So, if anyone finds an inaccuracy, please correct me. As well as with my poor English.

When we talk about light and lens, we are assuming some geometrical and ideal situations. The truth is that we can not reach that perfection when building something! Our lens has not the perfect curved surface that we desire. Somehow, in a very, very, very small scale, our lens hasn’t a perfect polished surface, but an irregular one.
This means that three light rays, with some geometrical point as source and crossing a lens, do not reach the target on the exact same point, due the surface different angles they have when entering and leaving the lens. And when we increase the number of rays, the area reached by them also increase.
To that area, not being a geometric point, they call the “Confusion circle”.
If this circle is too big we, the users, call it “out of focus” or “less defined image”.
This lack of definition increases with the curve of the surface. But also increases with the distance between the focal centre of the lens and the target. The bigger it is, the bigger the error, due the original difference of angle when emerging from each surface.
So, the lens designers try to correct this using several elements, each one with less curved surface. And adding some chemical treatment, or coats, to the surfaces, in order to get the most perfect curve they can.
That is why we get a bigger “deep of focus” when closing the diaphragm: we are using less light rays and, so, reducing the “confusion circle”.

But there is another thing that also may increase the “confusion circle”.
Light is composed of several wave lengths that, according to what they cross or what they are reflect from, we call “colour”. And each wave length has its own speed. This speed differences are used to measure astronomical distances.
But when light cross the border between two different materials, like air and glass or water, each wave length has its speed differently changed. And that is perceptible on their emerging angle. We all do that when creating a rainbow with a prism, as we did in school.
This light behaviour implicates different points for each wave length when reaching the target. And increasing the “confusion circle”.
Lens designers try to correct this by using different “refraction index”, using one different glass quality on each lens. And using different chemicals for the coats they apply over the surfaces. That is, they try to adjust the different wave length speeds.
What happens is, when doing that, they also change the overall contrast of the resulting image, as well as the colour saturation. That is why we have some lens with better resolution than others and some lens with “better colours” than others.

The admissible “confusion circle” differs with the use we give to the image. The ratio between the original target (film or sensor) and the final image size is the main factor. The bigger the ratio, the less admissible “confusion circle”. That is why we need for huge prints, and besides the pixel or grain issue, perfect lens. Those that has the smaller “confusion circle”. And, of course, the most expensive ones.
The image above was done some 20 years ago, on slide and using the most simple lens one can conceive: one magnifying glass, fixed on a extension and adjustable tube and all that fixed on my SLR camera. No correction lens, no multi coats over them. And the confusion circles became lack of definition.

You may now ask if all this is important. Or if a genius photographer (or painter) knows all about wave lengths, confusion circles and so on. They probably don’t, since they have what can not be learned: creativity! Or art, if you prefer.
But it may help us (at least it helps me) to understand what I am doing and trying to get further, using techniques instead of that spark of art that I don’t have.

As I said in the beginning, I’m not an expert on the subject and any extra information or correction is mostly welcome. As well as with my English.


Texto e imagem: by me

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