How does a lens work to form an image on the camera's sensor...

Started Mar 7, 2014 | Discussions thread
Leonard Shepherd
Leonard Shepherd Forum Pro • Posts: 15,935
Re: What is used for focus is

What is used for focus is a + shaped segment of the image (for cross type sensor) or a I segment if using a single direction sensor.

The segments are a little longer than the viewfinder AF boundary.

The sample goes through something similar to a beam splitter and is projected as 2 separate images on an AF detection line.

The camera AF then compares the 2 images and (if they are good for comparison) instructs the lens which way to turn and how far to get accurate focus.

The AF sensors detect only contrast (black and white). Different colours with similar black and white tones may not be accurately compared.

On a detail the camera sensor detects only in Black and White, though there is either a Red or a Green or a Blue filter above each pixel. The camera very cleverly compares detail from about 40 surrounding pixels before assigning a colour and density to each pixel site.

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Leonard Shepherd
We all aspire to take great photos but may always achieve this perhaps due to a lack of application, a lack of knowledge, or even a lack of talent.
The best photographers probably work quite hard at their photography.

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