Focal length - question about millimeters

TMHKR

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Hello.

I have a question about DSLR lenses that has always bothered me, and it's about the focal length millimeter values.

I know that the focal length is the length in millimeters between your sensor and the optical center of your lens, that is, a piont where the light rays collimate.

But I don't understand how this works for ultra wide-angle lenses, for example a 10mm lens. Since the optical center is (usually) inside the lens, 10mm is way too short to fit a DSLR mirror between it and the sensor, isn't it?

Since I'm obviously not good at optical concepts, can someone explain me what I'm missing here?

Thanks in advance.
 

480sparky

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They design the optics to physically move the optical center of the lens away from the focal plane yet still maintain the field of view of an equivalent focal-length lens.
 

john.margetts

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But I don't understand how this works for ultra wide-angle lenses, for example a 10mm lens. Since the optical center is (usually) inside the lens, 10mm is way too short to fit a DSLR mirror between it and the sensor, isn't it?

Since I'm obviously not good at optical concepts, can someone explain me what I'm missing here?

Thanks in advance.
The optical centre of the lens is very often outside the physical lens. With a 10 mm lens the optical centre is actually inside your camera while for a very long focal length lens, the optical centre will be some distance in front of the front of the lens. This is one of the reasons modern lenses have so many pieces of glass.
 

fmw

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Most wide angle lenses are designed using retrofocus design. They are basically inverted telephoto lenses. The purpose, of course, is to move the optical center away from the focal plane to make room for a swinging mirror. You can look up retrofocus lens design in your browser for more details.
 

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