What does mm mean in terms of focal length.

[AlphaAlex]

Okay, guess I should go back to the basics. Sorry if this has been asked too many times

I know 17mm is wider than 50mm and 200mm is considered a telephoto length. But what does the unit mm really mean please?

Thanks for looking

Al

 

[ojohn]

The mm is the optical center of the lens (in millimeters).

Imagine a pinhole in a card, 17mm or 50mm or 200mm from your sensor. The view captured by the sensor from that pinhole is the view you would get from a lens of that focal length.

"Focal length" literally means the distance between the pinhole and your sensor. With your glass lens, there will be glass elements in "front" of and "behind" that optical center where the pinhole would be.

For DSLRs, where for example a 17mm lens physically can't be only 17mm from the sensor, optical tricks are used so that the optical center (or "focal length") of the lens is still 17mm.

 

[WFulton]

I know 17mm is wider than 50mm and 200mm is considered a telephoto
length. But what does the unit mm really mean please?

As a kid, maybe you played with a magnifying glass lens outdoors to focus the sun into a tiny spot to burn things?

Maybe you held that lens 3 inches above the burning focus for it to focus and burn things. That 3 inches would be the focal length of that lens, when focusing something at infinity onto this tiny spot. Where it focuses. If the lens is a simple one element of glass like the magnifying lens, this is measured from the "center" of that piece of glass lens. It is harder to say just where this center is on complex lenses. It is not necessarily where the aperture is. But the idea is the same.

This center of a 50 mm lens is mounted 50 mm (about 2 inches) from the film plane or the digital sensor plane to the design center point of the lens. Where it focuses when focusing infinity.

A "normal" lens (a field of view considered normal) is considered to have a focal length equal to the diagonal of the film size or the sensor size, more or less, no strict limits. A longer lens (more mm) would be called telephoto and a wider lens (fewer mm) is wide angle.

http://www.dpreview.com/learn/?/Glossary/Optical/Focal_Length_01.htm

 

[AlphaAlex]

thanks!