Martin Tai,

Do you have a formula relating to the change in focal length for a given lens when the lens is close focused? Is it related to the distance travel from minimum to infinity? Does the advertised focal length correspond to infinity focus?

-- chris chen (chrischen@msn.com), March 12, 2002

Answers

Does the advertised focal length correspond to infinity focus?

Mostly. The nominal focal length is at infinity but actual can vary somewhat from nominal. Many older Leica lenses have a numeral on the focussing ring next the the feet/m engraving which indicates the deviation from nominal at infinity.

At closer distances assuming a non-zoom lens without internal focus or floating elements the actual focal length is related to the reproduction ratio. A 60mm macro lens at a 1:2 reproduction ratio requires 30mm of extension so the actual distance from the optical center of the lens to the film plane is 90mm. For a 1:1 reproduction ratio the extension is 60mm, the same as the nominal focal lenght of the lens.

For internal focus, front-group focus, floating elements and zooms, it depends on the particular lens design.

-- Douglas Herr (telyt@earthlink.net), March 12, 2002.

chris For practical purpose, the focal length of lens(except varifocal lens ) is regarded as a constance, it is the number on the lens. The formular govening focal lenght, object distance and image distance is Newton formular

(d-f)x(D-f)=f*f

Where D is the object distance, d is the image distance and f is the focal length.

When D= infinity d=f, ie, the image distance is the focal length

otherwise, d is larger then f, the lens must be extended.

(d-f) is lens extension, when object is at infinity, the lens extension =0.

At when d= 2f, d=D, the image size = object size.

-- martin tai (martin.tai@capcanada.com), March 13, 2002.