Question about aspherics FL

[AlexGT]

From where is the focal lenght "FL" of the aspheric measured?

Is it measured from the light source to the back of the aspheric lens or from the lightsource to the center of the aspheric lens?

Here is a pic of what I mean

Thanks!
AlexGT

 

[saabluster]

From where is the focal lenght "FL" of the aspheric measured?

Is it measured from the light source to the back of the aspheric lens or from the lightsource to the center of the aspheric lens?

Here is a pic of what I mean

Thanks!
AlexGT

The focal length is measured to the center. You will often also see BF or back focal length which is measured obviously from the back of the optic.

 

[AlexGT]

Thanks saabluster!

AlexGT