The meaning of tirage infra numbers (in mm) on SFOM lenses.

[Stolk]

I have noticed that some SFOM lenses have a specification printed on the lens, that other lenses do not.

Typically, you find focal length, aperture and maybe coverage printed on it.

But on these lenses, you will see printed:

Tirage Infra: +0.35mm

or:

Tirage Infra: +0.7mm

I am going to assume that it refers to infrared? Does that mean these lenses do not correct chromatic aberrations and thus are are not suited for full spectrum photography?

What does this distance refer to? A correction offset when focussing? A delta in focal length when it projects infrared light?

 

[OAPOli]

"tirage optique" in French means "back focus distance". "Infra" likely for "Infrarouge" i.e. infrared. This is the offset distance required to compensate for focusing infrared light.