LF lens infinity focus dimension

[luis_rives]

<p>I know that there is a specific name that lens manufacturers use in their specs for the infinity focus dimension of a particular lens, the distance from the film plane to the face of the front element when the lens is focused on infinity. Does anyone remember what that is called? Thank you.</p>

[paul_droluk]

<p>I think you're referring to "Flange Focal Distance" or FFD, which is the distance from the film plane to the mounting flange of the lens (normally the rear of the shutter). This measurement equals the bellows draw required for infinity focus.</p>

[dan_fromm2]

<blockquote>

<p>the distance from the film plane to the face of the front element when the lens is focused on infinity</p>

</blockquote>

<p>Flange focal distance is not measured from the film plane to the lens' front vertex. For lenses in shutter it is measured from the film plane to the back of the shutter. For lenses in barrel it is measured from the film plane to the back of the mounting flange.</p>

[luis_rives]

<p>Thank you for the clarification and the info. I actually was not sure where the dimension from the film plane was taken to, but knew the data existed.</p>

[Aoresteen]

<p>The<em><strong> Focal</strong> </em>length of a lens is measure from the rear focal node of the lens to the film plane. Depending on the design of the lens it could be inside the lens, behind the lens or in front of the lens.</p>

<p>You need an optical bench to do this.</p>

[nickc1]

<p>Adding to the above it has to be remembered that the focal length of a lens may differ from the marked value, and while individual examples are unlikely to vary much amongst themselves, only Anthony's suggestion of an optical bench can provide an exact answer.<br>

As a result a lens fixed at its nominal FFD may not give acceptable results, even for objects at infinity.</p>

[jose_angel]

<p>Popularly in LF photography, the film plane to shutter mount plane (lens board) distance has been taken as the "focal length" of the lens when focused at infinity. Obviously this distance rarely coincide with the real focal length in mm, but in most cases it`s pretty close.</p>

<p>Some camera rulers can be used "zeroed" at this point to easily know the added extensión when focusing closer than infinity. This way we can make calculations related to bellows extensions and compensation factors, magnifications, etc.</p>

<p>And BTW, I think have read anywhere an specification about the "camera length" from the film plane to the front rim of the "newest" IF Micro-Nikkors, in order to allow calculations related to the available working distance at different magnifications. It was about the Nikon DSLR system, nothing about LF cameras.</p>