how often do you need rail extension?

[stradibarrius]

At what focal length lens do you need an extension rail for a Sinar F2 4x5. I think a 12" rail is standard. I have a 180mm and 250mm lens that I use for table top shots.
Is there a focal length that if you go longer than X you need an extension rail?

[rdenney]

With a 210, it's a matter of how close I'm focusing. A 240 really prefers that I add a 6" extension. A 12" lens can't be focused without it.

Rick "data points" Denney

[adam satushek]

Others will chime in with better responses...but here is my experience. I seem to be able to get away with the 12 in rail (with caps) with a 210mm, if I'm focused about 15 or 20 feet away. This depends on the amount of movements that are used. If I were doing tabletop I'd want at least another 12in extension around. The closer you focus....the more extension you need.

But short (usless) answer, I always need a rail extension with my sinars, even with my shortest lens.....cus i store them on 6in rails :)