Calculating f-stop for Achromatic Meniscus Lens

[Jon Wilson]

I searched the forum, but could not find an answer....thus, can someone outline the steps in determining the f-stop for an Achromatic Meniscus lens when the barrel is removed? Is the procedure different from that used to calculate/estimate the f-stop of a petzval? I have a nice 14 inch f11 Dallmeyer landscape lens and am trying to determine its f-stop when the barrel is removed. Thanks. Jon

[Steven Tribe]

It will still be F11 unless the light through the barrel was restricted by a stop/iris.
In which case, it will be 14" devided by the lens diameter (in inches).
If this is the triple achromat meniscus, note that the thin ultimate lens has been known to become detached!

[Jon Wilson]

Makes sense Steven. Thanks. Jon