Does this mean that the term "focal length" does not necessarily refer to a physically measurable distance between a focal plane and some point within the physical body of a lens?
The reason I referred to the convex surface of the lens is because it is the one closest to the sensor; the surface we see from outside the camera is concave. I realize that the "thin lens" approximation is just that and not always applicable (and I don't think it applies to the Seek lenses), but I have not been able to find anything that has enabled me to understand how the Seek lens could be considered a 7mm focal length when it is impossible for any molecule of that lens to be any closer to the sensor than 9mm (maybe 8 with the focus restriction removed) given where it is mounted in the camera and the adjustment range of the mount.Edit:
No, wait, I think I've got it. It finally dawned on me a couple hours before sunrise today...The focal length Bill W is talking about is the "effective focal length" or perhaps more correctly the "simple-convex-lens-equivalent" focal length; that is, the focal length of a thin single or double convex lens having the same field of view as that of the actual Seek lens, or the same magnification relative to a "standard" convex lens. This is the standard convention used with zoom lenses on SLR cameras and I'm okay with that and never give it a thought, but it bothers me here, probably only because I am thinking about it and was trying to directly measure the physical focal length of the Seek lens. What makes this case strange is that the actual lens is farther from the sensor than the simple convex equivalent whereas with most telephoto zoom lenses the lens [group] is closer to the sensor than the simple convex equivalent lens. The "effective focal length" is more useful than the actual focal length of the lens because, as with SLR cameras, it enables one to use consistent F-stop and shutter speed values regardless of the complexity of the lens. So when someone asks what the focal length of the Seek XR lens is, what they "want" to hear is 7mm so they can then see that it has an f/1.4 aperture which they can compare with other thermal cameras.
It has been my intent to bring this subject up back on the lens repair thread AFTER I collected some more data and did some simulations with lens ray tracing software, but I have not met my prerequisites on that yet. I have been suspecting that the formula which yields that 7mm from the field of view & sensor pitch was only valid for the thin lens approximation, but
, again, I have not found anything that deals with that for different lens configurations...And with the above realization, maybe I don't need to bring it up any more.