Reviews > Ramblings of the Fierce Bear of the North
The new fluorite generation: AFS 600 mm f/4 Nikkor FL E
Les Olson:
--- Quote from: BW on December 31, 2015, 09:59:18 ---Thanks for your reply! That might explain the discrepancy I observed at close range. Maybe the closer, near focus limit on the 600 FL, might compensate? I had the feeling I was shooting with a shorter focal length at the time I did the comparison.
--- End quote ---
The shorter focal length is a result of close focusing. The basic lens equation is 1/f = 1/u + 1/v, where f is focal length, u is distance to the object and v is distance to the image plane. When u is very large - "infinity", as one might say with only slight exaggeration - 1/u is so small it can be ignored, and 1/f = 1/v. But if u is relatively small, 1/u is larger, so either v has to get bigger to match, or 1/f must get bigger and f must get smaller. V gets bigger if the lens gets longer, and once upon a time lens did indeed get longer when they focused closer. Now we have internal focusing, so the lens does not get longer when it focuses closer, so focal length predictably gets shorter at close focus, and the closer the closest focus the shorter the focal length.
In a complex lens it is not quite that simple, because physical lens lengthening is not the only way 1/v can change, but the principle is the same.
Bjørn Rørslett:
Actually it is much more complex because the Thin Lens Equation assumes internodal distance = 0, which almost always is not the case for a real lens. A zoom lens or a design with CRC moves the nodal planes around a lot thus making the internodal distance a fourth variable. Even further complicated because one really needs to measure the conjugate distances (u,v) from the nodal planes (pupils).
In practice, one keeps sanity and frustration in check by setting up any comparison to make detail magnification in the focused plane identical. For shorter focal lengths there is no real alternative although many testers apparently is unaware of such basic facts.
BW:
It`s complicated, but since the nodal point seem to move because of close range corrections, that might explain the discrepancy. The near focus limit, the physical construction and measurements of the lens also play a role. Maybe I shouldn't bother my head with such complicated stuff ::)
Bjørn Rørslett:
Agreed. One uses lenses for photography not equations.
Akira:
Even though the thin lens equation is messed up by the modern optical design tricks, the effectve focal length can be calculated simply based on the focusing distance and the magnification factor of the image projected on the image plane, I guess?
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