Reviews > Ramblings of the Fierce Bear of the North

The new fluorite generation: AFS 600 mm f/4 Nikkor FL E

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Les Olson:

--- Quote from: Akira on January 02, 2016, 03:57:04 ---Even though the thin lens equation is messed up by the modern optical design tricks, the effective 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?

--- End quote ---
The thin lens equation applies to complex lenses exactly the same as to simple lenses, in that for any complex lens there is an equivalent single thin lens.  In a complex lens the lens equation is harder to use because you do not know where the nodal planes from which U and V are measured are located.  A telephoto lens, like the 600mm, is a lens whose focal length is greater than its physical length in that the rear nodal point is in front of the lens.  The ordinary Nikon 70-300 extends its physical length by (roughly) 50mm in zooming from "70mm" to "300mm", so it is movement of the rear nodal point away from the lens that accounts for the extra (roughly) 180mm.  It is not that zooming causes the nodal point to move: zooming is the movement of the nodal point.

As far as measuring the focal length by measuring magnification and focus distance, yes you can, but the problem is accuracy.  If you use an optical bench, an accuracy of 1% - 6mm in the case of the "600mm" - would be a good result.  If you don't use an optical bench the results will be very approximate. 

From a photographic point of view, the important point is that a lens' focal length getting shorter as it focuses closer is not a design fault, it is a fundamental property of a lens.  There are ways you can mitigate that effect - lengthening the lens, eg - but they have drawbacks. 
 

Bjørn Rørslett:
Let us return to the topic at hand, please.

replica:
Any comments on the new beast's performance and general feel compared to your first 600/4 (AIS, manual focus)? Having seen an ad for an ai-s 600/4 tripped this question, although I am confronted with the lack of support gear for such a tool (street photography, maybe? ). thanks and a happy 2017!

Bjørn Rørslett:
The earlier versions were even heaver (> 6 kg) and it would be dangerous to your health to consider using such a lens without adequate support.

Even though the new lens is considerably "lighter", it still is 4 kg of high magnification optics. Thus it requires some kind of support than is better and tires less than hand-holding it.

paul hofseth:
And for those of us not blessed with two tripods to enhance stabilisation , but rash enough to use long, but old fashioned optics hand held with in-body stabiization cameras when the light is strong, we would then need to adjust the stabilization settings knowing the actual focal lenght.

Luckily, i do not own such exotics.

p.

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