Author Topic: The new fluorite generation: AFS 600 mm f/4 Nikkor FL E  (Read 3825 times)

Les Olson

  • NG Member
  • *
  • Posts: 247
  • You ARE NikonGear
Re: The new fluorite generation: AFS 600 mm f/4 Nikkor FL E
« Reply #15 on: January 01, 2016, 10:45:02 »
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.

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

  • Fierce Bear of the North
  • Administrator
  • ***
  • Posts: 7544
  • Oslo, Norway
Re: The new fluorite generation: AFS 600 mm f/4 Nikkor FL E
« Reply #16 on: January 01, 2016, 13:06:05 »
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

  • NG Supporter
  • **
  • Posts: 808
  • You ARE NikonGear
    • Børge Wahl-Photography
Re: The new fluorite generation: AFS 600 mm f/4 Nikkor FL E
« Reply #17 on: January 01, 2016, 21:16:55 »
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

  • Fierce Bear of the North
  • Administrator
  • ***
  • Posts: 7544
  • Oslo, Norway
Re: The new fluorite generation: AFS 600 mm f/4 Nikkor FL E
« Reply #18 on: January 01, 2016, 21:26:43 »
Agreed. One uses lenses for photography not equations.

Akira

  • Homo jezoensis
  • NG Supporter
  • **
  • Posts: 4941
  • Tokyo, Japan
    • Akira on flickr
Re: The new fluorite generation: AFS 600 mm f/4 Nikkor FL E
« Reply #19 on: January 02, 2016, 03:57:04 »
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?
"The eye is blind if the mind is absent." - Confucius

Les Olson

  • NG Member
  • *
  • Posts: 247
  • You ARE NikonGear
Re: The new fluorite generation: AFS 600 mm f/4 Nikkor FL E
« Reply #20 on: January 02, 2016, 13:35:07 »
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?
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

  • Fierce Bear of the North
  • Administrator
  • ***
  • Posts: 7544
  • Oslo, Norway
Re: The new fluorite generation: AFS 600 mm f/4 Nikkor FL E
« Reply #21 on: January 02, 2016, 13:58:59 »
Let us return to the topic at hand, please.

replica

  • NG Member
  • *
  • Posts: 3
  • You ARE NikonGear
Re: The new fluorite generation: AFS 600 mm f/4 Nikkor FL E
« Reply #22 on: January 03, 2017, 21:02:14 »
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

  • Fierce Bear of the North
  • Administrator
  • ***
  • Posts: 7544
  • Oslo, Norway
Re: The new fluorite generation: AFS 600 mm f/4 Nikkor FL E
« Reply #23 on: January 03, 2017, 22:00:13 »
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.