From: jacobson@cello.hpl.hp.com (David Jacobson)
Subject: Re: ? Burred backgrds w/ Nikon 105 or 85mm
Date: Tue, 10 Nov 1992 05:22:07 GMT
Organization: Hewlett-Packard Laboratories

In article <1dmgkdINN161@sixgun.East.Sun.COM> marc@.East.Sun.COM (Marc Thompson-Sun-Vienna VA-Systems Engineer) writes:
>I am looking at purchasing a new Nikon 105/2.5 AIS or 85/1.8 AF lens for
>my Nikon camera.  My main area of interest is fashion photography, and
>I am looking for a portrait and full body lens that would allow me to
>blur out the background as much as possible.  
>
>The two lenses that I am looking am considering are listed above. Does
>anyone know
>which of these lenses would do a better job at blurring the background
>when fully opened.

Let's assume that with both lenses you are at the appropriate
distance so you have the same size image of the subject.  For the
"near background" the lens with the smaller numbered f-stop will
defocus the background the best.  This is because, assuming you are
not in a macro situation and quite a bit closer than the hyperfocal
distance, relation between distance out of subject plane and circle of
confusion is proportional to M^2/f, where M is the magnification and f
is the f-number.  The focal length doesn't matter.

Now the question for "far background", where far background begins
about twice the lens to subject distance, is a lot trickier, and
somewhat subjective.  If the subject has a magnfication M, a point at
infinity will be rendered as a circle of diameter F M/f.

You can see two differences between this and the previous formula.
First the focal length shows up.  And secondly M is not squared.

Let's explore the case where the magnficiation is 1/25, ie the subject
area is approximately 24x36 inches.  The 105 f/2.5 lens results in a
point at infinity being rendered as a 1.68mm circle on the film, while
with the 85mm f/1.8 lens a point at infinity is rendered as a 1.88mm
circle.

So comparing the lenses you mention, the f/1.8 lens defocuses best in
both distance ranges, but this is not true with every pair of lenses.
In addition, the out of focus objects in the background are a little
larger with the 105mm lens.  Or conversely, less of the background is
visible.  The effect of this is subjective, but in my very naive
opinion, this makes the background more obscure; a small point
favoring the 105mm lens.

The following is a plot of circle of confusion diameter vs.
distance behind subject, both in millimeters.

           @@@@@@  85mm f/1.8
           ****** 105mm f/2.5

      #                                                          @@@@@@@@@@    
  1.5 #                                              @@@@@@@@@@@@              
      #                                      @@@@@@@@                          
      #                                @@@@@@                       *******    
      #                          @@@@@@                 ************           
 1.25 #                        @@               ********                       
      #                     @@@           ******                               
      #                  @@@        ******                                     
      #               @@@      *****                                           
    1 #            @@@      ***                                                
      #          @@      ***                                                   
      #        @@     ***                                                      
 0.75 #       @    ***                                                         
      #      @   **                                                            
      #      @ **                                                              
      #     @ *                                                                
  0.5 #    @ *                                                                 
      #   @ *                                                                  
      #  @**                                                                   
      # @*                                                                     
 0.25 #@*                                                                      
      #*                                                                       
    ##*######################################################################  
      #                                                                        
                  2000         4000          6000          8000        10000   

  -- David Jacobson


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