P3D Re: DOF Table on Realist lens cover varies among samples!

["Greg Wageman" <gjw@xxxxxxxxxx>]

From: Paul Talbot <ptww@xxxxxxxxx>


(I wrote:)
>> Perhaps they changed the size of the Circle of Confusion.  This is a key
factor
>> in DOF scales, and you're comparing apples to oranges unless you know this
>> parameter.
>
>Greg, do you mean "changed the parameter for purposes of the
>calculations" or that the CoC of the lenses actually changed?
>(I don't understand CoC in the first place--makes my head spin
>in confused circles ;) --, so I don't know whether it is a lens
>spec or an assumption used to come up with DOF.)


The former...  Only one plane is really "in focus" (the plane at the distance
indicated by your rangefinder, assuming it is working correctly).  However, at
small apertures, other parts of the scene are in "acceptable" focus.  You choose
a CoC that defines "acceptably sharp" images.  It relates to how much a point
really resembles a point at the focal plane (I don't have the exact definition
handy).  And as has been pointed out, it also relates to how much you're going
to enlarge the resulting image; choosing a smaller CoC will allow more
magnification without the image appearing fuzzy.
So it is an "assumption", but there are generally agreed-upon values for
particular situations.

>I would hate to think of David White doing something down and dirty
>like that!  ;)  It occurred to me to compare the DOF scales between
>my retrofitted early Realist and my later Realist.  The dials are the
>same!  (The later Realist has an exposure guide instead of a hyperfocal
>table inside the lens cover; I compared the markings on the dial.)


I presume you're referring to the markings on the pointer plate; the markings on
the focus knob only indicate the plane of focus and shouldn't change unless the
focusing mechanism or lens design changed.

     -Greg W. (gjw@xxxxxxxxxx)





------------------------------