Re: [OM] Finally, infinity, was: Blood moon

[Chuck Norcutt]

Finally.  After looking at the manual for an 800mm Nikon (which says 
nothing interesting) I found that the manual for the 800mm Canon 
illustrates the use of the infinity mark for "normal temperatures" and 
implies that there is more range for non-normal temperatures (which are 
not defined) and refers to the rest of the range as the "infinity 
compensation mark".  Anyhow, you can see it here on page 6
<http://www.usa.canon.com/cusa/consumer/products/cameras/ef_lens_lineup/ef_800mm_f_5_6l_is_usm#BrochuresAndManuals>

These infinity markings are the same type as on the Tamron lenses 
mentioned before but are not explained in any way by Tamron.
Chuck Norcutt


On 11/7/2014 7:47 AM, Chuck Norcutt wrote:
> OK, I looked.  You may well be correct that there is sufficient change
> of focus due to day/night temperature changes to warrant refocusing at
> night.  However, I don't think one can positively draw that conclusion
> from the two Tamrons you pointed to in the Rockwell link.
>
> I downloaded the owner's instruction sheets for both lenses and both
> have the following comments regarding the focusing distance scale:
>
> "The distance scale is marked for guidance purposes.  The actual focal
> point may slightly differ from the distance marked on the focal length
> index."
>
> "At infinity make sure the image in the viewfinder appears sharp.  The
> infinity position is made with certain allowances to insure proper focus
> under a variety of conditions."
>
> While that may well refer to temperature change effects there is only
> one marked infinity position and the fact that it may be slightly
> inaccurate is no different than the statement for other marked
> distances.  There is certainly no indication of how to deal with
> temperature or focus beyond infinity.
>
> Chuck Norcutt
>
>
> On 11/7/2014 12:01 AM, Moose wrote:
> > On 11/6/2014 8:08 PM, Chuck Norcutt wrote:
> > > I have to think about that one a bit.
> > Mercy! Look, don't ponder!
> >
> > > I've only heard of such things but never seen one.  It leaves me very
> > > curious since lens tubes both expand and shrink with temperature. If
> > > there is sufficient linear expansion to cause focusing error when the
> > > tube shrinks with cooling temperature there is also sufficient change
> > > in length to cause focusing error when the tube expands with
> > > temperature.
> > Thinking too simple still. Think complex zoom, with all those different
> > parts inside being changed in size by temperature.
> >
> > > The "past infinity" would occur with a shrinking tube but a "short of
> > > infinity" would occur with an expanding tube. It seems to me both
> > > situations are equally likely but I've never hear of a tube marked for
> > > both nor one with specific temperature recommendations.
> > >
> > > Does anyone have an actual lens so marked and with instructions how to
> > > use it?
> > I don't believe I've ever seen instructions talk about it. But who
> > knows; it's been a lot of years, I've read a few lens instruction
> > sheets/manuals, back when they had them. But I've ignored or never
> > received many more. IF I remember correctly, I was curious and looked it
> > up.
> >
> > Anyway, there are a couple of examples under the Tamron heading here.
> > <http://www.kenrockwell.com/tech/pma07-c.htm> I know I saw them before
> > digital, but on which lenses ... ??
> >
> > Infinity, Wherefore Art Thou Moose
--
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