Re: [OM] Blood moon

[Chuck Norcutt]

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
--
_________________________________________________________________
Options: http://lists.thomasclausen.net/mailman/listinfo/olympus
Archives: http://lists.thomasclausen.net/mailman/private/olympus/
Themed Olympus Photo Exhibition: http://www.tope.nl/