P3D Impact Of Internal Focusing on Lens Depth Of Field

[RDJ92807 <RDJ92807@xxxxxxx>]

Since depth of field is an important aspect of stereo photography, I wanted to
share some information and get a response from other stereo photographers.  I
recently purchased my first lens with an "internal focusing" mechanism...a new
Tamron 28-105/2.8 LD lens.  This lens has internal focusing, which seems to
allow the lens to have a short minimum focusing distance and a front element
which does not rotate....making polarizers easy to use.  HOWEVER, I seems
there is a down side to this new technology.....

I shot comparative photos with many lenses...to test sharpness, color, and
contrast vs the new Tamron.  The new Tamron 28-105 did very well at infinity.
For a zoom, its very sharp with good color at all zoom settings and in my test
range of f5.6-22.  However, I noticed that when this lens is focused
closer...say around 10-15 ft, the background landscapes were definitely LESS
sharp than all the conventional lens shot against it.  It appears that
internal focusing has the effect of less depth of field.....which is not great
for stereo slides.  I talked with Tamron techs and they agreed that this is an
issue with internal focusing lenses.
They indicated that it had something to do with moving the optics within as
opposed to lengthening the barrel as in conventional lens design.

Since many of the newest (and best?) lenses are turning to internal focusing,
I was hoping others on this list had also checked their lenses and may have
seen the same issue.  This issue was also raised in the March issue of Popular
Photography, which indicated that internal focusing can cause unusual optical
results at different focus settings and zoom settings.  Help!!!!  Robert


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