From: ka1gt@cbnewsm.cb.att.com (robert.m.atkins)
Newsgroups: rec.birds,rec.photo
Subject: Teleconverters (was Shoulder mounts for bird photography (long))
Date: 29 Sep 92 19:14:21 GMT
Article-I.D.: cbnewsm.1992Sep29.191421.19668
Followup-To: rec.photo
Distribution: na
Organization: AT&T

In article <1992Sep29.013233.10509@nmt.edu>, john@nmt.edu (John Shipman) writes:
> There has been a fair amount of traffic in the BIRDCHAT
> mailing list lately on bird photography.  In reply to this
> traffic, I have prepared two postings on little-known
> techniques (shoulder mounts and flash projection) that have
> proved especially useful to me; I thought you folks on
> USENET might also find them useful.

> 

Thanks for a very interesting posting. Though I am a dedicated tripod user,
I will grant that there are circumstances when the use of a shoulder stock
is the only alternative to regular hand-holding a lens and it can give a
significant improvement in sharpness. This thread ran in rec.birds a week
or so ago. My own preference is to use a tripod whenever
possible, but I have been convinced that the use of a monopod or shoulder
stock is a viable alternative when a tripod is just too awkward to be of use.

> 
> (Since everyone always asks, I prefer the doubled 300mm to
> the regular 600mm because it focuses much closer, 13'
> instead of 25', and I often get close enough to birds to
> care about this difference.  After considerable testing I
> concluded that there is no loss of sharpness with a
> high-quality teleconverter, as opposed to the $40 Vivitrash
> model.)

I have no doubt that you can take fine picture with a teleconverter, and
that you don't see any significant differences with and without one, but, and
please don't take this as a flame, there are a number of common misconceptions
about teleconverters which can give rise to confusion. Here are some facts:

1 - A teleconverter, when attached to a high quality lens MUST lower the
lens resolution (by a factor of 1.4 for a 1.4x converter and by a factor of
2 for a 2x converter). This comes from a fundamental optics analysis based
on diffraction arguments. Thus if the lens alone was the limiting factor
in image sharpness the above would apply. HOWEVER, since the film also
limits the image sharpness, the reduction in the overall image resolution is
less than would be predicted above. (note that "high quality" = diffraction
limited here).

2 - The only way in which the use of a teleconverter would not result in
a drop in image sharpness is if the original lens was not limiting 
resolution via diffraction effects, eg in cases such as:
(a) The film was severely limiting image resolution - i.e. high speed (400+)
film.
(b) The original lens had resolution limiting aberations (NOT diffraction)
which the teleconverter just happend to cancel out. This is highly unlikely.
More likely is that the effects of resolution limiting aberations would be
increased by the teleconverter.
(c) Camera vibration was limiting image resolution (often the case when a
tripod is not used).

3 - The depth of field of a prime lens + converter is exactly the same as that
of an equivalent prime lens. i.e. the DOF of a 300/4 + 2x converter is EXACTLY
the same as that of a 600mm f8 lens (despite what George Lepp has said). The
close focusing distance of the combination is the same as that of the original
lens (a significant plus, as the original poster stated).

4 - In general, teleconverters do not work well with Zoom or Mirror lenses.
There are some exceptions to this rule when high quality zooms are used.
1.4x converters degrade the image less than 2x converters.

Many tests (including a few of my own and extensive tests by George Lepp and
others) have shown that high quality teleconverters are very usable and can
give very high quality images when used with high quality lenses, HOWEVER in
all cases they do degrade the resolution of the original lens. Whether or not
you can see that degradation in your images may depend on your technique and
method of image analysis.

I echo the original poster's statement that cheap teleconverters are optically
poor. Generally the manufacturers teleconverters are best (and most expensive).
Of the cheaper brands, Lepp found the Tamron-F converters to be quite good
(at least the Nikon and Canon AF versions). I have the 1.4x Tamron-F for EOS
and I am quite pleased with it.

===============================================================
Bob Atkins  AT&T Bell Labs  email (direct)  att!clockwise!rma
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Newsgroups: rec.birds