I can't comment on the Sigma lenses (I've never owned one but had used
one or two a long time ago); I've used Tamron lenses on both film and
digital and they are absolutely terrific.  I currently have the 28-300,
the 19-35 and 24-135 and couldn't be more pleased with the results.  I'm
looking to buy the 90mm portrait/macro after my funds start building
back up a bit ;-).  I owned the "film" version of it about 18 years ago
and it was one of the best pieces of glass in my arsenal.  You can't go
wrong with Tamron glass but as I said earlier, I haven't used Sigma in
many years but I was still more impressed with Tamron back then.  Don't
really know what their [Sigma's] quality is like now.

By the way, all are currently being used on a Canon 20D.

Zach

My take on it is that Sigma make a lot of lens models, and Tamron fewer.  Sigma
have a few decent lenses; Tamron have a higher % of decent lenses.

Another way to say it is if looking for a third party lens, I would examine
Tokina and Tamron lenses before looking at the equivalent Sigma and I'd make
sure the Sigma was better before selecting it over the other two.

The 28-75 f/2.8 is likely to perform close to the 28-70 f/2.8 from Canon (which
is a fine lens, and should be at over $1100).  The Canon is better (and 3x the
price).

The 28-300 is a stay away lens except perhaps for travel where you want memories
but not necessarily the sharpest phots.  (I did see a travelogue slide
presentation done with a 28-300 (Sigma or Tamron) and I was impressed.  But the
photographer was much above average.)

Were I you, I'd consider something with more range than the 28-75 and better
optics than the 28-300 such as the Canon 28-135 IS.

Cheers,
Alan

this is an anti Sigma group, you should go ask on dpreview.com forums
instead.

I'm going to repeat something I read a while back, don't recall where.

It seems that some reliable sources have said that there is more
variation between lenses of the same brand and type, than there are
between the same types of lenses of different manufacture.  AIUI, modern
lenses are of such sophisticated design, and materials are so good, that
the variables are in manufacture and assembly.  IIRC, it was assembly
that was the main culprit.

Expensive lenses made by major camera manufacturers are more closely
QCed, which might mean that there are more rejects, and/or more
attention is paid to the work, don't know which.  I suppose that Leica
is the standard for lens quality, and because Leica rejects anything
less than perfection.  Probably Canon and Nikon follow somewhere nearby,
with other camera manufacturers close behind.

Third party lenses are a different story.  I remember reading that
Vivitar never built a lens, that they had them designed and manufactured
to their specs, probably price point.  Kiron built lenses for Vivitar
for a while, for instance.  Tamron and Tokina market some expensive
lenses and some inexpensive lenses, and the main difference aside from
bells and whistles, is quality control.

Don't know what the story with Sigma is, but I'd be willing to guess
that they have their stuff manufactured, even though they offer camera
bodies as well as lenses.  So the "third party" ideas would apply, in
that case.

So the conclusion is that you get what you pay for.  For inexpensive
third party lenses, one can sometimes get a real jewel, and sometimes a
real lemon, depending on the luck of the draw.  What you pay for seems
to be related to the probability that the lens you get is a good one.

I really don't know about all this, but it makes sense to me.

Will D.