Rightly so, it's very average.

Apochromatic. AKA "low dispersion", & various other names that mean
pretty much the same thing. Very broadly, it's a kind of glass that's
less prone to colour fringing than normal glass.

I don't understand your question.

I haven't used either lens, but in theory, the first will probably be
better than the second. OTOH, I strongly recommend avoiding the budget
priced Sigma zooms (& to a lesser degree, *any* brand of budget
zooms), because it's pretty much impossible to build a good quality
zoom cheaply. My standard  advice for new DSLR users on a budget is to
go for fixed-focal-length lenses, which tend to be cheaper than even
the lousy zooms, while being much higher in quality, plus; the fact
that you can't zoom means that you'll learn more about composition in
a given amount of time than you will if you start off with a zoom.
My first fixed focal on a Canon was the EF 50mm/f1.8 II, which is
extremely cheap, & much, much better optically than the 18-55 kit
lens, & most of my best photos have been taken with it. Since then,
I've upgraded to the 50mm/f1.4 (at about 4 x the price of the f1.8!),
but that was as much for the solider build of the f1.4 as it was for
optical quality. The 28mm/f1.8 isn't (IMO) as good as the 50/1.8, but
still way better than any cheap zoom, & a hell of a lot cheaper than
any zoom that can match its quality. The 85/f1.8 is around the price
of an average zoom, but the quality is outstanding - shoot with it
after using a kit zoom, & you'll laugh at the kit zoom & feel like
throwing it out the window.

Good question. I can't think of any offhand, but I'm sure many posters
here will come up with good links.'

Hello.

Got to be careful here because Sigma use both "DC" and "DG" in their lens
nomenclature and the two mean different things.

Sigma's brochure says that the "DC" means that the lens design has
"concentrated on the correction of distortions and aberrations.
Magnification of chromatic aberration is particularly conspicuous with
digital cameras... The optical designs and cutting edge technology
incorporated by Sigma eliminate flare and ghosting from the image sensor
...Vignetting is minimised whilst marginal illumination is ensured".

The main aspect of a DC lens is likely to be coatings on the rear glass of
the lens to cut down reflections between that glass and the sensor in the
400D's body. This reflection problem is not as severe with film because (I
guess) film emulsion isn't as reflective as a digital sensor.
In comparison with film, digital sensors are more susceptible to
vignetting - where the edges of the sensor receive less light than the
centre and so the image appears darker in the corners than in the centre. If
you've not seen this effect then use a wide angle lens with a telephoto
lens's lenshood. I think this problem is because each pixel in a digital
sensors is actually at some depth from the surface of the sensor (each pixel
is behind its own lens) whereas with film there is little depth to the
actual emulsion.

Sigma's brochure says that "DG" lenses are suitable only for digital bodies
using APS "C" sized sensors.
They'll be similar to Canon's EF-S lenses.
What has happened is that your 400D body will take "normal" lenses by both
Canon and Sigma. These lenses project an image large enough to cover a 35mm
film frame.
In addition, both these manufacturers produce lenses (EF-S in the case of
Canon and DG in the case of Sigma) which project an image large enough to
cover the 400D's sensor but not large enough to cover a 35mm film frame. The
idea behind this is to offer lenses which are lighter and more compact than
the lenses which cater for 35mm film  on the (understandable) reason that if
you use a 400D then you'll probably not also carry a 35mm film body (or a
digital body with a sensor the same size as 35mm film, this would be the
Canon EOS 1DS which is a lot more expensive than the 400D).
Personally, I'm happy to use these EF-S lenses. I used to use a Minolta
Vectis APS SLR and its lenses were designed solely for use with this APS SLR
so they were much smaller and lighter than their 35mm film equivalents
(their equivalent of the 500mm mirror and 100 - 300m lenses really were
light and small and a delight to take on holiday).

Your 18-55mm lens is an EF-S lens and you will find that you cannot fit it
onto a Canon film body (or an EOS 1DS body) .. well, not unless something
breaks!

You wrote that you are "confused between buying the 18-125 f3.5- 5.6 DC
(28-200mm in 35mm film terms) or  the 28-300 f3.5- 6.3 DG Macro". These
lenses would be similar to the 28-200mm and 44-400mm lenses in 35mm film
terms.

Let's see. Firstly, if you have used a 35mm film SLR then you'll need to
multiply these lenses focal length by 1.6 to get an appreciation of their
effective length when mounted on your 400D (because the 400D's sensor is
smaller than a 35mm film frame and this crops the view from any lens which
is effectively the same as increasing the focal length of the lens).
Secondly, the bad news is that this is a choice for you to make. I'd
approach it in this way:
- if I want to have a wide-angle lens (for landscapes, interiors, groups of
people) then I'd get the 18-125 as my first lens.
- if I want to have a telephoto lens (for birds, aviation, faces in a crowd)
then I'd get the 28-300 lens as my first lens.

A couple of thoughts - have you tried the Canon 10-22 lens. My girlfriend
bought one and I tried it and really would like to have one myself. For
buildings, landscapes, interiors and suchlike it really is a lot more useful
than a lens which only goes to 18mm ... but it may be outside of your budget
(I'm saving up the pennies).

If you do decide to get the 18-125 then you might prefer to get a fixed foal
length telephoto lens rather than a zoom. Might not be cheaper but may well
be smaller, lighter and with a wider maximum aperture than the zoom (which
are 5.6 or 6.3 at the telephoto end of their range).

Final comment is that I had a Sigma 105mm macro and my girlfriend bought a
Canon 100mm macro.
We compared them. I sold my Sigma and use the Canon instead. The Sigma's
focussing was slower, noisier and it was only too easy to put it partially
into Manual Focus mode, something which Sigma warn could damage the lens.

The other "catches" with Sigma is that they don't actually have licensed
versions of the EOS lens mount and their quality control seems somewhat too
tolerant. My assumption is that Canon, Nikon and suchlike place tighter
tolerances on their lenses and this in part accounts for their higher
prices.

Regarding the lens mount, Sigma do not buy a licence from Canon for their
use of the EOS lens mount so they "reverse engineer" which means that their
lenses will work with current EOS bodies but may not work with future
versions of the EOS mount. My girlfriend has a Sigma 500mm lens which works
with her EOS30 film body only in some of its exposure modes. Sigma have a
very good attitude to this - if a Sigma lens needs an upgrade to work fully
with an EOS body then they'll do it for the cost of postage (That's what
Sigma here in the UK offered me) if they have the necessary chips in stock.
If you didn't be buy the lens from new then there will be a charge of around
£35 - £40.

Regarding quality control, this is my own opinion having read m any reviews
and postings about their lenses. For most Sigma lenses I've seen both
glowing and critical reports. My guess is that Canon (and other camera
manufacturers) place a higher tolerance on their lenses hence (partially)
the higher prices.

Personally, having compared the Canon 100mm and Sigma 105mm macros I prefer
to buy Canon lenses. The Sigma macro didn't handle anything like as well as
the Canon macro and for my use that was important. When I'm taking photos of
flowers and insects in a garden or field then I've enough to do to cope with
breeezes and I really don't want to have also to cope with a lens which
hunts for the corret focus. Optically, the ersults weren't too far apart
though I recall being pleased that the Canon lens did produce betetr results
than the Sigma. Oh, I eventually buoght the Canon 100mm macro from my
girlfriend after she bought the Canon EF-S 60mm macro lens which she likes a
lot because it is a ligth lens to carry and use.

Hoep this helps but do take note of other comments - there are a lot of
Sigma lenses out there and a lot of happy users.

Regards,  Ian.-

I have bought 3 Sigma lenses over the years, and concluded that I could
not afford them, as I always ended up replacing them with Canon ones
anyway. I think it is fair to say that the Sigma ones, from all the
reports I have seen recently, have improved a lot, but I can no longer
overcome the prejudice of years!

See my other post.

Sorry, no idea.

For the reasons stated above, I don't know those lenses personally, but
many people find the very long zoom ratio lenses, like the 28-300, to be
moderate in quality. OTOH, many others (perhaps those only producing
pictures for the web, or small prints) find them satisfactory. BTW, the
designation "Macro" on such zoom lenses is just as much a lie as the APO
designation, if not more so. A true macro lens will focus down to life
size (on the sensor or film, not on the final print) and be optimised
for such work in several ways (flat field, quality optimised for work so
close the subject etc.). I do not know of any so-called "macro-zoom"
which even comes close to meeting these criteria, though the
slightly-closer-than-"normal" property can be quite useful.

Call me old-fashioned, if you wish, but what's wrong with a book? With a
web-site, you take it on trust that the author knows what he is talking
about; with a book, you know that quite a lot of people have checked it
and at least most of the information in there is likely to be correct.
And  the pictures are usually much better...

Since you are a Canon user, you will certainly find the Canon book "EF
Lens Work" (mine is EF Lens Work III, but may be a later version now) of
great interest. Obviously, it uses Canon's own lenses, but all the key
physical principles and the technologies involved are set out in
reasonable detail.

PS - there is no such thing as a dumb question when you are in genuine
need of enlightenment; we all start out with zero knowledge of lenses
and photography

David

Try the Canon forum here: http://www.photocamel.com/index.php