The Pentax lenses (and probably the Minolta ones, too) from 30 years ago
were probably prime lenses, and they had excellent optical qualities.  While
zoom lenses may offer more versatility and convenience, you will typically
see that their image quality (especially in the case of amateur kit lenses)
is not up to the quality of those great prime lenses of the 60s and 70s.

If you still have your old lenses, try fitting them to your camera, using an
adapter if necessary, and see the difference for yourself.

With regard to the classic Pentax normal lenses, have a look at this
assessment:

http://www.luminous-landscape.com/columns/sm-02-11-24.shtml

"mjmm" posted:
"...
The only reason I can think of is that the Sigma lens is
not as good as the lenses on the old SLRs. Is this likely to
be the
correct explaination?
..."

Probably. My personal experience is that *many* (if not
*most* ... or even *all*) Stigma lenses have low contrast,
and the pictures taken with them appear to have an overall
haze (flare)

For that reason ... I haven't used that "brand" for many
years.

It is difficult to compare slides taken decades apart under different
conditions.

However: modern zooms have less contrast than older prime lenses. There is
no way around the physical properties of light bouncing between the many
more glass elements of a zoom lens than a prime lens.

Photoshop cures many ills, real and imagined.

Not that I am a big Sigma fan but you did mention that some of your
slides were fairly old.........Kodak did change the Kodachrome formula
at one point, want to say it was in the late '80s or early '90s (sure
someone else here will know for sure). Have heard tales of some of the
hardcore Kodachrome pros (the late Galen Rowell comes immediately to
mind) buying up all that they could get their hands on and putting it in
the freezer because they did not like the new stuff as well.

I have a few Tamron lenses (all in their SP lines) and am pretty happy
for the most part. Any more all I buy is either the Nikon lens or, if
price is too big of an issue, the Tamron SP counterpart. Jeremy makes a
good point though, a high quality prime will generally out perform even
a high quality zoom.

BTW- if you want more "punch", you should really try Fuji Velvia. I
prefer the older ASA 50 stuff most of the time but the 100F is great
also. The 100F sometimes reminds me of the old Kodachrome stuff I shot
in the '80s.......

Hope it helps,
Bill

there are so many variables here it is difficult to explain the
difference.  Film technoogy has changed radically as has commercial
processing.   Lens technology has changed radically.  Some may pretend
that older lenses are superior to current lenses, which in some
specific examples of simple desings is probably true. Look at zoom lens
technology.   The zooms of the 70's and 80's were garbage compared to
todays zooms and cost 3 to 10 times as much in 1980 dollars.   While I
treasure my Nikkor 105 2.8  prime.... my 80-200 F2.8 blows away any
zoom and most primes built before 1990. Computers are a wonderful
thing.    I will say that my Nikon 28-85 requires careful exposure and
attention to stray light.  When I consider these factors I get great
exposures and saturated color.  I believe that your Sigma is probably a
signifcantly netter lens tan the average lens of the 70's/80's .,

BTW.. when critically comparing slides, get a light box (ie PortaTrace)
and a good loupe.  Examine the slides first  and then scan.  Scaning is
not a good way of judging the quality of the slide.  I examine my
slides, and adjust the analog gain depending on shadow density that I
observe when examing manually.  I find this a much more satisfactory
method than scan and fix.

Jim