MM, you are taking some really nice shots! The 3200 is a fine DSLR, and
can take pics as good as anything else out there in it's class.
If you don't have one, I would really encourage you to get a copy
of a good processing program such as Adobe Elements 10 (or whatever
version they're up to now) and Adobe Lightroom. With those two programs
you can do just about anything with an image that needs done, and
in all honesty the only program I use anymore is Lightroom v3.6. Once you
learn the program (which isn't difficult) you can take a good image, and
then improve it greatly with a few minor changes.
The Sigma 70-300mm 4.6-5.8 Macro is a fine lens, but I found it's
not quite as sharp at the higher ISO settings as the Nikon version
(which I use frequently). It cost about $145 new on Amazon (the
Sigma).
If you get another opportunity at that same shot (of the city), try it
next time at
f8, 300mm and ISO 400. At f11 the aperture really
starts to close down, and if you can open it more by moving up to
f8 (might even try f6.3?) and at that distance you should still have
plenty of focused depth.
Your composure is overall very good, however you've reached the level
where applying a couple of the basic rules of photography will bring
immediate improvement. The
"Rule of Thirds" is important with landscape
shots, as it brings balance to the image. In your tungsten shot of the bridge (below)
all I did was crop it to a panorama vs. the original 2x3 crop, and also just
slightly (like very little..lol) enhanced the colors of the lights on the bridge.
Cloned out a couple of eye catching distractions near the horizon, and also
leveled out the horizon. Less that a minutes work in Lightroom.
I firmly believe that photography today is 10% science (settings for the camera, etc.)
20% inspiration, and 70% artistic impression. Once you've selected the proper
camera settings for the shot, everything else revolves around your artistic viewpoint
of the subject. You frame the image you want to take, and then in post-processing
you can make that image reflect not only what you actually saw, but you can even infuse
your emotions of the moment into the finished image.
You have a great grasp of subject, and I believe you have the artistic skills
to take your photography to a much higher level if you choose. The site
"Cambridge in Color" has a superb set of tutorials that will help you along,
and I highly recommend them to any beginning or intermediate level
photographer.