I have money to spend on some gear. My current kit works well but a flash or new lens is always useful.
On the other hand, I am constrained to terrible editing tools like Gimp and Darktable. They are serviceable but are by no means industry standards like Photoshop, Bibble or Lightroom.
Which sort of investment will make the most difference in the quality of your work? Is it only a good idea to buy software if you know when and how you're going to use it?
Your existing gear is very good. You have a good camera body (D90) a fast f/1.4 lens and the Tamron is a good, sharp lens as well. I don't think you'll improve your images markedly by upgrading camera body or lenses.
A good quality editing tool like Lightroom or Photoshop, along with carefully chosen presets and plugins, can certainly help your workflow, save time, and possibly increase quality somewhat. But as a rule software isn't going to save images that aren't captured well in camera. Now it could be that your post processing isn't all that hot, but if that's the case you'd probably benefit from some instruction and practice more than better software.
Photography is all about light, and it sounds like you've identified that your biggest issue is lack of natural light. It would seem likely that you ought to look at artificial lighting, whether it's small flashes or larger lights. Even something simple like using a reflector might throw enough extra light on your subject to improve shots dramatically.
Two things that immediately jump to mind as required (for me) that you haven't listed are flash and a quality tripod and head.
Flash can be the thing that helps take your photos to the next level, whether used on-camera and bounced to just help bring a little more detail, or off-camera to really light the subject as you envision.
A quality tripod and head is the difference between a tool you loathe pulling out on those occasions you have to do it, and a piece of your kit you enjoy using whenever the opportunity presents itself. Of course, a tripod is always good to bring a little more sharpness and care to the photo as you take it.
I am a big believer in getting it right in the camera, and being light in post. That said, sometimes you have to do post processing.
I used Gimp and Darktable when my main machine was Ubuntu. I had to get a MacBook Pro for my day job (I write software) so I picked up copies of both Aperture and Lightroom 3.
Since then I use Aperture. I'm tempted by some things, like lens correction that LR3 has and Aperture does not, but the basic workflow of Aperture appeals to me.
I found that Darktable was a cool idea, but too immature. It was so slow that it constantly got in the way. And it would periodically just crash. It may be better, as I haven't used it in 10 months or so. But for me there was no going back.
I don't do much that requires the kinds of pixel tweaking that you can do in Gimp and Photoshop. I consider them to be graphics editing tools, and I look for photography editors.
I do the usual adjustments to exposure, details, saturation, etc. Any tool will do that, and most do it with minimal fuss. Crop and straighten, and minor retouching.
I fully believe that using either Aperture or Lightroom will let you be happier with the results. Of course, Aperture is Mac OS-X only, and Lightroom is Mac or Windows only.
Aperture is $80 from the Apple store, and LR3 is on constant specials for about $100 as LR4 is to be announced in a week or so. If you run Windows, get LR3 and take advantage of the upgrade special price that is sure to happen once LR4 is out.
There is really very few other places you can spend $100 and make as much of an improvement.
I recommend you read the Strobist blog, he has lots of specifics on how he takes shots, including a large number of shots taken in the golden hour and augmented with flash. You can get a nice Strobist starter setup for $300 from MPEX
If you feel like you are spending too much time on photo workflow, you need something like Lightroom. But Darktable is a lot like Lightroom I understand, so I'd put a software upgrade at the back of the list.
Personally in your position I would buy things in this order:
Notably a tripod isn't in that list, and it's because the subjects you're saying you work on most are likely not to go well with the slower more considered style of photography that a tripod entails.
My suggestion that you upgrade the body is likely contentious. Clearly there is nothing wrong with the D90 at all, it's a great camera. But it your work mostly takes place around sunset, light is at a premium. Only some subjects will suit a flash and for the rest it's all about fast lenses (which you have already) and sensitive sensors.
Finally, there are techniques for using a strobe out of doors. Here's a shot I took with one around sunset using a simple technique from Neil's book:
What do you like to shoot? Are you hobbiest or looking to go pro? Answering some of these questions will narrow down your options.
But I'll attempt to answer the question assuming you're not a pro:
If you feel like your workflow is not where it needs to be then your choice is obvious. Since you are producing pictures with your current setup, then experiment more with that setup. And use/buy software, not to get better QUALITY photos, but to help you develop your 'style'.
For past 2 years I've been shooting personal stuff with Nikon D300 and 35mm f/1.8 lens. And only that. Reason, I wanted to limit my self with equipment, but expand on the creativity and thinking outside of the box. And to my surprise it helped me make better compositions at portrait orientation, which at the time, I felt it was my weakness. By limiting focal length, I had to move around to create composition, made me create all kind of crazy compositions. But most importantly it helped me let lose and just think creatively.
As far as what software to get. Lightroom is for someone doing some editing, a lot of syncing and batching; Photoshop for extensive editing.
Hope that helps,
