There is a long standing convention in photography that if a building has straight sides, then that is how it should look in a photograph. Almost any textbook you pick up about photography (especially architectural photography) will mention and stress this point. Now, while making sure your verticals are vertical is a skill any photographer should have (and one that we will detail in this article), there is an important point to make.
On many occasions, converging verticals in a photograph look damn cool! Look at these dramatic examples:
If you like the dramatic effect of converging verticals then, by all means, continue shooting that way. It's difficult to imagine that the visual appeal of Gotham City or Blade Runner's Los Angeles could be maintained without the occasional converging vertical and similar results can be had in real life by intentionally chasing this visual effect (usually by getting up close with a wide angle lens or tilting the camera to look up). Sometimes the entire visual effect of an image relies on defying some convention or other.
However, if you'd like to straighten up those verticals (and it's probably a skill you should at least have as a photographer, even if you don't plan to use it) then let's look at what you can do. This article is more about shooting skills than trying to dictate what you should do or how you should shoot. Keeping the verticals vertical can be done like this:
Step Back – Getting further away from a building, especially if you're using a wide angle lens, will change the perspective and make the vertical lines straighter. Sometimes this isn't going to be possible (such as in dense cities) but if it's possible, then getting away from the building and shooting from a distance is usually the best option to straighten up the vertical sides of the building.
Level the Camera – Tilting the camera up, especially with a wide angle lens (but with others too), will give you converging verticals. Try to keep the sensor parallel to the buildings. This is as simple as keeping the camera level and resisting that urge to tilt it higher. Again, this isn't always possible, but it's something to keep in mind if you want straighter vertical lines.
Get Higher – Following on from the last point about keeping your sensor parallel to the face of the building you are photographing, sometimes one of the easiest (or only) ways to do that is to get a higher vantage point – often by taking the photograph from another building so that you can get high off the ground.
Think Foregrounds – If you are using a wide angle lens, and cannot get further away or climb a nearby building, then have a look around for an interesting foreground. If you can find something of interest to include in the foreground of your shot, then it will look as if the composition is intentional and you will still have nice, straight verticals and an interesting foreground to boot.
Get a Shift Lens – A shift lens basically allows you to control the mid point or center line of an image so (for our purposes) you can fit more in without the problem of converging verticals. You can read more about the wonders of shift lens here, but if you're doing a lot of architectural shooting it's a piece of equipment that you should seriously consider.
Get Your Post Production Skills Kicking – It is quite possible to use many different pieces of photography post-production software to straighten up your verticals. As always, it's probably best to do as much as you can “in-camera” while you are shooting, but sometimes, if that's just not possible, then you will need to go with the software option. There is a nice little tutorial on straightening verticals in Photoshop that you can read here and one for GIMP users here.
This is basically a process of elimination. If one strategy doesn't work, then move onto the next (or combine them) and you will start to see that the vertical lines get closer to straight. As we mentioned at the beginning, if you're looking for dramatic converging verticals to add impact to your architectural photography then you may not even use these techniques, but it's nice to have the skills to shoot straight verticals if you ever change your mind.