Almost anyone who has attempted to photograph tall buildings understands the issue of converging verticals. The building being far bigger than you are, you need to shift your angle of view to fit it all in the frame. With what result? The vertical lines are no longer parallel. What can you do?
Well, you can straighten the lines either before or after you take the shot. Straightening the lines optically is the better solution if you want to avoid photo manipulation during post processing. Doing so will allow you to retain 100% of the details. However, straightening the verticals during post-processing is the cheaper and more convenient option, but you will likely sacrifice some small loss of detail.
Straightening the converging verticals before you take the photograph means that you need to frame the shot so as to capture the parallel lines a head-on as possible. There are few ways to do this.
1. Elevation Let’s consider, for example, photographing a ten-story building. If you want straight verticals, you’ll need to find a building opposite or some other means of raising yourself to a higher level and photograph the ten-story from there. In this case, it would be ideal to take the shot from the fifth floor of the opposite building. This will give you straight lines. The driving concept here is that the camera needs to be on a level at about the center of the object you want to photograph in order for that image to portray parallel lines. This technique applies to many things you photograph, such as in taking whole body shots or advertising products to which symmetry is crucial.
2. Tilt-Shift Lens If you can’t get access to a viewing point of sufficient height, then you can use a tilt-shift lens. A tilt-shift lens is designed to control the perspective and compensates for your lower vantage point. The downsides are: tilt-shift lenses are expensive, they only have manual focus, and they require a bit of tinkering as you figure out how to use them. Once you master the tilt-shift lens, however, you'll find that they allow for some creative effects and framing.
Using a tilt-shift lens might be a more expensive means of capturing quality images, and it is not always convenient to gain access to a high shooting point. These techniques require much more time in planning and executing the shot, and can require you to bring along more gear. Travelling photographers probably won’t find all of this convenient enough for them.
If you have already taken the shot and you wish to fix the converging lines, then you still have plenty of options to choose from. Post-processing is required since you will be manipulating the image.
1. Photoshop You can straighten the lines using Photoshop. You have the option of transforming/distorting by perspective with the free transform tool. This way, you’ll be able to straighten the lines by hand. It helps if you pull down some guidelines to help you with aligning everything. Keep in mind that if you stretch things out too much, then you’ll probably lose some details because of enlarging the image. It is always better to push in with the perspective correction.
2. DXO Viewpoint 2 DXO Viewpoint 2 is software which specializes in perspective correction. It corrects perspective by using two, four, or eight parallel guiding lines. You use these built-in lines to tell the software which lines you want straightened out, and it takes care of the rest. It is a very powerful tool, and it does its job perfectly. When you use more than two parallel lines in DXO Viewpoint 2, it automatically corrects the horizon line angle, as well. As far as I know, there's no other software to compare with this one.
3. Freeware Any other photo editing software that allows for perspective transformation can also get the job done. So if you are the freeware kind of person, you can use GIMP instead of Photoshop or DXO Viewpoint. GIMP will do the job as well as Photoshop, but it does take a little more work.
Other things you need to have in mind when using software for correcting converging verticals is that there will always be some loss of picture quality. Not that this has to be a deal-breaker; it's simply the way this stuff works. You can’t expect perfect results from it, every time. Also, if you shoot with the intent to correct in post-processing, leave some room for manipulation in the image. Shoot a bit wider than usual because chances are it will get distorted to the point that you’ll have to crop too much. Excessive cropping will give you a weird aspect ratio.
There is one last thing you should remember: when straightening verticals by software, the original size and height-to-width ratio on the building may be altered, so you might need to correct that ratio, too. DXO Viewpoint 2 lets you do that on the spot by moving a slider or two. But you’ll need to do it by hand in Photoshop. It is helpful to keep an eye on the size and ratios while in the process of perspective correction. This is because you will already be straightening the verticals by hand in Photoshop.