Horizontal lines converge just like vertical lines, but often this is just part of the natural perspective of the building getting smaller in the distance. There are however a couple circumstances where getting horizontal lines parallel is pretty important / frustrating. Here’s an example of one of those scenarios. Below is a photo of Double Monk shoe shop designed by Antony Martin. The shelving unit creates a grid of lines both vertical and horizontal. The frame is also tight, which means lines are very close to the four edges of the photo, which our eye uses as a reference to decide if they are parallel. The challenge therefore is getting all four sides to be parallel with their respective edge. Now we could try and stretch the image around in photoshop, but we can also do this in camera.
The starting point is to understand that horizontal lines and vertical lines are controlled and dealt with separately. I’ve already covered vertical lines here: Correct perspective in architectural photography and why buildings fall away and I’m going to assume you’ve already mounted your camera to a tripod and levelled it with a bubble level. (read that linked post if that didn’t make sense)
This second image just illustrates that once your camera is not aimed straight onto the subject that parallel horizontal lines become a non issue. We don’t expect the horizontal lines to be parallel with the top and bottom edges of the frame. Instead we end up with leading lines and a sense of perspective. (The vertical lines however are still parallel.)
Why do horizontal lines converge?
If the plane of the camera is not parallel with the subject then the distances of A and B (see diagram below) are different. This results in perspective kicking in, and things that are further away getting smaller.
To overcome this you therefore need to make sure point A and B are equal, which will result in the camera being parallel with the subject.
The challenge though is figuring out when your camera is parallel. In theory you want to measure point A and B and make sure they are the same, but this isn’t a practical solution.
What you can do instead is use the preview screen of your camera (Live View) and a grid overlay (you’ll need to check your camera’s settings to figure our how to turn this on, but most modern cameras now have this feature). The grid will give you a reference as to whether the lines of the subject are parallel, or converging. If they’re not then rotate the camera left, or right until the lines become parallel. Obviously this requires the camera to be mounted to a tripod and a geared three way head is recommended for fine movements. What you’ll find is that even very subtle adjustments will effect the horizontal lines.
Positioning your tripod
Framing your shot while maintaining parallel horizontal lines can be challenging because normally we use the left and right movement of the tripod to aim our shot, but in this context we’re using it to get parallel lines. Which means you can’t simply rotate the camera to get something in, or out of the shot. So where you plant the tripod to begin with is critical. One way of getting around this though is to use some form of sliding rail, either a macro rail, or even a larger movie type slider. This allows you movement from side to side and avoids having to reposition the entire tripod and start the process from scratch.
One last thing to consider is lens barreling. If you’re lens has a lot of barreling then try as you might your not going to get perfectly parallel lines in camera. The lens is bending the lines which will show up in your live preview. Once you import your photo you can correct this with software like Lightroom, which has presets for most lenses.
Other places where this technique is useful
This technique is useful when you’re directly face onto a subject that has a clear horizontal line at the top or bottom of the frame. I find this occurs a fair bit with interiors because of joinery and the tight crops found inside buildings. You might also find it useful for window grids, certain types of cladding, or when shooting a full elevation directly in front of the building.