For most people new to photography it is only natural for them to pick up their camera and hold it in the horizontal perspective, for one it's the most stable and comfortable way to hold the camera. So, in turn, all their images are horizontal even though some may have looked better vertically. The question from most new photographers usually is, "how do I know when something may look better in a horizontal or vertical orientation"? I answer them first by saying, "when you have the time, photograph your subjects both ways, this way when you go back to review your images you can choose what looks best to you”. This may seem like a simple solution, and it is, but the first step is to train your photographic eye to see your subjects both vertically and horizontally, and the only way to do this is to actually take the images and review them. Of course there are a few subjects that generally work out better vertically, and you can start with these in mind if you don’t want to shoot everything both ways. Vertical lines, trees, flag poles, tall buildings all work well vertically, of course it is not a “rule” that all vertical lines must be shot vertically, just a starting point to get you to turn your camera vertically. Portraits are a second type of image that may look better in a vertical orientation. For me it depends on how many people I’m working with. 1, 2 or 3 people I will shoot vertically more often than a larger group which I will shoot horizontally.
Below are three vertical images all shot with the Tamron AF18-270mm VC PZD lens. The first is a group of very tall sailboat masts shot vertical to show off their height, I cropped out the boat because it was docked in an area that was crowded and it got lost amongst everything around it. The second once again, I used the vertical line of the flag pole and the vertical lines of the water being shot out of the boats engines to create a strong vertical image. Lastly, my interpretation of a portrait, which of course, doesn’t always have to be human. I went on an Eco Tour where the guide held up (with the help of one of us) a lobster to show us how to recognize if it’s a male or female (still don’t know) lobster. I will be honest, I shot this both vertically and horizontal, I ended up liking the vertical better because of the way the arm coming in from the bottom left leads you up to the (vertical) lobster, which then leads you to the arm on the right. The key is to experiment and shoot your images many different ways, the more ways you shoot them the better your eye becomes.