On The Level: Making Peace with Crooked Horizons
You see photos everywhere with trees or buildings leaning over. It's a common problem. There are a number of ways to prevent this or fix it of course but still you see pictures with crooked horizons. I'm betting some of you have shot some yourself. In the interest of helping to straighten things out, here are some thoughts on the matter.
A variety of bubble levels
Your best bet is to level your camera before you shoot so you don't have to deal with it afterwards. Yes, you can straighten things out in post processing, but that takes time. It also costs you image resolution. If you rotate an image in Photoshop you naturally get one that has sloping sides. It may now have a level horizon but not just the horizon got turned when you rotated it. Cropping the result will cut off some from each side leaving you with a smaller image with fewer pixels than you started with. Your twelve megapixel camera just created a ten megapixel image. Congratulations. The way to avoid cropping is to get things level before you press the shutter release rather than after.
So, the use of some form of bubble level in the field is a good idea. But there's more than one way to approach this task. I mean, where precisely do you put the level? The usual answer is that you affix it to the camera hot shoe. Quite a few companies make bubble levels specifically for this purpose. Some have a level oriented along only a single axis, some on two axes, and a few now with bubbles in along all three axes. I've long been a user of the double bubble kind but was always frustrated when I flipped the camera from horizontal to vertical orientation since that meant I also had to flip the bubble level ninety degrees to get the two axes back to being useful. To fit three bubble levels the triple kind is obviously a bit larger, but I find it quite convenient not to have to fiddle with the thing once I've mounted it on the hot shoe.
With a bubble level attached atop your camera, a simple glance can tell you if it is crooked or not. But if the base of your tripod head itself isn't level, panning from side to side will take your camera through an arc that also isn't level. As such, your hot shoe bubble level may tell you your camera is level one minute, but annoyingly no longer level the next. All you have to do is pan left or right and you're off balance again. You can't solve this problem with a level in your flash shoe. However, some tripod legs come with a bubble level for this purpose on the top platform where the tripod head mounts. My trusty Gitzo has one. This way, I can check that the panning bed is level when I adjust the legs. Once this is done, I know that I can pan my camera without messing up the horizon.
The science of leveling a tripod head. From bottom to top: Gitzo level on legs, Acratech level on leveling base, and Kirk Enterprises level on tripod head.
Getting the tripod legs set up just right though can be difficult. Think about it for a minute. You're trying to level the panning base in two directions but the tripod has three legs. Raising or lower just one leg will have an effect on how close to level you are according to the bubble level for both horizontal axes. This can lead to a cyclical need to tweak the height of one leg, then the next, then on to the third leg, or perhaps back to the first, and so on. Regardless of the sequence, by the time you finally do get the legs adjusted to level, you are likely to find that the overall height is now wrong. If you end up a couple inches too high or low you'll have to start the whole leg adjustment leveling dance all over again after you fix the height. With some subjects an inch or two of height won't make much of a difference, but with macro or certain landscape subjects with a foreground very close to the lens front, it can make or break the shot.
Which means we have yet another problem, doesn't it? Some tripods address this problem by allowing you to tilt the center column to an arbitrary angle to level the panning base even when the legs themselves aren't quite level. But I hate tripod center columns since they don't allow me to splay the legs out completely to achieve a ground level position. The presence of a center column means a tripod can't get any lower than the length of that center column. Instead, I use a leveling base made by Acratech that sandwiches in between the top of the leg set and the bottom of the tripod head panning base. It works basically as a pivot point to allow the head to be moved by a up to ten degrees in any direction. Thus, the legs themselves only have to be close to level for me to be able to make up the difference with the leveling base. And yes, it has a built in bubble so I can check that I get the thing level when I need to. Just like so many other toys for photographers (I mean "essential equipment" for photographers) the Acratech base is machined from anodized aluminum so it's rock solid without adding much weight.
I can hear some of you hoping that all this isn't really necessary since you can just eyeball it, but in practice that isn't really as easy as you might think. In particular, if you're not shooting exactly at eye level (and you rarely really want to shoot at eye level unless you're photographing something the same height you are) then you must be leaning over to look through the viewfinder. And if your head is leaning to one side it's actually quite difficult to tell if things are level. No, this is one case where technology triumphs over man. Bubble levels may not be all that high tech, but they work more reliably than our built in sense of balance in this case.
Of course sometimes you might want things crooked. Sloping horizons are all the rage in advertising photography. They're supposed to make an image look more dynamic but have in fact become so commonplace that they're in danger of becoming clichéd. Still, if you do want a horizon that isn't level, there's no reason why you can't just ignore your bubble levels from time to time.
If you inadvertently do end up with a crooked image all is not lost of course. It just means there's one more think you'll have to take care of in post processing. And personally I'd rather do what I can to get it right in the field so I don't have to try to fix it later.