Perspective comes in different forms, the most obvious & basic of which is the use of the word in relation to the appearance of relative orientation in a three dimensional space. At first, it may seem irrelevant to photomanipulators, (because, hey, the photographs already are perspectively sound,) but putting more than one photograph in context requires that you learn to see what makes sense and what doesn't.
The Technical Stuff
Perspective (from Latin perspicere, to see through) in the graphic arts, such as drawing, is an approximate representation, on a flat surface (such as paper), of an image as it is seen by the eye. The two most characteristic features of perspective are that objects are drawn:
- Smaller as their distance from the observer increases
- Foreshortened: the size of an object's dimensions along the line of
sight are relatively shorter than dimensions across the line of sight
These two are the most basic things that you can do to make a setting appear three-dimensional. I purposely say "a setting" because when using photomanipulation as your chosen medium, you usually don't need to worry much about making things appear three dimensional by themselves: the photograph does that for you. But it is still important to know how to recognize each element's perspective so you can consequently integrate a number of photographs and have the perspectives match! I cannot stress enough how important this part is and while it may usually be something that you do on gut-instinct (it just looks wrong if not done properly), a more analytical approach can help you in tricky situations.
How to see perspective in an image
1) The Horizon
The first thing you need to remember is that every picture has a horizon line. And that horizon line is completely horizontal. Even if you may not be able to see it because it's obscured by tress or mountains or the shot is simply aimed too high or low to have it in view.
You can easily see the horizon in the first and last images. In the last picture, the fog does obscure the horizon, but you can still guess
easily enough where it should be. In the forest image, you can guess it, too, even though that is a bit trickier as there is no sky to help you out. The horizon line in this case is in the upper third of the image, probably right behind the last line of trees or a bit further up even. With the mountain range, it's near impossible to tell where the horizon should be as you have no indication of how high up the photographer was standing in relation to those mountains. My guess would be the horizon is in the top third, but without foreground it's hard to tell beyond a gut feeling.
2a) The Vanishing Point
On that horizon line I mentioned above, there is something called a Vanishing Point. In its most basic form, it comes alone and all real-life-parallel lines will lead to it - this is called a one point perspective
The railroad tracks are a prime example: all lines on the ground seem to lead to one imaginary point on the horizon. And we know from experience railroad tracks don't get thinner (not ever or it would defy their use!) and yet this looks completely natural and normal. The same goes for the row of trees: we know all those trees are the same height (they're artificially planted and, well, that's just what park officials like to do: keep them all the same so it'll look nice) and yet, if you draw a line through their bases right where they leave the ground and where the first branches appear both will lead to that one imaginary point on the left of the horizon.
The next step is a two point perspective
, which uses two vanishing points. In the case of two vanishing points, you are able to find parallel lines (well in real life they are parallel) that will point to two different vanishing points. Those lines are at a right angle
in real life, for example the base of the columns in the picture below: one set of parallel lines (red) can be seen vanishing to the right while the other side will have a vanishing point to the left (cyan). Both points should fall on the horizon line
(shown in green/yellow) and thus give us a means to find out where that horizon line is by "connecting the dots" - and all without seeing any actual horizon at all. Btw, this holds also true for the upper parts of the columns, above the horizon.
If you look really closely, you can even see a slight three point perspective
in the columns: The lines that should go from the floor to the ceiling along the columns (like the one vertical line shown above) will in a 2 point perspective all be at a right angle to the horizon - just like that one line shown above. But if you look closely, you can see that the columns further back seem to be "falling" slightly toward that center line. If you were to draw lines like we did on the floor from floor to ceiling, the third vanishing point would be where those meet - and they would meet somewhere very far above the picture but on that vertical line - because that line is actually at a right angle with the horizon.
You can apparently (and I hadn't even known that for a fact prior to some research I did for this article) have even more than three vanishing points
(where those lines start to curve and do strange things, but it looks pretty cool if you're out for a wow effect), but let's just stick with the ones above for the sake of simplicity. If you need some more explanation on vanishing points, I suggest you visit GriswaldTerrastone's gallery - there are tons of examples there on how to draw perspectives with vanishing points
that I'm sure will help with the understanding (the following is just a very small excerpt!):
2b) The further away = the smaller
As you can see in the photographs above, anything that is further away from you will get smaller than it would be if you were standing right in front of it until at the very horizon, things just become a tiny dot. Like in these examples:
So? What do I do with this?
Once you know how perspective "works", you'll find it much easier to compose your manips. And once you can find out (by using vanishing points and lines or by instinct, whichever works for you) where that horizon is supposed to be, you can actively place your stock objects in the right heights/distances and scale them accordingly. To that end, please take a look at the following tutorial by kiolia Placing figures in perspective
I hope this helps illustrate the relevance of knowing what you're doing
Time to see if you can spot perspective and the occasional play with big vs. small things -- and beyond! For example, what happens if we use an unusal perspective? Say from above or from very close to the ground and up?
But yes, of course
To get a feeling for perspective, I would strongly suggest not only looking at your backgrounds in that regard, but also looking at objects/people you want to place in your scene more closely. Ask yourself: do the perspectives match? Look at the background on the object/people stock and try to figure out if the horizon lines would match with your chosen background (remember that tutorial about matching perspectives earlier). To that end, I can only advise you to not use pre-cut stock until you get a feeling for the perspectives that are involved. Especially with people stock and objects that are not rectangular it can be very hard to tell what kind of perspective (and lighting situation as well, but more on that at a later installment!) it was shot with.