Establishing perspective from a reference photo

 How to find correct angles of perspective from a photo.

The first thing you always do is look for the horizon line and vanishing point. Here are some clues to look for:

(EYE LEVEL can be the level of the camera, not your eyes)

1. Find a spot where angles level off to a more horizontal line

2. Is there a person in the picture? The horizon line is possibly through that person's eye, if he is standing and is about your height.

3. Are there windows in the picture? You can guess-timate where someone's head might be in a ground floor window. If the windows are all in a row, even better!!! You can draw a line from the top of the window, then another at the bottom of the window, and find out where they converge.

4. If your view point is from an upper story window, look for any lines that would be parallel in reality, and find out where they converge.

5. What about cars or other vehicles that you can compare a person's height to? If you can see the top of the car, your eye level is above it. If not, it is below or at that level.

Here are some pictures to illustrate:

To find my eye level, I could find parallel lines, such as the windows on the buildings, the sidewalk and street edges. There is a person in a backpack, but his head is way higher than the level of other people's heads. He is either very tall or the street slopes downward a little, maybe both. But I would judge my eye level about on the x on the backpack, and the vanishing point to the left of the x.

Another one-point perspective. These are usually the easiest to find eye level and vanishing point.

You can see the tops of the windows are above my eye level; the bottoms, which slope upward to the center, are below my eye level. I could just follow the street lines until they converge. I would judge the horizon line to be just below the head of the person in the street.

Here's a perfect example of two points perspective, with vanishing lines on the left and right sides of the photo, off the page. I have some clues as to possible eye level. Look at the line through the center of the windows. They look fairly horizontal to me. To verify, I would extend the line of the base of the house to the left; then extend the angle of the roof to the left, until the two converged. That would be one vanishing point, and you can draw a horizontal line through that all the way to the right of the picture. 

Here is a bridge I frequently walk across. Just looking at the picture, one point perspective, it would seem my eye level is somewhere in the trees. Oops. Look again. The top bars of the bridge are nearly horizontal. That is my eye level. So you can tell this bridge is going uphill.

Same bridge, but looking downward. If I drew lines where the bridge floor converged, it would be about the level of the girl's knees. Plus, the top bar of the bridge is at her eye level, yet it slopes down instead of horizontal. 

The path is pretty level until it gets to the bridge. Since I walk it often, I know those posts, standing next to them, reach my face. The road takes a curve, too. My vanishing point is somewhere on the level of the posts, but to the right of the bridge. 

Several things to notice, though. One is that the road continues to narrow in the distance. And the road is darkest near the viewer and lighter in the distance.

Here is a picture with 3 perspective points, called OBLIQUE perspective. In one and 2 point perspective, all vertical lines in reality are still vertical. In this picture you have a vanishing point on the left (out of the picture), one on the right, and you also see these very tall buildings get smaller as they reach the sky. That is your third vanishing point.

This has no real linear perspective. All horizontal lines appears horizontal; all vertical lines appears vertical; there are no shapes with much volume. 

This we didn't have a chance to cover, but we will. Stairs. Always a conundrum.

There is an eye level. When looking at stairs, eye level is where you can no longer see the top of the step, only the side face. When drawing stairs, you need to establish 2 things: YOUR eye level, and the vanishing point of the stairs, which makes it a 3rd vanishing point.