Perspective Control in Images - Focal Length or Distance?
- A Tutorial -
This article is all about controlling perspective in images.
For the discussion in this article, the definition of perspective that we will use is "the size relationships of various elements in an image that are different distances from the camera".
Examples of why perspective control in images is important to us:
- A photograph showing a scene with a family standing in front of a mountain range is one case where perspective is important to the aesthetic quality of the image. If the mountains are so small in the image that they do not seem impressive, the image may not be appreciated as much as one with the mountains appearing larger behind the family.
- A portrait may have the subject's nose looking noticeably larger than it looks in real life (as compared to the subject's ear, for example) or the relative sizes of various people in a group photo may seem unrealistic.
The reality is that focal length has absolutely nothing, in itself, to do with perspective in images. Neither the apparent "compression" of distance between foreground and background elements of a scene captured with a long focal length lens nor the "distortion" of features at close range captured with a short focal length lens has anything to do with the focal length chosen to create the image. It is distance, and distance alone, that controls perspective, either in a scene viewed by a person or in an image made with a camera. The choice of focal length affects one thing and one thing only - framing of the subject in an image.
The focal length/perspective myth has its origins in the unconscious confusion of cause and effect common among those adept in any field of endeavor.
Experienced photographers who do portrait work, for example, often choose a lens of a particular focal length to use for a head-and-shoulders portrait and a lens of a different focal length for a full-body shot. That's because their experiences "tell" them what focal length they will need to use to frame the subject at the particular distance needed to provide decent perspective. They are not selecting the focal length to achieve the desired perspective - it's purely experience providing a shortcut in the thinking process for the photographer. In reality, it's the distance that the photographer chooses through habit born of experience to place between the camera and the subject that controls the perspective - whether or not the photographer actually realizes this fact.
85mm is an often recommended focal length for use on a so-called "full frame" camera (35mm film format - 24mm x 36mm) as a "portrait" lens . This is not because of any effect the 85mm focal length has on perspective, but because when used at the optimum distance for pleasing perspective, 85mm is the focal length required for framing the typical waist-up portrait when used on a "full-frame" camera.
These facts are often hard for photographers to understand, so we will illustrate them for you.
The following three photographs were made with my Canon 20D, with the camera in precisely the same position for all three images, roughly 13 feet from the subject (one of my wife's MANY teddy bears
). The background was approximately 7 feet behind the subject for all images. Image #1 was made with a 200mm focal length. Image #2 was made with a 70mm focal length. Image #3 was made with a 24mm focal length. All three images were shot with the aperture at f/8. The second and third images were cropped to represent the same framing of the teddy bear as in the first image. If you examine the three images closely, you will discover that the sizes of the objects in the background have not changed at all relative to the size of the teddy bear - even though there is nearly a 10:1 difference in focal length between image #1 and image #3.
>> #1 - Shot with 200mm focal length at 13 feet distance to bear - full image. <<
>> #2 - Shot with 70mm focal length at 13 feet distance to bear and cropped to match above. <<
>> #3 - Shot with 24mm focal length at 13 feet distance to bear and cropped to match above. <<
Now for the coup de gras. This final image, #4, was taken with the same 24mm focal length and the same f/8 aperture that were used for image #3, but the camera was moved close enough to the subject to keep the framing of the teddy bear similar to the first three images. In other words, I used the so-called "foot zooming" technique to frame the teddy bear. The distance from the subject to the background is unchanged from images #1-#3. Examine the background size, relative to the teddy bear's size. Also, note that the teddy bear's feet are larger as compared to his head than in the first three images. All of these perspective differences are due to one thing - the difference in the distance from the camera to the subject and background.
>> #4 - Shot with 24mm focal length at 2.5 feet distance to bear - full image. <<
Lesson 1: Hopefully you can see from this presentation that "foot zooming" is definitely NOT the way to create an image with controlled perspective. The illustrations here prove this. When I "foot zoomed" with the 24mm lens (to make image #4), there was a tremendous difference in the resulting perspective in the image due ONLY to the change in position of the camera. Of course, if you are shooting an image of a flat surface (such as a painting on a wall), "foot zooming" to frame the flat subject is going to provide roughly the same result as changing a focal length. However, for real-world three dimensional subjects, the knowlegeable photographer should totally forget the concept of "foot zooming".
Lesson 2: Some photographers will argue until they turn blue that they pick a focal length to control the perspective of their image. Indirectly, their logic is correct. However, their reasoning is flawed as we have proven here. The fact is that when a photographer chooses a longer focal length, the tendency is to move farther from the portrait subject to get the desired framing. The movement of their position is what changes the perspective, not the choice of focal length. Hopefully, if you have read this article carefully you will now understand this hard fact.
There is one other interesting element about the images, but we won't get into details of the topic in this article. The interesting element is depth-of-field. Note that images 1, 2, and 3, have a very different apparent depth-of-field. The background is more blurry in the first image than the second, and the background is more blurry in the second image than the third. This is simply because of the different focal lengths used. The distances, apertures, and camera format (the size of the film frame/digital sensor, which controls the "circle of confustion) are all identical.
You must realize, of course, that all three images are presented at significantly different magnification (relative to the 22.5mm x 15mm size of the in-camera images). This is because images #2 and #3 are radically cropped to keep the teddy bear at the same size. All four images were shot with the aperture set at f/8 because there was absolutely no change in the lighting for the four images.
THE BOTTOM LINE SUGGESTION....
Many inexperienced photographers choose focal lengths merely to be able to frame a subject from whatever camera position they feel is convenient at the moment. They probably don't even realize that there is a huge composition advantage in finding a better vantage point for the shot. The reasons are that distance between the viewer (or camera) and subject is what changes perspective and a different angle, combined with a perspective change can potentially make a huge difference in the quality of composition in a photo.
When I am trying to be completely "in control" of my images, I will - when possible - choose my camera position based on what it does to the perspective. Then, and only then, I will choose a focal length to fill the camera’s frame with the intended image.
Here's a simple example of how perspective control can work for you:
Let's assume that you are taking a photo of some friends in a scene that has mountains in the background. You stand 20 feet from the people and view the scene. A 50mm lens will let you fill the frame with the group of people and some of the background quite nicely, so you take a shot. Then you realize that the mountains are rather small in the background.
Back up to to 40 feet (twice the distance) from the group of people and view the scene, you will see that the mountains are now larger relative to the people - twice the size they were before, in fact. However, the people are smaller in your viewfinder. You now need a 100mm lens to keep the people the same size as in the first image, but the mountains now appear twice the size that they were in the first shot.
Why is this? It's because the additional twenty feet that you put between yourself and the people is insignificant relative to the fifteen miles between your viewing spot and the mountains.
Acknowledgements: The concept of doing this article was an idea by "Perry Ge". The text of this article was written with the help of editing and ideas by "xarqi" and "Wilt". "Wilt" has added his sections below.
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