Lenses and Perspective

Photography and Perspective

As a photographer you should realize that perspective is not a function of lenses. The “wide angle perspective” we talk about as photographers is an effect that is simply an exaggeration of the relationship of a foreground object to the background or other parts of the image.

Perspective is simple. It is the relationship of foreground to background objects based on the position of the photographer relative to the scene. If you want to change the perspective, you have to physically move in relation to the subject.

Lenses simply allow us to include (wide angle) or exclude (telephoto) parts of the scene in front of us. The misconceptions of distortion come from the peculiar “effect” that we see in the two dimensional representation of the subject if it is different than what we normally perceive. Our eyes are essentially fixed focal length lenses. What we consider “normal” is simply based on the way we perceive reality. Get close to a subject with a wide angle lens and you create a distorted visual relationship, but not a change in perspective.

 

Architectural photographers do this with buildings as the spacial relationship creates dramatic receding lines. If the subject is far from the camera as in a landscape, the effect may not be noticed. Using an object close to the lens in a landscape image can be used to dramatic effect as the near to far relationship helps the three dimensional feeling.

Ansel Adams illustrated this in “The Camera” (Previously “Camera and Lens”) photographing a fountain with a building in the distance. The relationship between the two is not changed with a change in focal length which is essentially cropping the image at the capture stage. To change the perspective you need to physically change your position in space relative to the closer object. The difference in perspective then appears as a change in the relationship to the background regardless of the focal length used.

This is pretty easy to learn. Choose two objects with a distance between them and make some photographs with different lenses or both ends of a zoom. Do the same at a closer distance to the foreground subject. With changes in focal length the relationship of the foreground subject and the background will remain the same. When you change your position in space relative to the foreground the perspective of the images will change compared to the first position. The foreground to background relationship in each set of images will remain the same regardless of the focal length.

To further illustrate the point, crop your wide angle lens image to match the amount of the scene captured with the longer focal length from the same position and you will see that the final image is identical. The lens focal length is simply including or excluding a portion of the scene. It does nothing to change the perspective.

In practice this becomes important as the first thing you should do when deciding how to photograph a subject is to determine the perspective or “point of view” you want or need based on the foreground to background relationship of the subjects in the image. Then the focal length of the lens is simply how much of the scene to include in the capture.

Diffraction Limit Sharpness vs Depth of FIeld

There is a limit to the resolution of any optical system. That limit is called diffraction and I first ran into the term with astronomical telescopes. In photography, according to optical theory, any lens devoid of aberrations should perform best wide open. In the real world our lenses generally perform best (sharpest) stopped down slightly from wide open, performing best in the range of f/4 to f/8.

In film days we typically closed down our lenses to achieve added depth of field. The reduction of the circle of confusion created by the out of focus image on either side of the point of focus makes the image appear sharper over a greater area. This is still true, providing we do not exceed the diffraction limitation of the lens. The effect of diffraction limitation has a more obvious effect in digital capture than it did in film for reasons I will not pretend to fully understand. The result is an image that loses critical sharpness even at the point of focus. This is because the disc resolved by the lens exceeds the size of the capturing pixel resulting in loss of resolution. That means that optimum sharpness and depth of field are a trade off after you reach the limiting aperture.

The pixel size on a DSLR is a result of the capture resolution, and the size of the pixels related to the lens determines the effective diffraction limitation, not the lens itself. Therefore, the limits change as you put a lens on different camera bodies with differing sensor resolutions. The diffraction limitation of a particular lens of mine is nearly the same on a 10Mp APS-C body as it is on a full frame 22Mp body because of the difference in the size of the pixels themselves.

While it may be helpful to know what your limitations are for a particular camera there are many other factors that will have a greater destructive effect on your images. All optical maximums depend on solid support, shutter speed, ISO noise, and other factors even including the characteristics imparted by the raw converter used to process the file.

For further information I recommend visiting http://www.cambridgeincolour.com/ and searching for diffraction limitation. There is more information there than you probably really want, but the interactive charts can be useful in ballparking the optimum apertures for your camera.