Camera's and Lenses:
By William E. Steinman:
November 29, 2004:

A while back I wrote an essay about a new camera I purchased. It is a Canon EOS Digital Rebel® with a 6.3 MegaPixel resolution. As I said at the time, the real reason I bought this particular camera is, it is not a camera. It is a camera back. It comes without a lenses, but it can use any of Canon's EF series of interchangeable lenses. In the following discussion I will not specifically say camera back. It will be implied. When I purchased this camera, I also bought two lenses. One is a zoom/macro lense that gives me a range of 28 to 105 mm. The second is a 100 to 300 mm zoom lense. Together these two lenses give me a range from 28 to 300 mm. At the time, I thought that was all I would need. Since then I have learned a few basic things.

My first edification came as a result of trying to take a wide angle, panoramic shot. It was at our family reunion and was to be a photo of all of my family. If you ever saw the size of some of those buggers you would know it had to be a wide angle panoramic shot. So I set my lense to 28 mm wide angle. I even bought an infrared remote for the camera so I could be in the photo. Fortunately, my son also has a Canon 35 mm film camera. His camera responded to the same remote so we were both in all of the photos.

When I displayed what I had shot on my computer using Adobe Photoshop® I realized I had not captured the whole family. The subjects at the ends of the group were not there. Fortunately, the photos from my son's camera did have the whole scene. Then I began to wonder why? What did his camera have that mine did not. I soon found out.

Even though the Digital Rebel was not hyped as a 35 mm camera, I was thinking of it that way. Now I want to say very clearly that it is not a 35 mm camera. I have discovered it is really a 22.7 mm camera. My error was in not realizing the difference. The Digital Rebel does everything a 35 mm camera does except for the field size. The width and height of field are necessarily smaller. In order to understand what that means I had to give myself a primer on cameras and lenses. This was one of those situations that required some research.

My learning curve was unfortunately complicated because of the photographer's jargon. I have never encountered such convoluted jargon before. This is worse that the jargon of the medical establishment by an order of magnitude. It makes computer jargon seem like English. I see no good reason for it. A lense is one of the simplest devices ever developed by man. All a lense does is collect and focus light rays onto a surface. In addition, a camera is nothing more than a light proof box with a hole in it.

So, here is the skinny to the extent that I have been able to decipher it. These definitions are from Microsoft's Bookshelf 1996-97.

Lens from the Microsoft dictionary.
A ground or molded piece of glass, plastic, or other transparent material with opposite surfaces either or both of which are curved, by means of which light rays are refracted so that they converge or diverge to form an image.

Lens from the Microsoft encyclopedia.
A device for forming an image of an object by the refraction, or bending, of light. In its simplest form it is a disk of transparent substance, commonly glass or plastic, with its two surfaces curved or with one surface plane and the other curved. Generally each curved surface, called convex if curved outward and concave if curved inward, of a lens is made as a portion of a spherical surface; the center of the sphere is called the center of curvature (C) of the surface. All rays of light passing through a lens are refracted except those that pass directly through a point called the optical center. A divergent lens (thicker at the edges than at the center) bends parallel light rays passing through it away from each other. The image formed by a diverging lens is always erect (upright), smaller than the object, and virtual (located on the same side of the lens as the object). A convergent lens (thicker at the center than at the edges) bends parallel light rays toward one another; if they are parallel to the principal axis of the lens, they converge to a common point, or focus (F), behind the lens. The image formed by a converging lens depends on the position of the object relative to the lens's focal length (distance between the focus and the optical center) and its center of curvature.

focal length:
The distance from the surface of a lens or mirror to its focal point. Also called focal distance, focus.

focus
A point at which rays of light or other radiation converge or from which they appear to diverge, as after refraction or reflection in an optical system: the focus of a lens. Also called focal point. b. See focal length.

From Britannica 2003 Ultimate Reference Suite.
Camera Obscura:
Ancestor of the photographic camera. The Latin name means "dark chamber," and the earliest versions, dating to antiquity, consisted of small darkened rooms with light admitted through a single tiny hole. The result was that an inverted image of the outside scene was cast on the opposite wall, which was usually whitened. For centuries the technique was used for viewing eclipses of the Sun without endangering the eyes and, by the 16th century, as an aid to drawing; the subject was posed outside and the image reflected on a piece of drawing paper for the artist to trace. Portable versions were built, followed by smaller and even pocket models; the interior of the box was painted black and the image reflected by an angled mirror so that it could be viewed right side up. The introduction of a light-sensitive plate by J.-N. Niepce created photography.

Now, is that more that you really wanted to know? So, let's decipher it.

As we said, a camera is nothing more than a box with a hole in it such as the camera obscura defined above. It is still possible for a hobbyist to build this kind of camera. It requires only a light proof box of some kind with a pin hole in one end. The pin hole, if small enough, will act as a convex lens and project an inverted image on the opposite end of the box. If a piece of unexposed film were placed at the image end, the outside scene would be imposed on the film. Think about it and you will see that our super duper Canon EOS Digital Rebel is just the same box with some modern refinements. A sketch may be helpful here.

In our sketch, notice the light rays will cross each other at the focal point so the image projected onto the image plane will be inverted. We can notice that, but we need not worry about it. the inversion is resolved in developing the film or in storage of the digital image.

The most important thing about the image plane is the focus or sharpness of the image. How sharp it is depends on the lens characteristics and the distance of the lens from the subject we want to photograph. To adjust for that, the focus mechanism actually moves the lense toward or away from the image plan. Of course, the focal point must also move. In most modern cameras this can be done automatically. After all, we are not all Ansel Adams'.

Now I have also sketched the relative sizes of the 35 mm film of my son's camera versus the 22.7 mm digital receptor of my Digital Rebel. You can see immediately that I will get a photo of about 2/3 the size of a standard 35 mm. Therein is the whole difference and the primary reason I only got part of the scene. For sure, before you say it, I should have noticed in the viewfinder that I was not gong to get the whole thing. The viewfinder does work and It would not show me something that would not be in the photo. I simply screwed up!

It's really a catch 22 choice. I could move the camera back and get the whole shot. However, If I did that the remote would not work. Its range is about 16 feet. There is another solution. It's called a 20 mm lense. That is about as wide angle as I can get without going to fisheye. So, I did buy a 20 mm wide angle lense. With that I will get a photo about the same size as my son's camera with a 28 mm lense.

There is also another solution to the problem, but it is a very expensive solution. It is called the EOS-1DS Digital Camera. It has an 11.1 million pixel receptor. That gives it a full 36 X 24 mm image plane. It is made by Canon so it would use the same lenses I have. However, at a little over 7 grand, it is priced a bit out of my range. I'll just wait a while. I am sure the price will come down and I will already have the lenses. Here's to the future.