[genr2b]

How is it possible that a 35mm camera could work with its normal lens, but also with a telephoto lens of 100mm?

[CommDweeb]

This is a context problem. The meaning of many technical terms are intimately tied to the context they are used in. In this case 35mm refers to the size of the film you can put in a camera, where as the 100mm refers to the focal length of the lens. These are related, but not the same.

To understand this relationship you have to have a third thing, a subject. Lets say that we want to take a picture of a 2 meter man and we want his image to cover 2/3rds of the film. This means that we need an image of the man that is around 23mm high.

If you calculate this exactly, the math gets a little complicated and you tend to loose what is happening in the numbers, so lets just estimate how far the camera needs to be from the subject to get a 23mm image on film through a 100mm lense. If fl is the focal length, d is the distance from the camera to the subject, hi is the hight of the image and hs is the hight of the subject, then the relationship can be described as hi/fl = hs/d. In plain english the hight of the image divided by the focal length is equal to the hight of the subject divided by the distance. This only begins to work where distance is at least 10 times the focal length, or about 1 meter for a 100mm lense.

Re writing the above equation we get d = fl * hs / hi. This means that d = .1 * 2 / .023 which is about 8.7 meters. If change to a 50 mm lense this distance becomes 4.3 meters. If we use 70 mm film with a 100 mm lense, we have to double the size of the image on the film to get the same proportions, so we could either change to a 200 mm lense or halve the distance to 4.3 meters.

I hope this helps