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We are trying to find out how cameras work. We found information about why cameras need light,but wonder what is the difference between video film and rolled film. We also were wondering how the pictures get on the film roll, how much light is needed to take a picture,and what makes the film develop. Finally, our teacher says that digital cameras like one he has dont use film. How do we find out more about the differences????
Question Date: 1998-11-19
Answer 1:

You probably know that our eyes work by detecting light, and this is how we see. Cameras are devices for capturing images, and they also work by detecting light. An ordinary film camera uses a film which is coated with salts (usually things like silver nitrate or other silver containing compounds). When a photograph is taken, the salts which are exposed light undergo reaction, and this provides a memory of the picture taken. For black and white film you only need one type of salt, but for color pictures you need three (since there are three primary colors). When the film is developed, the devoloper uses special chemicals to turn the reacted salts into a negative, which contains a reversed version of your picture. The negative can be projected onto a special piece of paper which has other chemicals on it that react in the presence of the light, thus creating your final picture.

Electronic cameras and video cameras both use electronic devices instead of chemical reactions to detect light. These electronic devices store your pictures in electronic form. Video cameras store their images on a magnetic film. When you play the tape, your VCR turns the electronic recording on the magnetic film (the VCR cassette) back into an image on the television.

I don't know a lot about digital cameras, but I think that they ususally also store their images in magnetic form. I think that the magnetic storage device is a little cartridge rather than the cassette used for a video camera. You can ask your teacher more about his camera. Anyway, you can give the magnetic device to a company and they can either turn this into a photograph on a piece of paper or into one that you can view on a computer.

I have a somewhat complicated question for you to think about, so you might need to ask your teacher for some hints. Using what I've told you about how ordinary film is turned into a photograph, how video cameras work, and how digital cameras work, I'd like for you to think about how the film company turns a magneticly recorded image (from a digital camera) into a picture on a piece of paper. There are a couple of different ways to do this, so you may be able to come up with a few ideas.

For more information on digital cameras you might want to look at the kodak.com
web site. Especially:
Unfortunately, some of these pages get rather complicated, but I think that you should be able to learn some more at this site. Good Luck!

Answer 2:

There are two popular imaging systems in common use today -- chemical reaction-based cameras and digital image sensors. All cameras first have a lens system for forming images (even your eye has one...). The lens causes incoming light to form an image on a surface-- i.e. regions of bright and dark on a surface that is sensitive to light.
The fundamental difference between the types of camers you mentioned is how the surface is sensitive to light. (First, you might try to make an image with a positive lens -- even a pin-hole will make an image).
Conventional cameras use photo-sensitive film which undergoes a chemical reaction when light shines on it. -- This reaction makes it possible to later use other chemicals to create colored compounds where the light struck. This second process is calleddeveloping. You can experiment with photo-bleaching by placing some dark paper cut-outs on colored paper (different colors work differently) and setting them in the
direct sun for several days. The shaded regions will remain colored, but the regions exposed to light will usually bleach to lighter shades. (Purple and blue-violet colors usually bleach fastest).

The simplest process for film photography is the silver-halide (AgBr, AgI etc) system in which specially prepared crystals are set in a gelatine emulsion on a plastic base (this is the film). When exposed to light (particularly blue light) the crystals undergo a reaction which changes the chemical bonds of the silver in the crystal. Later, the developer chemical reacts specifically only with the silver which is in the different oxidation state -- the reaction makes pure silver in a finely divided form, so fine it appears black. Then another chemical removes all the other silver halide from the film and it is then dried. Areas which were originally light in the image have the largest amount of silver -- and are dark. This makes the negative image or the negative. -- Given a negative, can you think of how to make a positive image??

Digital cameras and cam-corders use a silicon sensor which stores the image light and dark areas as electric charge which can be read out electronically. This has several advantages - you don't need film or the time to develop and print the image -- you can store the picture electronically so it can be sent as email etc. You will notice that pictures from digital cameras haven't reached the quality of the film-based ones -- but that will come as the price continues to drop.

More information on cameras and image formation can be found:
http://www.kodak.com -- lots on digital imaging; look under education
http://home.sol.no/~gjon/pinhole.htm -- pinhole cameras
http://cse.ssl.berkeley.edu/sii/segway -- all about light and astronomy

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