|MadSci Network: Other|
Hello Haley - this is a question that has more answers today than it did ten years ago! Photographs then were exclusively made using film whereas today we also have digital cameras. I'm going to assume that you mean film cameras in your question - but you might want to ask it all over again, for digital ones… The most important part of a camera is its lens. This is designed to focus the image you want to capture onto film. Given that the image may change quickly - things move, the light changes and so on, the camera also needs to have a shutter. As the name implies (think of a window shutter) this hides the film until we want to expose it to the focused image through the lens. The shutter allows exposure of the film to be very short - fractions of a second. So you look through the viewfinder, compose your picture and press the button. Click! - the shutter opens and closes very quickly. You then wind on and you're ready for the next one. So what has happened to the piece of film that was exposed? Well - the light has caused some minute, invisible changes to happen. Tiny crystals of silver salts within the layers of the film are made to be incredibly sensitive to tiny amounts of light. This sensitivity is such that it takes only a few photons (what scientists call "particles" of light) to cause an effect. The effect is to produce a few atoms of metallic silver on the crystals that were exposed. Those that were not exposed - still have no atoms of silver. We can't see the difference, so we need some method of distinguishing between exposed and unexposed crystals. This is called processing. The first part of processing is the developer and it is this bit that does the distinguishing. It is called the developer because it develops up only the crystals which have tiny amounts of silver on them so that they become completely converted to silver. The unexposed crystals remain unchanged. Now we can see the difference. The developer has made the difference much much bigger - in fact between a hundred and a thousand million times bigger! In traditional black and white photography we use the developed silver as the image - it is black - not silvery - because it is lots and lots of tiny clumps of silver - not a shiny sheet. Before we have a good, lasting image, though, it is important to remove the unexposed crystals. This part of the process is called fixing. A chemical bath dissolves the crystals without affecting the developed silver. We then wash the film and dry it - now processing is complete and we have our negatives. Negatives? Why negatives? Well the light (which is the opposite of dark) has created silver - which is black - or dark. So the picture is the wrong way round - what was light in the scene is now dark; what was dark - is now light. To make it come out the right way, we shine light through the negative onto another film (usually coated on paper) and process just the same. This inverts things again - making it come back the right way round. Colour photographs are just a bit ore complicated. We still use the silver salt crystals, and still develop them up to silver. In this case, though, we use the developer chemical when it has reacted (forming the silver image) to also for a dye image. Developer that has reacted will do this, whereas developer that hasn't - won't. Processing not only removes the unexposed silver salt crystals, but also the silver image - leaving behind only the dye image. This is a lot to take in already - so I'm not going to go into how the dye image gives us back all the colours we had in the original scene - that can be the topic of more questions, but you can probably already find the answers in the Mad Sci archives. Here's some of links to similar questions I have answered before : http://www.madsci.org/posts/archives/feb2000/950540434.Ph.r.html http://www.madsci.org/posts/archives/feb99/920252721.Ch.r.html http://www.madsci.org/posts/archives/nov99/941422207.Ch.r.html Thanks for the question!
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