MadSci Network: Other

Re: how are photographs made

Date: Tue Aug 8 17:01:09 2000
Posted By: Harry Adam, Research Associate, Research Division, Kodak Limited
Area of science: Other
ID: 965350884.Ot

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 
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 

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 :

Thanks for the question!

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