Photography

[MargeB]

What do the f numbers represent in photography and how do you use them?

[Didwot]

You will probably get more help if you post the question under "how it works".
But...
In photography the f-number (focal ratio) expresses the diameter of the diaphragm aperture in terms of the effective focal length of the lens. For example, f/16 represents a diaphragm aperture diameter that is one-sixteenth of the focal length (or the focal length is 16 times the aperture).
The greater the f-number, the less light per unit area reaches the focal plane of the camera film.

[KebabMeister]

In basic terms if you used f1.7 or f2.8 then this would be ideal for portrait photos since you'd be focused on the subject BUT the background will be out of focus.

If you used f16, for e.g., then the background will be in focus.

It all depends on how close the subject is and how far away the background is.

A good photography website should explain topics such as aperture, shutter speeds and depth of field.

[Tim Baxter]

Page 1.

The f number tells you how wide the aperture of your camera's iris is.
The wider the aperture the more light gets on to the film.
There are light sensitive chemicals on the film. They need a certain amount of light to react to and this enables them to 'store' the image. So, in dull conditions possibly not enough light will hit the film and the iris (aperture) needs to be opened so that enough light hits the film to make the chemical react and store the image.
One way round opening up the aperture is to use a 'faster' film, ie. it takes less light to make the chemical react so a small aperture can be used which will let in less light but because the film is faster it will still store the image.
The best cameras are those with a very small aperture (look up 'camera obscura1 on Goooogle). This means that they will let in very little light (very small T number)onto the film which in turn means that there is less de-focussing of the image. In a perfect (non artistic) world there would be a perfectly focused image of everything within the camera shot. This means that everything from your thumb in front of the camera to the peak of mount Everest would be in focus. However, because you need to let in a certain amount of light the image will always have parts of the subject out of focus because too much light has been let in.

[Tim Baxter]

Page 2

Now we come onto what the previous posts have said. There is a trade off between your desire for focus and how much light you let into the camera and onto the film. For your average snapper (like me) all I care about is capturing 'the moment' but professionals will 'create' a sense of atmosphere by deliberately using the camera settings to de-focus certain parts of the picture. They can do this by opening up the iris (increasing the f number) and letting more light in to de-focus the image.
So we come on to the 'depth of field' as previously referred to. The wider you aperture (bigger the f stop) the more light you let in and the de-focusing increases. This is referred to as the depth of field, e.g the distance from your camera which remains in focus even though more light has been let into the camera. If you focus you camera on an object in good light approx 20' from your camera with a low f setting then it is probable that everything in front and behind the object will be in focus. If you open the aperture to let more light in then defocusing will occur and it is probable that only the object at 20' will be in focus.
Having discussed the aperture you need also to consider the exposure time. This again a trade off against the aperture because you are looking for a certain amount of light to be admitted to the camera to affect the chemicals which store the image. It's quite simple, if you let in more light by opening the aperture then all you need to do to compensate is decrease the exposure time (shutter speed).
The advantage now is that a faster shutter speed (lower exposure time) will capture action shots better because the moving image has not been photographed on a long enough exposure for its motion to 'blur' the image.
So there you have it, the 3 trade offs, Shutter speed, aperture size and film speed. Good hunting/shooting.

[landie]

Basically a film needs a certain amount of light to expose it correctly. This is achieved via a combination of aperture size (f numbers) and shutter speed. The lower the f number the wider the aperture and the faster the speed of the shutter.

The effect of wide apertures is reduced depth of field (the amount of the picture in focus, which can be both a good and a bad thing. Narrow apertures give good depth of field (brings everything into focus) but is no good for action shots as the shutter will be open a long time and the shot will be blurred.

It's a question of horses for courses and juggling light against speed, one stop on the aperture ring which is in effect a doubling (or halving depending which way you go) of the amount of light entering the lens, will cost one step on the shutter speed dial to maintain correct exposure.

Don't forget to take account of the film speed in your camera especially after changing films to a different speed rating.

[NetSquirrel]

Probably not a lot of help to you after such informative answers, but I took a couple of pictures so you can see the effect of different aperture (f) values on depth of field.

 

1. Widest aperture (f/5.6) and shortest exposure time (1/10 of a second)

 

2. Medium aperture (f/14) and medium exposure time (1/2s)

 

3. Narrowest aperture (f/36) and longest exposure time (5s)

 

If the coins or the camera were moving in these shots, the 5 second exposure would obviously show the most motion blur.

[NetSquirrel]

That middle one should have been http://img230.echo.cx/img230/5926/depthoffield26tq.jpg

[Didwot]

So in plain English it is like this.
Small aperture, less light gets in, and the longer the exposure time, BUT the greater depth of focus.
Big Aperture, more light gets in and the shorter the exposure time, but a small depth of focus.

[Tim Baxter]

Ahhhh Didwot, if only you'd said that in the first place.....

[MargeB]

Terrific answers, thanks very much.