Photography - How to make sure enough light enters:

Aperture and exposure 

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Think of a dark room, or a dark cinema you just walked out from.  

Your eyes first have to adjust to the light outside before you can see properly again.  

This is the aperture, like your eyes, which has an automatic aperture (the iris), they open and close to regulate the brightness of an image reaching the retina. 

A photo also requires a fair amount of light to record a good image, and unless your camera can be set to take darker or lighter pictures, the camera will restrict pictures to natural light.   

There are different factors that affect the amount of light reaching the film.  

1. The duration and diameter of the exposure

These 2 must be related to each other for the correct exposure for each negative, but the control of the variables is also just as important.  The aperture is calibrated in f-stops. 

 The f-stops follow a standard sequence: 1.2; 1.4;2;2.8;5.6;8;11;16;22 etc.  

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The bigger this number is, the smaller the aperture will be.  From one f-number to another it is called a stop and this represents halving or doubling the brightness of the image. 

The aperture control 

A ring on the barrel of the lens controls the aperture.  If the ring is turned, a circle opens or closes to vary the light passing through the lens.   

An example of overexposed, correct exposed and underexposed

Over exposed images

The picture above is an example of an overexposed image.  This means that too much light was allowed to reach the film during the time that the shutter was open. 

Over exposure (too much light entered)

Correct exposure

The photo above is an example of the correct exposure.  Not too much or too little light was allowed to reach the film.  The right amount of light entered when this photo was taken.  

Correct exposure (Just enough light entered)

Overexposed

When the aperture is smaller, like in this photo above, the photo will be underexposed.  This means that too little light reached the film, thus, the reason for the photo being so dark. 

Underexposed (Too little light entered)