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C3 Manual: Setting the lens aperture
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The aperture setting determines the amount of light that comes through the lens. The smaller the aperture number (known as 'f-stops') on the dial, the more light comes through the lens. The aperture dial can be set in-between the marked numbers if necessary.

Over the years, models were made with slightly different ranges of f-stops. Your lens should have one of the following ranges:

3.5, 4.5, 6.3, 11, 18
3.5, 4.5, 6.3, 9, 12.7, 18
3.5, 4, 5.6, 8, 11, 16

If yours is marked 3.5, 4, 5, 6, 7, and 8+, you have a Matchmatic C3. The numbers on the lens correspond to the following f-stops:

3.5 = 3.5
4 = 4
5 = 5.6
6 = 8
7 = 11
8+ = 16

There's not much difference between 3.5 and 4 (or 4.5), but as you go up the scale above 4 or 4.5, each higher number lets in one-half as much light as the previous one.

The aperture setting also determines what range of distance will be in sharp focus in your picture -- known as 'depth of field'. At the smaller-numbered f-stops (3.5, 4), the range of sharp focus will be shorter -- objects in front of and behind the subject you focus on won't be as sharply focused as the subject. With the larger-numbered f-stops (11,16), objects in front of and behind the subject will be in sharper focus.