Slide 7 of 17

NOTES

SHUTTER speed numbers like 1000 and 500 are all a fraction of a second of time for how long the shutter will let in light. These numbers are 1/1000 or 1/500. Looking at this as a FRACTION we see that the larger the number the less time the shutter will open to let in light. Our rule in photography is to not allow this number to go below the FOCAL LENGTH of the lens. For a normal camera this is 50mm so our minimal shutter speed is 60 or 1/60th second. For a wide angle lens of 28mm it can go to 1/30th second or 30. For a telephoto lens of 200mm length the speed moves up to a faster 250 or 1/250th of a second. The shutter speed numbers each double the amount of light they let in as we move down the dial. 1000 - 500 - 250 - 125 - 60 - 30 and so on, each lets in double the amount of light. We will see why this is important when we look at the aperture.

APERTURE numbers or f-stops are also a fraction. Thus the number f 16 is really 1/16th and f 2 is really 1/2. What are they a fraction of? If you take a normal lens like a 50mm lens the size of the opening in the lens that lets in the light is compared to this number. Thus f2 represents a lens opening that is 25mm wide. As we change the focal length to 200mm an aperture of f2 now represents an opening of 100mm in size. Big difference -- yes and no. The larger the lens the greater the amount of light we will need to get an equivalent amount of light hitting the film. This is more technical than we want to cover for now - so - let's just remember that this opening is a FRACTION and that the smaller the number the MORE light we will have. Each aperture or f-stop also lets in double amount of light of the next one on the setting chain, just like the shutter speed dial. So as we look at the numbers we see these: 16 - 11 - 8 - 5.6 - 4 - 2.8 - 2 which as it turns out allows for a doubling of the amount of light in each one. They do not look like they do in the math because we are talking AREA in the size of the circle or HOLE that lets in the light. Anyway....they KEY is that each is half or double its neighbor just like the shutter speed numbers.

EQUIVALENT EXPOSURE

If we have a normal light meter setting of 500 at f11 for a subject and we want to open the shutter up for a LONGER time to allow for blur and thus we set the shutter to 250 we now must adjust the aperture to KEEP THE LIGHT THE SAME as it falls on the film. The shutter was set to let in MORE light so the aperture must be set to let in LESS light to keep the amount equal. So, we move the aperture to f16 and now we have an EQUIVALENT exposure. If we move the shutter to 125 the aperture would move to f22 which is about as small as most cameras will go. All of this works because each setting of the aperture AND the shutter are each half or double the one next to it.