Equivalent Exposures Getting the correct exposure requires selecting a shutter speed and f-stop combination that lets the correct amount of light reach the film. We control the amount of light reaching the film by adjusting the size of the lens opening, and the length of time the shutter stays open. For any given amount of light, there are several combinations of the two that will allow the right amount of light to reach the film. If we make the lens opening bigger, we need to reduce the time the shutter is open. If we leave the shutter open longer, we need to make the lens opening smaller. A common analogy for getting the correct exposure is to think of each different ASA speed needing a certain number of ‘gallons’ of light to get the correct exposure. Think of the film as a bucket that needs to be filled. The size of the bucket, that is, the number of ‘gallons,’ is determined by the ASA. ‘Fast’ films, those with large ASA numbers, need less light than ‘slow’ films to get the correct exposure. Doubling the ASA means that the film needs half as much light to get the correct exposure. i.e., ASA 100 film needs half as many ‘gallons’ of light than ASA 50 film.   When we fill a bucket, we’ve got two ways to control the amount of water going into the bucket. We can change how far we open the faucet, or we can open the faucet a set amount and change how long we let the water run. If you open the faucet to let just a trickle of water out, it takes a long time to fill the bucket. If you open the faucet wide, it takes less time to fill the bucket. On your camera, the lens f-stop is the same as how far you open the faucet, and the shutter speed is how long you let the faucet run. There are many combinations that give you the same amount of light. Shutter speeds are expressed in fractions of a second. 35 mm cameras usually have speeds from about 1 second to 1/1000 or 1/2000 of a second. On your shutter speed dial, they’re likely to be marked 1, 2, 4, 8, 15, 30, 60, 125, 250, 500, 1000 and 2000. Since they’re fractions, big numbers mean small times. Lens openings, usually called ‘aperture’, are found by dividing the focal length of the lens by the aperture. We talk about “whole stops” (aka “full stops” or just “f-stops”). Opening the lens a whole stop doubles the amount of light that reaches the film. Closing the lens a whole stop halves the light reaching the film. Whole stops end up being funny numbers, i.e., 1.0, 1.4, 2.0, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45 and 64. Your lens probably doesn’t have all of those numbers. We often define our lenses by the largest f-stop. Commonly, the largest opening on our ‘standard’ 50 mm lenses is 1.4, 1.7 or 1.8 (lenses don’t have to be built to match whole stops). Commonly, zoom lenses have smaller largest openings, often f 3.5, 4.0, 4.5 or even 5.6. Most 35 mm lenses have a smallest lens opening of either f-16 or f-22. OK, doubling the time lets in twice as much light, halving the time cuts the amount of light in half. Opening the lens one stop doubles the amount of light, stopping down one stop cuts the light in half. So, if we find the correct exposure with ASA 100 film is 1/125th second at f-16 in full sunlight. We know that we can adjust the camera settings to any of these to get the same amount of light: Shutter speed  Aperture 1/30th f32 1/60th f22 1/125th f16 1/250th f11 1/500th f8 1/1000th f5.6 1/2000th f4 1/4000th f2.8   Let’s look at the same idea when the correct exposure is 1/60th at f-8. These settings all let the same amount of light reach the film:                   Shutter speed  Aperture 1/4th f32 1/8th f22 1/15th f16 1/30th f11 1/60th f8 1/125th f5.6 1/250th f4 1/500th f2.8                                                        Understanding this relationship between shutter speed and f-stops is important to using different shutter speeds and f-stops to your advantage.