Back to basics #4
Using shutter speed and aperture together
By Rod Barbee
Actually, the title should probably be “How shutter speed and aperture affect each other when making an exposure”. But that’s too long.
When you make an exposure, you set an aperture and a shutter speed. This puts a certain amount of light on the film. But did you know that you can get that same amount of light on the film with different settings?
Let’s say you take a shot with an exposure setting of 1/250 at f/8. Next, you take another shot using a shutter speed of 1/125 while keeping the aperture at f/8. What happens? Well, you let in more light with the slower shutter speed, making the exposure lighter than the previous shot. One stop lighter to be exact. For the next shot, you keep the shutter speed at 1/125 but you change your aperture to f/11. This lets in one less stop of light. Now the exposure on film is back to where you started. So an exposure of 1/250 at f/8 looks the same on film as an exposure of 1/125 at f/11.
What does this tell you?
There is a reciprocal relationship between the shutter speed and the aperture. To maintain the same exposure value, if you change one you’ll need to change the other in the opposite direction. If you add light with the shutter, you need to take away light with the aperture. In photography, we call this reciprocity.
What this all comes down to is that you have choices and options. Say you’re shooting a landscape and you determine an initial exposure of f/8 at 1/30 second. You realize you need more depth of field. You simply change the aperture to what you need and change the shutter speed a corresponding amount. If you need f/22, what will the shutter speed be?
f/22 lets in three stops less light than f/8 so you need three more stops of light with the shutter. That would be 1/4 second.
Of course, you’ll need to be on a tripod for this. But that’s for another discussion.
For now, just remember that for any given lighting situation you have many choices that will allow the same amount of light to hit the film.
Play around with your camera, set a base exposure and changing either your aperture or shutter speed and then change the other control so that you bring your meter back to zero. Do this enough times so that you’re comfortable with the concept of reciprocity.
In the next few Back to Basics columns, I’m going to start getting into the meat of determining “correct” exposure. Up until now, you’ve been learning some of the basic concepts that will help you understand the overall picture of exposure. (I know, I know. It’s a bad pun.)
So stay tuned and keep playing and practicing with your camera. The better you know your equipment, the sooner you’ll be making prize-winning images.
© Rod Barbee. All Rights Reserved