Strap on your vocabulary boots, folks. It might get ugly.
In the last homework assignment, I mentioned shooting in Auto…of course, this is where you allow the camera to decide how to expose the picture. In general, this will allow for accurate photos, if you don’t mind not being able to control your camera. But if you want to learn to take better pictures, you need to learn how to control your camera, and the effects these decisions will have on your pictures.
When we talk about exposure, there are three things that we have direct and immediate control over:
Shutter speed: how long the camera takes to make an exposure. Think of the camera as a door. How long does the door need to be open for you to walk through? This is a fairly straight forward number. It is the amount of time the shutter is open, such as 1/500 of a second, or 1/250. Or 1/125. Or three minutes.
Aperture: the size of the opening the exposure is made through. Think of the camera as a faucet. You could turn it on full blast, or turn it on just a trickle. One way or the other, you can still sill up a cup of water. Aperture numbers, also called f-stops, look like f/1.4, f/2.0, or f/5.6. They generally range from f/1.4 to f/22 or so. Some camera will shoot up to f/32 or even f/64, and some expensive lenses can shoot down to f/0.9. Lower numbers represent larger holes, and higher numbers are smaller holes (the f-stop number physically represents the number of aperture diameters that can fit between the film or sensor, and the lens focal node). If you look at the f-stop numbers, they are roughly the square root of 2 apart from each other, signifying that each f-stop corresponds to an aperture that is either double the area, or half the area, of the next f-stop. Double the area, double the light.
ISO (you could also call this “sensitivity”): The abbreviation ISO stands for International Standards Organization, so there is no photography knowledge there. But it means how responsive your camera is to light, and it is expressed with numbers such as 100, 200, 400, 800, and 1600. It used to be that 400 was considered high, and 1600 was really high. Digital technology has radically changed the ISOs at which we can shoot, and now ISO 3200 and above is not unreasonable. This opens up a whole new world of low-light photography, and this, more than anything else, is the legacy of the digital photography revolution.
Way back when, cameras worked in whole stops. A shutter speed could be 1/125 or 1/250, but nothing in between. ISO could be 400 or 800, and nothing in between. (Apertures have always been a continuous function, so it has always been possible to set “partial” stops with the aperture). Modern cameras, both film and digital, can now conveniently be set in-between stops, making for more accurate exposures.
Together, these three factors make up what is called the “exposure triangle”, and every picture you take balances the benefits and drawbacks of each.
A bit more on the term “stop.” A stop is the basic unit of exposure and, in all functional senses, it is a relative term meaning either double the light or half the light. Each corner of the exposure triangle can change the amount of light coming into the camera, and we can get the same exposure with different settings. For example, if we start with a shutter speed of 1/125, we can let in an additional stop of light—double the light—by leaving the shutter open twice as long, or 1/60 of a second (I am aware that is not exactly double…let’s not worry about that right now).
However, we could also cut the amount of light in half by changing the aperture from one size to half of the size, say from f/5.6 to f/8. Half of the hole, half of the light. You have now reduced the exposure by one stop.
So if we increase exposure with the shutter speed, but reduce it the same amount with the aperture, we are, in the end, still letting in the same amount of light. Although there will be some changes to the picture (more on that next time), the exposure will be the same.
Homework: turn your camera off of Auto, and start adjusting your shutter speed, aperture, and ISO manually. If you have not already, read your camera’s manual, or at least the parts about setting shutter speed, aperture, and ISO.
Go out and shoot.