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What does "exposure of the image" mean and how does it affect my digital camera's photos?

Adjusting Your Digital Camera's Exposure

Mark Comon (Vice President, Paul's Photo, Torrance, CA) gives expert video advice on: What does my digital camera's "manual exposure" mode allow me to do?; How do I change the shutter speed on my digital camera?; Which is worse to overexpose or underexpose pictures on my digital camera? and more...

What does "exposure of the image" mean and how does it affect my digital camera's photos?

In photography, the term "exposure" refers to how we control the lightness and the darkness of the image. The exposure is controlled by the camera's light meter. Whether you control the light meter yourself or you let the camera do the light metering for you, it all sets the F-stop and shutter speed. I know for a lot of you, those are scary words, but we always do F-stops and shutter speeds on every picture to get the light right or the lightness and darkness and exposure. A good way to think about exposure is just like steak. Some people like their steaks rare; some people like their steaks well done. Exposure or lightness and darkness in the picture is a combination of the F-stop, which is the size of the hole in the lens, and the shutter speed, which is the length of time that the shutters open. So, if you think about it, if we leave the shutter open longer, we're getting more light to the film or more light to the digital sensor, so the picture gets brighter, or lighter. If we shorten the exposure (give less light to the film or to the digital sensor), the exposure gets darker. Longer shutter speed: more exposure, more light; shorter shutter speed: less exposure, less light.

What is an "f-stop"?

"F-stop" means fraction and the f-stop number is the fraction of the actual opening in the lens compared to the focal length of the lens. So, let's say we have a lens with a focal length of 5 mm and we have an f-stop on the lens of 2. 5 divided by 2 is 2.5. The actual diameter where the light comes through the lens is 25 mm across. If we have an f-stop of 1, for example, on that 5 mm lens, 5 divided by 1 means there is a 5 mm hole that the light comes through. That's what the f-stop number actually means.

What does my digital camera's "manual exposure" mode allow me to do?

The manual mode on your digital camera is where the real photographers hang out. In the manual mode we set "F" stop and shutter speed. If you really want to control the light, the exposure, how the picture works you need to learn to use manual. It's not just for the propeller heads and the guys who still shoot film. Manual mode is still viable today even with digital because it's really how you control the look and feel of your picture.

Why would I want to change the aperture on my digital camera?

Why would you want to change the aperture your digital camera? Well, why would you want to change gears in a car? The aperture is really important to control the picture. The aperture lets in the light, and the light is the most important thing to your picture. Without light, we ain't got a picture. So we set the aperture to control both the light and the amount of focus or depth of field. You set a wide opening, like f2 or 2.8, blur the background and have your subject razor sharp. You shoot a medium aperture, 5.6 or 8 so the subject is sharp and background is slightly out of focus but still recognizable. We shoot f stop 11 or 16 or 22 for a landscape picture when you want the flowers in the foreground, the river, and the mountains all in focus.That's why we set the aperture. To me the aperture is far more important to great pictures than the shutter speed, because it controls the look and feel of the picture.That's why in my camera, 9 percent of the time I'm in f-stop priority, controlling that f-stop, because it's so important to the picture.

Why would I change the ISO on my digital camera?

You change the ISO on your digital camera to control the camera's sensitivity to light. In bright light we need the camera to be less sensitive, to give us a better quality picture. In low light where there's less ambient light, we need more sensitivity in the camera. So we raise the ISO from 1 to 2 to 4 or maybe 8 if you have to, to get enough light in so the picture looks good. Now, what's the payback? As you raise the ISO we get more noise or grain in the picture, and less color. So, you want to set the ISO as low as possible without having the ISO too low so we make blurry pictures.

Why would I change the shutter speed on my digital camera?

We change the shutter speed on our camera to affect the action-stopping ability. If I'm shooting a picture with my camera handheld, I need a shutter speed of 125 or 250. If I'm shooting moving subjects, I would change my shutter speed to a shutter speed of 500 or a thousandth of a second to stop the moving subjects. If I'm shooting pictures in low light, where I need more light to come in through the shutter, I'd set shutter speed to thirtieth or a fifteenth of a second. Now, when I set to a slow shutter speed like thirty or fifteen, action is going to blur. So, I use thirty or fifteen when there's low light or when I want the action to blur. Medium shutter speed: 125 or 250 for normal pictures. Fast shutter speed: 500 or 1000 for action. Thirtieth or a fifteenth of a second to blur action or under low light.

Which is worse to overexpose or underexpose pictures on my digital camera?

You want to get the exposure right on. With a digital camera, though, overexposure is death. If you overexpose your image, you have no data because everything is thrown into the white. So on a digital camera, you want to get exposure right down the middle if possible. But, if there has to be an error, you have to go toward underexposure because the dark areas can be salvaged somewhat in Photoshop. Light areas are lost and gone forever.

What is my digital camera's "program mode"?

The exposure modes on your camera allow us to control how we set the light. The basic mode is the “p” mode (program mode) and what I tell people is if in doubt “pm” the camera. Which means use the p-mode; fully program, the camera sets the f-stop and shutter speed in determining the exposure. The advantage of p-mode is you don't need to know anything. It's just a little bit above the green auto or “idiot proof” mode. In p-mode, the camera sets stop and shutter speed and you as the photographer have no control of the action stopping ability or depth of field of the picture you create.

What is my digital camera's "aperture priority" mode?

On your digital camera you have the choice of “A-mode” or aperture priority. In the aperture priority mode (it's a way we determine the exposure) you the photographer picks the aperture or f-stop. The camera will choose the shutter speed for you. Aperture priority is the automatic mode that I use most with my camera. So, I pick the f-stop, whether it's f2.8 to blur the background, f8 for medium focus, or f16 to make everything in focus. I shoot 9% of my automatic pictures in aperture priority, because that's how I think about the picture the best.

What is my digital camera's "shutter priority" mode?

On your camera, shutter priority can either be S or T mode depending on your camera. In shutter priority mode, you pick the shutter speed and the camera sets the F stop. The advantage of shutter speed is we set the number that's most convenient or most comfortable to us. Then the camera will pick the other number, the F stop. I don't particularly care for shutter priority because shutter priority is limited depending on the light. And you can actually make pictures in shutter priority that are too light or too dark. That's why I recommend as you get going in photography, to learn to use the aperture priority mode. You will be a lot happier and a better photographer for it.