The concept of exposure, in relation to digital photography, can be tricky. There are three factors that contribute to what is commonly referred to as the Exposure Triangle. This post is just an introduction. I’m considering more in-depth posts for aperture, ISO, and shutter speed, if there is any interest. Leave a comment below and let me know if you found this post helpful and/or if you’d like to see more posts on digital photography. My examples and descriptions are specifically for digital photography, although the principals apply to film photography, as well.
In photography, exposure is defined as the amount of light that reaches a digital camera’s sensor or the frame of film inside a traditional camera. There are three things that can affect exposure: aperture, shutter speed, and ISO. In this post, I’ll briefly describe each one and how it affects your pictures. If you’d like to know more about the specifics, leave a comment below and I’ll go into more detail in later posts.
There is an excellent graphic depicting the Exposure Triangle on photographer Michael P. Young’s blog. I’ve posted it here, with his permission, for your viewing and learning pleasure.
The aperture is the opening through which the light passes on it’s way to the sensor. The aperture designation, measured in f-stops like f/2.8 or f/22, is inversely proportional to the opening size and the amount of light entering the camera. So, to put it more simply: the smaller the number (f/2.8), the wider the opening and the more light that reaches the sensor. A larger number, such as f/22, indicates a smaller opening and therefore less light reaching the sensor. Aperture also affects the depth of field. A large opening (small value) gives you a shallow depth of field and anything that is not the same distance from the camera as the focal point will appear blurry. A small opening (large value) will pull more of the background and foreground into focus.
Shutter speed is the amount of time that the shutter is open and the sensor is exposed to the light. Longer, slower shutter speeds let in more light than shorter, faster ones. Faster shutter speeds are good for freezing movement and action. These are great for photographing sports, or hummingbirds (one of my favorites). With slower shutter speeds, movement will cause blur. That’s not desirable when holding your camera by hand; I sure don’t know anyone that can hold a camera perfectly still for very long at all. But, if you have a tripod or can set your camera on a stationary surface, like a table or even the ground, you can use a slower shutter speed and still get a clear picture. This is handy for blurring water in a waterfall, or photographing in very low light conditions, like at night.
ISO is the sensitivity of the sensor to light. A smaller number, like 100, makes the sensor less sensitive to light, so only the brightest lights are captured and the image is crisp and clear. At a larger number, such as 3200 or 6400, the sensor is more sensitive to light, and will often pick up stray light, making your image look grainy.
All Together Now: Exposure
Aperture, shutter speed, and ISO work together to determine the exposure of your photo. When set to auto, your camera decides the best values for the three settings based on lighting conditions. But once you know what each means, and how changing it affects the image, you can tweak the settings to fit your specific purpose. Here is a real life subject that a friend of mine had problems photographing: the moon.
When you take a picture of the moon with your camera’s settings on auto, you end up with a solid black picture with one blurry white spot. Overall, there isn’t much light available at night, so your camera automatically increases the exposure. The moon, however, is very bright in the night sky, so that part of your image is overexposed. To compensate for the brightness of the moon, you need to reduce the camera’s exposure. You can do this by changing any combination of the three components of the exposure triangle. I do this simply by trial and error. First, I increase the shutter speed, thereby decreasing the time that the sensor is exposed to the light, and take a photo. If the resulting image is still too bright, I move the ISO down to 200 or 400, and try again. If the moon is still too bright, I can increase the aperture value, thereby decreasing the size of the opening and the amount of light that gets to the sensor. Keep in mind that when you decrease the exposure enough that you can see the moon clearly, the rest of your image will be very underexposed and show up as only blackness. Any trees or stars that may have appeared alongside your bright, blurry moon in the original photo will not be bright enough to show up when the moon is clear.I work very much by trial and error. I start with the camera on auto and look at the photo that results. Then, I can decide how I want to change the image from there: more/less motion blur, more/less of the image in focus, more/less grain, and more/less brightness. Then, I shoot it again. This is fine if you like experimenting and have the time. It can be bothersome if you are photographing something that is extremely short-lived, like two toddlers playing nicely together. Conditions like these are over very quickly, so you must make your best guess at the settings, or just go with auto.
Most of the time, shooting under normal lighting conditions, auto settings will work just fine. But hopefully, you’ve learned a few things here about the exposure triangle, and will know how to tweak the camera’s settings on those occasions when you want a more unique photograph. Another great site for learning about all topics photographic is Digital Photography School. It’s where I go when I have questions… 😉
Do Try This At Home
Take a picture of something moving pretty fast, like a car or train, with your camera set to auto. See the motion blur in the image? See if you can take a clearer image by increasing the shutter speed. (Remember: increasing shutter speed will decrease your exposure. Remember the two ways you can increase the exposure while keeping your faster shutter speed?) If you want to, post your before and after pictures on your blog, website, twitter, facebook, etc., and comment below with a link! I’d love to see how you guys do. Be sure to include any questions you have in your comment!!
Does this clear things up a bit? Do you still have questions? Be sure to ask, and I’ll respond to all comments. I ♥LOVE♥ feedback!!
Be sure to check back for follow-up posts. If there’s any interest, I will do three, one specifically on each aspect of exposure: aperture, shutter speed, and ISO.
Have fun (and don’t forget to take your camera!),