let’s talk about exposure

Ok, so here we go. For the first part in this series, I have decided to discuss exposure (not the indecent kind).

Exposure is kind of the corner stone of photography. I mean, that’s what photography is– exposing film (or a light sensitive chip) to light. There are three variables that determine the exposure of an image (not counting post-processing). They are ISO, Shutter Speed and Aperture. The “exposure triangle” is a useful way that I have seen it explained before. I drew a little diagram with a marker of the exposure triangle. Please pardon my terrible penmanship, but I thought it was nicer than stealing someone else’s graphic, and faster than fumbling with Illustrator (not my forté)–

All three of these settings can usually be controlled manually on most modern digital cameras, even inexpensive ones. This allows the photographer to take total control of the way that the image is exposed. There are several sought after effects that can be achieved by manually dictating the way that your camera exposes the image. First, the very basics–

ISO

Historically, ISO was a number that was used to measure the sensitivity of the emulsion on film. The lower the number, the slower that the emulsion reacted to light, making it take longer for the film to expose. This same idea translates to digital cameras. The ISO setting on a digital camera controls the sensitivity of the chip. The settings generally range from ISOs 100 to 3200 (or more), depending on what kind of camera you are using. The lower the number, the less sensitive the chip will be, and the longer it will take to achieve desired exposure. Conversely, when using a high ISO  number the chip will be very sensitive.

There is another factor that should be taken into account when selecting ISO. The lower the number, the richer and creamier (term of art, hahaha) the image will be. The higher the ISO, the grainier the image will become. The graininess that occurs at high ISO settings is commonly referred to as “noise.” Some times images are shot to purposely include noise for aesthetic effect (our friend Kat in Charleston, last summer) . Most of the time it is not desirable, because it can muddle the sharpness of the image.

As a rule of thumb, I try to keep the ISO at 400 or lower. But depending on lighting, that’s not always possible.

Shutter Speed

Shutter speed is pretty obvious, right? It is the setting that determines how long the shutter is open, allowing the film or chip to be exposed to the light. Fast shutter speeds are great for freezing time (fashion week fall 2010), like when shooting sports or if you are shooting outside on a really bright day. Slow shutter speeds are cool for showing motion (a great example, shot by my good friend Zach Tolbert), or for when shooting in low light. Shutter speed settings can range anywhere from 30″ to 1/2500″ (or more). In layman’s terms, those numbers translate to thirty seconds (a super slow speed, for shooting at night or something), to one twenty-five hundredth of a second.

About 90 percent of the time, I find myself shooting between 1/80″ and 1/640″. Many times, this setting will show up on the camera menu display as a single number, not a fraction. For example 1/80″ will just be 80. Higher end cameras will also generally have a “bulb” setting. Bulb will keep the shutter open as long as the shutter release button is pressed down (or press once to open, press again to close). The “bulb” setting is what people use to get those crazy long exposures of stuff like the stars swirling in the sky, etc..

Something to keep in mind– To obtain sharp images, it is best to keep the camera perfectly still while the shutter is open. So, shutter speeds slower than 1/80th of a second while shooting hand-held (not on a tripod) might run the risk of blurriness caused by camera shake, especially when large amounts of caffeine have been ingested.

Aperture

The aperture is the diaphragm behind the lens, that can be adjusted to control the flow of light coming through the lens. The size of the opening is measured in “f-stops” and is generally expressed as a fraction, because… that is what it is. An example of how f-stops are written is “f/4” or “f:4”. The “f” part of the fraction is referring to the focal length of the lens (we will get more into that later). So for example, if I’m using a 50mm lens, then “f” is 50. If I’m using a 50mm lens set to f/4, then I know that the aperture size is 1/4 of 50 millimeters.

Ok, if all of that sounds a little too much like math class, then just remember that the lower the f-stop, the larger the aperture is, and the more light you are letting through. So f/1.4 is letting in a lot of light. F/22 is only letting in a little pinhole of light. As with shutter speed, the f-stop may appear as a single number (not a fraction) on the camera menu. So, f/4 might just show up as “4” or “4.0.” A helpful way to think of it might be to remember that the higher the f-stop number the more light you are stop-ping from getting to the sensor.

One important thing to keep in mind when choosing the appropriate f-stop, is that the aperture size also affects the depth of field. Depth of field is what determines how much of your image will be in focus. We will discuss this more later. For now, just try to remember that it is affected by aperture (or f-stop).

For those who are more visual learners (like myself), I’ll use an example that one of my grade school photography teachers used, and has stuck with me all of these years. Again, please pardon my less than stellar drawing skills and penmanship–

Ok, in the amazing graphic above, we are using the metaphor of filling a bucket with water, as getting the correct exposure of an image. Just like the exposure triangle, we have three main variables. How large is the bucket? Compare the bucket size to the ISO setting. If the bucket is small (if the ISO is a high number), the bucket will fill up fast. Conversely, if the bucket is large (if the ISO is a low number) the bucket will take longer to fill.

Shutter speed can be compared to how long we have the tap open for.

Finally, compare the aperture to how large of a hose we are using. If we are using a fire hose (large aperture), the bucket will fill up much faster than if we are using a garden hose (small aperture).

The object is to use the exposure variables to offset each other, when we are seeking a certain result. Let’s say that we want to shoot with the aperture wide open to create a really shallow depth of field (more about that later). But since we are letting all of that light in, we are going to seriously overexpose the image, unless we compensate by choosing a fast shutter speed and/or drop the ISO.

Now let’s say that we are trying to shoot a hummingbird and freeze it’s wings motionless in the image. So, we need to use a really fast shutter speed. Unless we have a really bright light source, we are probably going to underexpose the image if we fail to boost the ISO and/or open up the aperture.

Hopefully some of this makes sense. I’m sure that there are plenty of other photographers who have helpful insight on understanding exposure. So, please feel free to contribute in the comments.

Should I be giving assignments?

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View all parts of this series here.