I’ll post installment two of the series on filters in the next “Thursday Tips.” For now I want to pivot to a subject covering the principle elements of establishing exposure. I’ve received a surprising amount of correspondence asking about these elements and what they “do,” so I thought a brief piece covering the exposure setting triad was in order. This technical matter isn’t exactly scintillating, but it’s very important to understand at a fundamental level. If you understand this stuff, it’s much easier to make desired adjustments in the field, on the fly. I realize that this material will seem elementary to experienced photographers, but for those of you reading this that fall into that category, think back to the time when all this was Greek to you…unless of course you are Greek…in which case it probably seemed English to you…or something. :)
For the purposes of this discussion, we will define exposure as a measure of the amount of light that falls on the photographic medium (e.g. digital sensor or film) during the process of taking a photograph. In essence, there are three components involved in establishing exposure. While the three work together, they fulfill very different purposes. The three elements are shutter speed, aperture and ISO. I’ll discuss all three in this piece and try to explain the priority of each. But first, I want to briefly touch on how all three relate to one another as they pertain to establishing a desired exposure. (How to go about establishing what the desired exposure should be is a related topic, of course, but is too long for this presentation.)
Shutter speed determines how long the shutter is open to allow light to reach the camera’s sensor (or film) and is expressed as a measure of time—seconds, to be specific. Each time the shutter speed is halved or doubled, a “stop” or “exposure value” (EV) of light is said to be lost or gained respectively. So, for example, a 1/50 second shutter speed is exactly one more stop/EV as a 1/100 second shutter speed. 1/20 second is exactly one stop/EV less than 1/10 second, and so on. Each additional stop/EV added represents a doubling of light: 1/10 second doubles the amount of light reaching the sensor compared with 1/20 second. Dropping a stop, halves the amount of light reaching the sensor.
Aperture, in photography, refers to the size of the opening of the lens diaphragm and is expressed in f-stops. The larger the f-stop number (e.g. f/22), the smaller the opening. (In case you’re wondering where the f-numbers come from: the specific f-number is the ratio of the focal length of the lens to the aperture diameter. So, a 100mm lens, at f/8, would have an aperture of approximately 12.5 mm: 100/12.5 = 8. At f/16, the same lens would have an aperture of 6.25 mm. Note that as the f-stop number gets larger, the size of the opening gets physically smaller.) A full list of f-stop values can be found here. Every full stop on the scale—from f/2.8 to f/4, for instance, or from f/11 to f/16—represents a “doubling” of the amount of light reaching the sensor (or halving, if you go the other direction). Note the similarity in this regard to shutter speed. (Notice that we’re talking about a stop/EV when discussing doubling/halving, not the doubling or halving of the f-number itself. For example, f/8 does not allow twice as much light to reach the image sensor as f/16; f/8 is two more EVs than f/16 and would represent a quadrupling of the amount of light to reach the sensor.)
ISO is an abbreviation for International Organization for Standardization. In photography terms, the ISO number historically referred to the sensitivity of film to light. More sensitive film needs less light—relative to less sensitive film—to establish a specific exposure, all else being equal. The more sensitive the film, the higher its corresponding ISO number. This principle, as a matter of convenience in adaptation as much as anything else, was applied to digital photography. Technically, the digital sensor isn’t more “sensitive” in the manner that film is as the ISO is increased (in actuality the digital signal is amplified as you increase the ISO with a digital camera), but as a practical matter one can think of it the same way. The higher the ISO number in digital photography, the less light is needed to achieve a given exposure, all else being equal. Every doubling of the ISO number represents the equivalent of one stop (or exposure value—abbreviated as EV).
So, the commonality between the three measures of the exposure triad is the “stop” or “EV.” To obtain a given exposure, any of the above three factors can be adjusted with equal exposure results. Increasing the aperture by one EV will have the same effect on exposure as increasing the ISO or shutter speed by one EV. A one-EV decrease of any of the exposure triad factors works the same way: a one EV decrease of, say, ISO, has the same impact on exposure as a one-EV decrease of shutter speed or aperture.
So, consider the values of a given exposure:
Shutter speed: 1/50 sec.
If I add one exposure value to the shutter speed (by halving it to 1/25 sec.) and I subtract one EV from the aperture (to f/11) while keeping the ISO the same (100), I’ve achieved the exact same exposure as above.
Shutter speed: 1/25 sec.
The two charted exposures are identical. Similarly, if I raise the ISO by one EV (200) to retain the same exposure, I would have to adjust either the shutter speed or the aperture—or some combination of the two—by giving up one EV. I could raise the shutter speed to 1/100 sec., or close down the aperture to f/16 or I could lower one of the values by 1/3 EV and the other by 2/3 EV, for example, to obtain the same effect.
Now, let’s say that a given exposure appears too dark. For that, more light is needed. Either the shutter needs to be open longer (longer shutter speed) or the aperture needs to be opened more (smaller f-stop number) or the ISO needs to be raised…or some combination of two or all of these things. For an exposure that appears too light, the opposite is true—less light is needed. I’m sure you can tell what your options are in that instance—the opposite of the specifics listed in this paragraph.
So, you may ask, if these factors are interchangeable in terms of exposure value, what difference does it make which you choose to adjust? Well, it turns out that it does make a difference, because each factor of the triad does more than simply allow a certain amount of light to reach the sensor; there are other critically important implications.
Let’s start with shutter speed. You may desire a certain effect in the photograph you’re taking. For instance, you may have a fast moving object—a vehicle, or a person or an animal—that you want to render as sharp. That’s going to require a very short shutter speed—perhaps 1/1000 of a second. You may be able to get a properly exposed photograph using a longer shutter speed and the appropriate combination of aperture and ISO, but who cares if it doesn’t render the image the way you want? In the above example, you would end up with a perfectly exposed blur, which isn’t what you want. In the image below, I was trying to render the swirls in the water in a certain way, which required a specific shutter speed (in this instance, 10 seconds). Faster or slower shutter speeds didn’t produce the effect I was looking for, so the shutter speed needed to be approximately 10 seconds. The aperture and the ISO could be adjusted to gain the proper exposure and the desired effect, but not the shutter speed.
Then there’s aperture. The smaller the aperture (i.e. the greater the f-number), the greater the depth of field: that’s an optical truism. There are some practical limits (as you make the aperture smaller and smaller, one must deal with diffraction, which can decrease apparent real world image sharpness), but the smaller aperture, the greater depth of field. Consider the image below. To obtain the desired depth of field, I had to stop the lens down to f/16. I was free to manipulate the shutter speed and the ISO to obtain the necessary exposure, but there wasn’t that flexibility with the aperture.
When it comes to ISO, to the extent possible you should use your camera’s base ISO (check your manual to determine what that is; with most DSLRs it’s either 100 or 200). Why? As you increase the ISO, the level of objectionable noise in your image rises and the dynamic range of the sensor declines. The noise problem has become less and less of an issue with modest jumps in ISO with each generation of digital sensor, but if you don’t need to raise your ISO, you shouldn’t. Why would you ever need to raise your ISO? Can’t you always raise either the shutter speed or the aperture or both? Unfortunately, no. People who shoot in dark environs that need to maintain a minimum shutter speed often find themselves in this quandary. Even at the maximum aperture of the lens being used, they may not be able to reach the necessary shutter speed. Think of the wedding photographer in a dark church, or a sports photographer in a dimly lit arena. It even happens to the nature photographer, from time to time. The shot you see below is such an example. I was photographing on an overcast, breezy day. It was relatively dark and I needed to maintain a certain aperture for depth of field (at least f/8) and achieve a certain shutter speed (at least 1/40 sec.) to keep the leaves on the tree from being blurred. The only way I could achieve the necessary aperture/shutter speed combination? You guessed it—the ISO needed to be raised.
Once you understand the relationship between the elements that determine exposure, it becomes quite easy to determine what your priorities are given the kind of photography in which you’re engaged. In fact, if you’ve ever wondered why there are shutter priority and aperture priority shooting modes on many cameras, it’s likely that you now understand the reasoning. Of course, by using these modes you’re still trusting the camera’s meter to render the scene properly, but that’s a rant for another day.
Thursday Tips is written by Kerry Mark Leibowitz, a guest blogger on 1001 Scribbles, and appears every other Thursday. To read more of his thoughts on photography, please visit his blog: Lightscapes Nature Photography.