Is there a difference between exposure and shutter speed, or are the terms interchangeable? I read that "If you use a quick shutter speed, you can just raise the exposure to compensate." Is this statement invalid, or is there a difference?
The term "exposure" is used for a number of different but related things in photography. I can see how this might be confusing. Here are six different ways in which it is used:
- The combination of all factors which make a photograph have a certain overall brightness. The key factors are shutter speed, lens aperture, and sensor or film sensitivity (a.k.a. ISO), in combination with the light levels in the scene itself. Sometimes, post-processing can also be involved, as in "pushing the exposure" when developing film or with an "exposure" slider in a RAW development program. This is the sense used in the common phrase "exposure triangle" (which I don't recommend, but for other reasons).
- The amount of light allowed to to fall on an area of the sensor or film. This is related to the previous sense, but is more strict. It's arguably more technically-correct, but I think the wider definition is more common these days. Here, though, ISO sensitivity (and definitely post processing) aren't included — it's the light in the scene combined with the size of aperture you use combined with the time the shutter is open. In this sense, one might say that the same exposure yields different results at different ISOs.
- Exposure time — A synonym for shutter speed. Or, technically, exposure time is the result of a certain shutter speed. Sometimes one might hear just "exposure" for this, but usually this sense is qualified with the word "time", or in combination with some unit of time (as in: "Exposure: 30 seconds"). "Shutter speed" is itself jargon, since we don't really mean the speed at which the shutter travels.
- The result of a single click of the shutter. An HDR image, for example, may be said to be composed of multiple exposures — several images taken in a row.
- The independent effects of two different sources of light within a single shot. For example, in the "dragging the shutter" technique, the resulting image may be said to combine the flash exposure and the ambient exposure — two "exposures" overlaid in the same image.
- Occasionally, a term for a photograph, usually used in a semi-technical or jargon-ish way. This is often referring to unsorted, unedited negatives or proof prints, or a collection of images just dumped off a memory card. Or, even potential photographs, as in "a 36-exposure roll of film".
Exposure value is a common technical term which uses the first sense in the list above. You can read more about that in the question What is the EV scale?.
Confusion between sense #1 and sense #2 above sometimes results in long, pedantic flamewars between people who both think the other person is wrong, and don't realize that they're actually talking past each other.
In the example sentence you give, "If you use a quick shutter speed, you can just raise the exposure to compensate", I think the term is simply accidentally misused and that they probably meant "ISO sensitivity". Raising the exposure (or exposure value) doesn't really make any sense in that context.
Briefly, exposure is a combination of factors all of which together tell you how much light accumulates on the sensor to make the picture. Shutter speed is only one aspect of exposure. The three major factors are ISO (sensor or film sensitivity), f-stop (how much light the lens lets thru), and shutter speed (how long the light has to accumulate on the sensor of film).
Photography is all about exposure, both in experience and light captured. When you heard:
If you use a quick shutter speed, you can just raise the exposure to compensate.
They may have been referring to Exposure Value.
As stated before, you can change the exposure in a few different ways. Most basically Aperture and Shutter speed. Assuming you know what both are, you can shoot in aperture priority or shutter priority. Or fully manual where you control both. You also can change the EV (Exposure Value).
If you can manually change the aperture and shutter speed to control exposure, why bother having EV?
Well, in aperture priority, changing the EV actually changes the shutter speed (or ISO) accordingly. You might want a particular Depth of Field.
In shutter priority, the EV value will change the aperture (or ISO) accordingly. Again here, you might want a blur effect where the DoF doesn't really matter.
And again, ISO changes the sensitivity of the sensor. In the film days, a roll of film was graded with an ISO value. Most commonly 100, 200 or 400. So it's great in this digital world you can just press a button!
A low ISO value means it's not very sensitive to light so you can have a brighter exposure. A high ISO value means it's more sensitive to light, so you can have darker exposure. But, the higher the ISO, the more grainy the results.
Exposure is a combination of three things:
Aperture - the size of the opening in the lens through which light passes
Shutter Speed - the time which the sensor is exposed to the light
ISO sensitivity - the sensitivity of the sensor to the light it is exposed to.
So there isn't really a difference between shutter speed and exposure, but it's part of what makes an exposure.
In the text you read, when using a faster shutter speed, the sensor won't have so much time to "see" the scene, so you might consider using a larger aperture (a smaller f-number), or increasing the ISO sensitivity of the sensor, to compensate.
When one learns photography they are taught that they need to "correctly" expose the film or sensor in order to achieve an image that captures a range of light. The subjectivity of what is "correct exposure" aside,
Exposure is the quantity of light allowed to pass through the lens and shutter in order to be captured by the film or the sensor. There are Two controls available to the photographer that they can use to control the amount of light that passes through the lens and shutter to be recorded :
The size of the opening in the lens which is called the aperture, we call the different sizes apertures fstops,
The length of time that light is allowed to pass through the shutter, we call this shutter speed.
Changing fstops/apertures changes the size of the opening (larger or smaller) letting in more or less light. Changing shutter speed changes how fast the shutter opens and closes (slower or faster) which in turn lets light pass for a longer or shorter period of time (thus letting in more or less light).
Changing the ISO does not change the amount of light that has passed through any particular combination of fstops and shutter speeds. It only changes the sensitivity, or in other words the amount of light that is needed to capture an image.
Consider the following situation. You are in a dimly lit room, your camera is set to ISO 100 and the widest aperture available on your lens is f2.8. Your light meter is telling you that you need a shutter speed of 1/4th of a second for the f2.8 to get a correct exposure (aperture priority mode). So far so good. Now, if you want 1/60th of a second because you want to stop a motion, you can't have a correct exposure with the same settings. You need 4 more stops of light to get a shutter speed of 1/60 => you need to change how sensitive you camera is : switching to a faster speed film (higher ISO) or changing to a higher ISO with your digital camera. By switching to ISO 1600 you now have gained 4 stops of light sensitivity. You have a correct exposure with a shutter speed of 1/60 and an aperture of f2.8.
In my opinion, since the advent of digital cameras many people are getting confused about ISO because with digital cameras, one changes the ISO (but does not put in a different sensor) and sees that it affects the exposure and (some people) mistakenly thinks that it is changing the amount of light that is passing through the lens/shutter. In the past you had to physically change the film in your camera and set a dial to match the ISO of that film, your light meter would then be calibrated to that films ISO (light sensitivity).
The bottom line is that changing the ISO does not change the amount of light passing through any given set of aperture/shutter speed combos. It ONLY changes the sensitivity or the amount or light that is required, to achieve the correct exposure for the conditions you are photographing in.