Understanding Exposure (Shutter speed, Aperture, etc)

You can easily find a great deal of reading material on this subject, but here is my own little tutorial. It is gearing towards those with a SLR (Single-Lens-Reflex) camera because compact "point and shoot" cameras often don't provide full control of the camera and also aperture adjustments don't have the same effect on these camears due to their very small sensors.

First I will talk about making exposures with a generic SLR camera (film or digital) and then discuss a slightly modified technique that people often use on a digital SLR to make the most of the digital format (commonly called "Exposing to the Right").

A camera controls the amount of light captured in 3 ways: 1) Shutter speed, 2) Aperture, and 3) Film Speed (ISO). On a digital camera the term "film speed" is not quite appropriate but still the concept is the same and people will still often say "film speed". You are most likely to want to adjust the Shutter speed and Aperture often, while generally you just want to keep the film speed low, so we will save the discussion of film speed for the end of this tutorial. (For now, keep your film speed at the default setting and forget this setting exists.)

To force yourself to think about Shutter speed and Aperture, always take pictures using one of these modes: All SLR cameras will have icons on the "model dial" for Shutter priority, Aperture priority, and full Manual. (Check your camera manual for the icons - note that Canon uses "Tv" to mean Shutter priority.) Your camera may have many other modes such as Program (fully automatic), Landscape, Sports, Portrait, and so on, but using these modes will prevent you from having to think about what your camera is actually doing. On a professional camera, modes such as Portrait and Sports don't exist because they are completely unnecessary if you understand how to use Shutter priority, Aperture priority, and Manual mode correctly.

Shutter speed is normally talked about in fractions of a second (ex. 1/100) and sometimes whole seconds. In your camera's viewfinder, a shutter speed of 1/4 is displayed just as 4. A shutter speed of 4 seconds is displayed as 4 " (with a double quotes afterward to indicate whole seconds). Shutter speed is the amount of time that the camera is permitting light from the lens to hit the film or digital sensor. A fast shutter speed is good for "freezing" the action of a subject in motion. A slow shutter speed is good for purposely blurring the motion of a subject (note with a slow shutter speed it is generally desired to have the camera on a tripod). A shutter speed of 1/30 for example will let in twice as much light as a speed of 1/60 since it is two times slower. When you decrease the shutter speed by a factor of two you are increasing the amount of light by "one stop". Remember that fractions get larger as the number on the bottom gets smaller. The following table shows a series of shutter speeds, each one stop apart from the next. Note that for example, 1/8 is 2 stops faster than 1/2 because it is two boxes to the right.

(slower shutter speed / more light) 2 sec 1 sec 1/2 1/4 1/8 1/15 1/30 1/60 1/125 1/250 1/500 (faster shutter speed / less light)

Aperture (or f-stop) refers to the size of the opening in the lens of the camera when a picture is taken. As you are focusing the camera, the lens is "wide open" with the largest aperture supported by your lens. But when you take the picture the lens will "stop down" to your chosen aperture. This means the opening will get smaller (unless the aperture of your choice is the same as the maximum aperture). Every lens has a maximum aperture (the widest it can open) and a minimum aperture. Generally the maximum aperture is more important and so the lens will be advertised with this number (ex. 24-70mm f/2.8). The aperture of "f/2.8" is a fairly wide or large aperture and also means the lens is "fast" because the ability to let in a lot of light means you can have a faster shutter speed (more on this in the next paragraph). With aperture, smaller numbers (ex. f/4) mean a larger opening. Large numbers (ex. f/16) mean a small opening. Any time you decrease or increase the amount of light by a factor of two you are changing the exposure by "1 stop". The following table shows the most common apertures. These are each one stop apart. So for example, going from an aperture of 2.8 to 4 is "one stop". However, unlike shutter speeds, the numbers are not multiples of 2 - they are multiples of 1.4. But they are still changing the amount of light by a factor of two, just like the stops for shutter speeds. It is helpful, if possible, to memorize these numbers, and to be able to know in your head for example that "the aperture 2 stops smaller than f/5.6 is f/11".

(larger aperture / more light) 2.8 4 5.6 8 11 16 (smaller aperture / less light)

[Insert a shot with large aperture / shallow depth of field, and another of the same scene with a small aperture / large depth of field]
[Also insert shot(s) of a camera lens with the iris open vs. stopped down]

The point of "stops": I have explained that both Shutter speed and Aperture can be thought of in terms of stops, in other words both can be used in increase or decrease light by the same amount of light (called a "stop"). The point is that if you want to decrease the Shutter speeds by 1 stop, you can maintain the same overall light level by also increasing the Aperture by 1 stop (or vice-versa). The means your picture will remain consistent in term of brightness while you are change these other settings - which affect how your picture will look in other important ways. I have already explained how shutter speed can affect the look of a photograph (either by "freezing" motion or blurring motion). So how does Aperture affect the look? - It affects "depth of field".

Aperture and Depth of Field: A large aperture causes a small depth of field. This means that only a small area in front of the camera lens will be in focus. Suppose you focus your camera at a row of trees 20 feet in front of your camera. There are some small bushes 10 feet in front of your camera and some mountians 1 mile in front of your camera. Only the trees will be in focus given the small depth of field. With a smaller aperture you will get a larger depth of field and you may be able to get everything (bushes, trees, and mountains) in focus. There are charts you can use to look up this information (refered to as depth of field calculators or depth of field charts). It is also possible to use a feature on most SLR's called "depth of field preview" to see exactly what your camera will see at your chosen aperture so that you can tell whether the mountains, etc. will be in focus or not. It is fairly obvious why a small aperture (large depth of field) can be useful - everything (or nearly everything) will be in focus. There are also reasons why a photography might prefer a large aperture and small depth of field. It may be to draw the viewers attention to just one part of the photograph - the viewer will always be drawn to what is in the sharpest focus.

Making compromises - using priority modes: It may also be that the photograher does not particularly care what the aperture is but chooses a large aperture to allow more light so that a faster shutter speed can be used - because what the photography cares most about is freezing some motion. Again, Aperture and Shutter speed, while independent settings, both affect the amount of light so often you want to change both at the same time to keep the amount of light consistent. Often you must make a compromise. You may desire both a fast shutter speed and a small apeture (large depth of field) but this may result in a photography that is too dark, so you have to pick which one is more important to you. This is where the Shutter prioirty and Aperture priority modes of you camera come into play. When you are about to take a picture, there are many things to consider, but one basic choice is which is more important for this shot? - Shutter speed or Aperture. Then pick the appropriate mode on your camera. Suppose you pick shutter priority and you pick a shutter speed of "1/100" (just "100" for short). Your camera will display the aperture it has picked for you given your choice of shutter speed. If it's too large you might compromise a little on your chosen shutter speed and decrease it to "1/50" so that your aperture is a little smaller (greater depth of field).

Your camera's light meter and the choices your camera makes for you: I just said that in Shutter priority mode, you pick that Shutter speed and then your camera will pick the Aperture. It will be vice-versa for Aperture priority mode. How does your camera decide what the other setting should be? - All modern cameras have a built-in light meter. By default, your camera assumes that you want the overall brightness of the image to be medium gray. (Actually the technical term is "18% gray" - but don't let that confuse you. You would think that "medium gray" would be "50% gray" but no - it's 18%.) So your camera adjusts the other setting to give you medium gray. (Pretend it's a black and white photograph, even if you are shooting in color. For example, your scene has a barn which is maroon [on the dark side], some grass [which is medium], and some blue sky [on the light side]. Overall this scene is naturally pretty close to medium gray.)

Exposure compensation: Now in some cases you may not want the overall brightness to be medium gray. For a winter scene (with snow) the overall brightness should generally be very light gray (almost white) if you want your photograph to look natural. For a night scene (dark) you probaby want the overall brightness to be dark gray. You can tell the camera your preference by using the "exposure compensation" setting. The default value of "0" means medium gray. +1 means light gray, +2 means very light gray. Similarly -1 means dark gray and -2 means very dark gray. You camera will probably let you pick values in between theses numbers also (such as +1 1/3).

Manual mode: While the two priority modes are often a little faster to use (because you only have to make a decison for one setting), Manual mode is also easy to use and gives you extra control. It requires you to pick settings for both Shutter speed and Aperture. Since the camera is not deciding anything for you, there is no exposure compensation in Manual mode. However the camera will still tell you (via a needle looking display) whether it thinks the settings you have chosen will cause an overall brightness of medium gray or something lighter or something darker. [Insert image of "needle" here.] For example if it shows the "needle" at +1 then the camera is telling you that your shot will end up with an overall brightness of light gray. This may be exactly what you want or you can adjust either the shutter speed or aperture so that the needle points to 0 (medium gray) or whatever level you think is appropriate for the shot.

Common situations where Manual mode might be preferred over a priority mode:

Film speed (ISO): As I said earlier, generally you want to keep film speed low. High film speeds cause a grainy look with film and a "noisy" look with digital - both cause many small specks to appear as part of the image and most people don't find the affect pleasing (some people do like film grain but it's generally agreed that digital "noise" does not look good). The main reason to ever increase film speed is to compensate for a low light situation and allow you to use a shutter speed and/or aperture that you could not otherwise use. Increasing the film speed is similar to increasing the amount of light, although you are not really increasing the light, you are just making the film or digital sensor more sensitive to it. The point is that by increasing film speed by one stop, you can then use a faster shutter speed or make the aperture one stop smaller and still have the same overall exposure level. Here is a table of typical film speeds (each one stop apart):

(slower film speed / less sensitive to light) 100 200 400 800 1600 (faster film speed / more sensitive to light)

"Exposing to the Right" (using a digital camera): Normally the "correct" exposure (combination of shutter speed, aperture, and film speed) is the one that produces the image that looks the way you want it to. However when using a digital camera and shooting in RAW format, there is often good reason to "expose to the right". This means that you let in as much light as possible, potentially overexposing the shot beyond what looks appropriate, up to the point where you hit the right side of your histogram. (See the next paragraph about histograms). This has the effect of increasing your signal to noise ratio and producing a slightly high quality image. You then must process the image to make it look "correct" for display purposes. However, for example, if you are shooting in low-light conditions and want a very fast shutter speed to freeze motion, you should not change the shutter speed for the sake of exposing to the right. The artist aspects of the image are more important than signal to noise ratio. Same goes for aperture and film speed. Only adjust them if you can do so in a way that does not affect other important aspects of the image.

Histograms (on a digital camera): (TBD)

Experiments to try with your camera: (It would be easiest if you try these outdoors on a sunny day so you have plenty of light. If you are indoors, use a faster film speed. But maintain the same film speed for all shots.)

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