Basic Digital Photography: Taking Control of Exposure
In a previous article, lighting was identified as the element which most determines the quality of an image. Working with available light and using internal flash and external light sources were discussed. In addition to the light present, the amount of light on the image sensor, or exposure, is determined by the opening/closing of the aperture and the length of time that the shutter is open. To better understand exposure, this article will discuss aperture, shutter speed, and the concept of “bracketing”.
Aperture is essentially the opening that allows light in through the lens. You will see aperture indicated in “f-stops”. The smaller f-stops represent larger apertures and therefore more light coming in through the lens. To state it more directly, the larger the aperture the brighter the light obtained. It is also important to realize that aperture also influences some of the focusing within a shot, known as depth of field. Technically, a camera can only focus on one item within a shot with items becoming less in focus the further they are from that point. The depth of field is the range or distance of items that are in focus; thus, an image with a narrow depth of field generally has a smaller area in focus. A wide depth of field is used when wanting detail throughout the shot but a narrow depth of field is desirable when a single object or person is the focus of the image. When using a larger aperture, depth of field is minimized while reducing aperture maximizes it.
Shutter speed is generally understood as the length of time the shutter is open. Slower shutter speeds allow for more light and faster shutter speeds less light. Therefore, a night shot often utilizes a slower shutter speed in addition to a larger aperture to maximize the light coming into the lens. Whenever a slow shutter speed is utilized the risk of camera shake is significantly increased and thus, tripods are often necessary to avoid blurring. Fast shutter speeds, such as 1/250, produce very sharp images and can act to freeze action. A slow shutter speed produces less defined images; images that demonstrate movement by blurring action. Just as with aperture size, the shutter speed selected is not “right” or “wrong”, it is merely which will create the type of image wanted.
It is important to understand that when adjusting shutter speed or aperture size it is often necessary to adjust the other to accommodate the resulting change in lighting. For instance, if a faster shutter speed is chosen to freeze action aperture size may need to be increased to obtain adequate lighting for the shot while realizing that there will be a narrower depth of field.
Digital cameras offer the user control over aperture and shutter speed. For the novice, a Fully Automatic Mode in which the camera decides on the shutter speed, aperture, white balance, and focus is most handy. This feature is often sufficient for producing excellent photos but there are limitations. For instance, the camera will not sense when the user is attempting to take a shot which captures faster action. Use of a shutter speed that is too slow in this instance will result in blurring. As described above, it may be advisable to adjust the shutter speed, and possibly the aperture size, for this type of shot.
Many digital cameras also possess a Programmed Mode which allows the user to select the situation that is most appropriate; for instance portrait, sports, or landscape, and the camera makes the decisions from there. As discussed above, adjustments in the shutter speed to allow you to create special effects may be desired but not congruent with what the camera assumes is wanted. Most digital cameras offer a Shutter Priority setting that allows the user to adjust the shutter speed while the camera determines the aperture and other features. In other instances, increased depth of field may be desired and achieved by selecting a smaller aperture. This is possible using the camera’s Aperture Priority setting which will allow aperture alone to be adjusted. The Manual Mode allows both the shutter speed and aperture to be manually controlled. Obviously, this mode allows for the most control of shots but requires a significantly higher level of skill.
A final word about getting the best exposure with a digital camera is about a technique called “bracketing”. Bracketing is used when it is difficult to determine the optimum exposure because the lighting varies within the scene. For instance, taking a photograph of a dog sitting in a snow covered field presents challenges based on the extreme contrast and reflection. Bracketing is simply the practice of taking several shots of the same scene with different exposures. Most digital cameras make this very easy to do as they can automatically vary the exposure, taking a shot at the metered setting and others which are under exposed and over exposed. The user then compares the images, selects the best one, and deletes the rest. During the editing process it is even possible to combine the best parts of multiple shots to get the best composite image.
Although automatic settings produce great images, even a novice can adjust shutter speed and aperture to capture action and create special effects within their photographs. Digital cameras make the process easier and less expensive through progressively manual controls and the option of deleting photographs that simply aren’t desirable.
Christine Peppler
http://www.articlesbase.com/digital-photography-articles/basic-digital-photography-taking-control-of-exposure-87402.html
Technical photography question using ISO to control exposure?
Ok, lets say I am using a little point and shoot digital camera…it has mainly automatic settings… but for some reason, even in landscape mode, where I would assume it would pic a low shutter speed and higher fstop number, very frequently it will pick something like 1/500 f2.8 instead of 1/60 f8…(as an example)
Here is my question…everything else being equal, if i take a picture
and the setting says 1/500 f2.8 and I am using iso 100, if i increase
the iso, say to 200, will i accomplish my goal of forcing the camera
to pick maybe something like a 1/250 f 3.5, and if I further increase the ISO, will the camera be forced to select something like 1/125 f5.6???? IE, force the camera to select a lower shutter, higherfstop number by increasing the ISO…
If I am correct, WHY would increasing the ISO cause this to happen…I am missing a basic fundamental of the relationship of how ISO can control exposure….
It’s really up to the camera. You have the right understanding in the relationship, but if your camera is already doing 1/500 f2.8 in landscape mode, I can’t tell that it won’t just go to 1/1000 f2.8 if you bump the iso up.
Honestly, your best bet is to just try it. I’d think that once it maxes out its shutter speed it would start closing up the aperture, but it’s already a little puzzling that it chose 2.8 for landscape mode.
It sounds like you get it, but since you asked for a "why" even if you were correct: ISO controls how sensitive your film is. The higher the number, the more sensitive. So if you make your film more sensitive, the only way to compensate for it is to either have a faster shutter speed or a smaller aperture, so that less total light hits the sensor. The hope is, as you guessed, that the camera would finally pick a smaller aperture instead of a faster shutter speed and give you a better depth of field, which is typically the priority for landscape photography.
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As Ramon said, you have the math right, however the camera will do what ever the spot meter inside tells it to. Best bet is check the manual for the location in the viewfinder where the spot meter "looks" for the light setting. Some point and shoots have setting for many points and some have only one a general spot meter. On one of mine I can set up the meter to look in the corners or the center.
This is why I still perfer film, I find it easier to force whatever setting I want.
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The higher the ISO number, the more sensitive the camera is to light.
If you increase the ISO to 200, the camera could go from 1/500 at f2.8 to 1/1000 at f2.8. After all, the camera knows it needs only half the shutter speed. Or it could go 1/500 at f4 instead. Same exposure.
If the camera has a manual mode, that’s the only way that you will get control over the exposure. Otherwise, it will always be a tug-of-war between you and the camera’s tiny pea brain.
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All cameras are just dumb boxes. Any assumptions you make about what a camera that only has Programmed Auto Exposure will or will not do in a certain situation or Scene Mode are invalid. The camera’s programming is for a correct exposure. It is incapable of reasoning and deciding "I’m taking pictures of a landscape so I’d better use f16 and a slow shutter speed." All it can do is find a combination of f-stop and shutter speed that will produce a correct exposure.
First, a camera that uses Programmed Auto Exposure will tend to select a higher shutter speed to help minimize the effects of camera movement.
Second, increasing the ISO will result in more digital noise which will give you poor image quality, especially with a digicam.
Correct exposure is a triangle composed of f-stop, ISO and shutter speed.
ISO is a measurement of the sensitivity to light of a light sensitive surface, whether film or digital sensor. A low ISO (50, 100) is very insensitive and requires a lot of light. It also gives you the best picture quality. A high ISO (400, 800) is more sensitive and requires less light. It also causes picture quality to suffer as digital noise increases.
The aperture is the opening in the diaphragm inside the lens and controls how much light is admitted. A large f-stop (f1.4, f2) admits all the light. A small f-stop (f11, f16) admits very little light. Our shutter speed is determined by the ISO we set and the f-stop we choose. The aperture also helps determine our Depth of Field (DOF) in conjunction with the focal length of the lens being used. DOF is defined as that area of acceptable focus in front of and behind our main subject.
The shutter speed determines how long the light admitted by the f-stop we chose is allowed to expose the film or sensor, based on the ISO we chose. If we use ISO 50 and f16 our shutter speed will be much slower than will be if we use ISO 400 and f2.8. It also determines whether motion in our scene will be stopped (fast shutter speed) or blurred (slow shutter speed).
One of the best ways to understand the exposure triangle is to apply the venerable "Sunny 16 Rule." It was developed when cameras were 100% manual and often did not have a built-in light meter. Here is what it says: "On a sunny day, set your aperture to f16 and your shutter speed to 1/ISO." So on a sunny day using ISO 50 we would shoot at f16 @ 1/60 sec. (Remember, shutters were mechanical back then and your only choices were 1, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000. Today’s cameras have electronically controlled shutters which are essentially "stepless" and will more closely match the 1/ISO component of the Rule). A further limitation was most cameras had a maximum shutter speed of 1/1000.
ISO 50
f16 @ 1/60
f11 @ 1/125
f8 @ 1/250
f5.6 @ 1/500
f4 @ 1/1000
ISO 100
f16 @ 1/125
f11 @ 1/250
f8 @ 1/500
f5.6 @ 1/1000
ISO 400
f16 @ 1/500
f11 @ 1/1000
As you can see, using a camera that does not allow you to choose Aperture Preferred or Shutter Preferred is very limiting. You are at the mercy of the camera’s programming and, as previously stated, in Programmed Auto Exposure the camera will always choose a higher shutter speed over a smaller f-stop simply to minimize the effects of camera shake.
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37 years of enjoying and learning about photography. Most all of my shooting is done in Aperture Preferred Mode because I want to control the f-stop for depth of field.