Monday, January 25, 2010

#3 Beyond Auto – Understanding Basic Exposure Controls

[Note to the reader:  this entry is intended to be a review for students that already have some knowledge of camera controls.  If you are a beginner, don't try to take all of this in at one time.  I'll be more than happy to address any questions you might have on any topic you find confusing.   You can contact me at for more info.  It usually takes several weeks of basic photography classes to allow all of the following info to sink in.]

Most digital point and shoot cameras and pretty much all consumer and prosumer level DSLRs have numerous “scene” modes that allow the novice photographer to capture acceptable images even in some of the most difficult lighting situations. When set to one of their full auto modes, many of these cameras can analyze the scene and do a better job of setting exposure than many beginners. So if the cameras have such good onboard computers and software, why should the artist take the time to learn the how to manually set the camera’s exposure controls? Why not let the camera do all that “exposure” stuff and concentrate on finding interesting subjects and getting the best composition? First let’s back up a bit and go over what we mean by the term “exposure.”

Back in the film days, in order to capture an image you had to expose a piece of film to produce a negative. Because of this the term "exposure" is often used as a synonym for what digital photographers refer to as a "capture." When you press the shutter button and create an image, that original data file and the image it represents is the "capture." The other, and for the moment more important meaning of the term “exposure," is how dark or light an individual capture turns out to be. With our photo editing software we can adjust the exposure long after the original capture. So why can't we just shoot pictures and worry about exposure when we edit? Well, while we can adjust the exposure to some degree in post processing, if the image is so light that the light parts of the image go white or so dark the shadows go black, we cannot restore information that was not included in the original capture. If you take a really dark image and brighten it up in your image editing software, you’ll see that the image comes apart before it is lightened nearly enough. Even if you image is just a little too dark, it can come out looking “flat” (too low contrast) when it is adjusted to an acceptable brightness.

The most advanced cameras will still fail to choose the very best exposure in every situation. Even at their best, automatic camera controls (when they work properly) produce “average” results. While an “average” exposure might be more than adequate for the casual snapshot photographer, anyone pursuing photography as a serious means of creative expression is well served by taking the time to develop a thorough knowledge of camera controls and how to use the more advanced camera modes or manual mode to choose the very best possible exposure and allow for the possibility of making extremely large, high quality prints.

Once you understand the basics of exposure control and how to operate your camera in manual mode, you’ll not only be able to utilize the “A” (aperture priority) and “S” (shutter priority) modes on your camera but you’ll also be able to understand how those “scene” modes actually work in the first place. Knowing how they work will allow you to decide when you can get away with using them and when the situation calls for more precise, hands on control. This knowledge also opens the door to some very specialized types of photography such as night photography which are, if not totally impossible, extremely limited in automatic exposure modes.

The Three Factors

Many creative people find the technical side of photography a bit daunting. The plethora of bells and whistles on today’s cameras, the menus, modes and functions can make using a DSLR seem as difficult to the novice as being called up from coach (without any previous training) to land a modern jet airliner. The reality of the situation however is much simpler. There are actually only three controls that one needs to understand to set a camera for the proper exposure. The first is the ISO or sensitivity to light (the digital equivalent of film speed). This is basically how much light is required for the sensor to register an image. The second control is the aperture. This is how much light is allowed to get through to the sensor and is controlled by making the hole the light passes through at the back of your lens larger or smaller. The third is the shutter speed. This is how long the shutter is open exposing the camera's sensor to the light.

The best “exposure” is the proper balance between these three controls. If you change one of the controls, you can still get the same exposure by changing one or both of the others to compensate. Different settings for each of the controls produce specific effects on the final image that your camera’s sensor records. By getting the exposure just right at the time of capture, we maintain as much shadow and highlight detail as possible and make our post capture editing a much smoother and quicker affair. While the screen can be quite forgiving, a bad exposure can make it impossible for you to get a good looking print.

When speaking of exposure adjustments, photographers often talk about "stops."  Basically one "stop" is twice as much or half as much as you started with.  When adjusting sensitivity for instance, ISO 200 is one stop faster than ISO 100.  One stop faster than ISO 200 is ISO 400.  Aperture (where the term originated as the f stop) works the same although the irregular sequence of numbers can be a bit confusing.  If you're aperture is set to f/5.6 then f/8 is "stopping down" one stop.  Half as much light is allowed in at f/8 as at f/5.6.  Stopping down from f/8 in one stop increments goes f/11, f/16, etc..  If you want to open up to let in twice as much light as f/5.6 then you'd set the aperture to f/4.  Shutter speeds as a bit easier to understand.  If you're shutter speed is set at 1/60", then 1/30" is one stop slower and 1/125" is one stop faster.  Most modern DSLRs can be set to allow adjustments in either 1/2 or 1/3 stop increments.  

ISO (Sensitivity)

Before the advent of digital photography, photographers were very limited as to controlling the sensitivity of the films they used to capture images. Film speed (sensitivity) numbers were set by the ISO (that’s the common short name for the International Organization for Standardization). The term ISO is still used today in digital photography, but now it refers to how sensitive your camera’s sensor is to light. The numbers used to indicate ISO mean that at a given ISO your digital sensor has sensitivity equal to that of film of the same number or “speed.” On a bright, sunny day, a low sensitivity setting (100 to 200) will work just fine (and is usually preferable) because there is plenty of light to work with. The darker it gets, the more we must increase sensitivity if we want to maintain the same setting on the two other controls. The effect of the increasing sensitivity is usually lower color saturation and increased digital “noise." Digital noise shows up as random specs of color in the darkest shadows.

These days’ manufactures are racing to produce higher and higher image quality at higher and higher sensitivity settings. Generally, just about any decent camera of recent years should perform just fine at 400 and below. With a little noise reduction software, higher sensitivity settings are often quite acceptable. The easiest way to choose your sensitivity setting is to set it as low as you can and still set the other controls to produce the desired overall effect and proper exposure. Testing is always the best way to determine which ISO to use. Test your higher sensitivity settings and note where the noise in the shadows and quality of color become unacceptable for you. With the application of a post production noise filter, you might be pleasantly surprised at how high your ISO can be and still produce acceptable images.

Since it is the random specks of color that are the most offensive element of the noise inherent in images made at the highest ISO settings, you will probably find that if you intend your final image to be monochrome (such as black & white or sepia), the higher ISO settings might be more acceptable than they would be in a final image that you want to present in full color.


The second control that determines exposure is the aperture. Behind your lens is a diaphragm which by opening or closing creates a larger or smaller hole for the light to pass through. The size of this opening is what we refer to as the "aperture." The larger the opening, the more light gets let in. As you increase the amount of light that reaches the sensor, you can use a lower sensitivity or a faster shutter-speed and achieve the same overall exposure. Apertures are indicated by numbers written as follow: f/8, f/11, f/16, etc... Beginners are almost always baffled by the numbers; the larger the number, the smaller the opening. Use the way it’s written to help you remember. If you think of the “f/#” as a fraction then they will no longer seem “backwards”. The math behind calculating the actual numbers and the reason that they seem to progress so unevenly are beyond both the scope of this article and my knowledge of the subject. I seem to remember it has something to do with logarithms (a word which tends to cause my brain to shut down).

So why not use a large aperture all of the time to take advantage of the higher image quality at lower ISO’s and the sharpness of faster shutter-speeds? Aperture influences your images “depth of field.” The “depth of field” of an image is the closest point in front of the camera that is in sharp focus to the furthest point in front of the camera that is in sharp focus. Sometimes we wish to make the background of an image blurry (out of focus) so that it doesn’t distract the viewer from our main subject. To do this we use a large aperture to achieve a very shallow or narrow depth of field. If we want as much of the image to be in sharp focus as possible, then we’ll choose a much smaller aperture often resulting in a long shutter-speed requiring the use of a tripod.

Keep in mind that the very smallest apertures are often not the very best for producing the sharpest overall image because of distortion inherent in lens design. Many photographers choose to avoid the very smallest apertures (highest number) especially of inexpensive zoom lenses. The only way to know for certain which aperture is best is to test your lens for yourself and note any problems. You may have to make test patches of a very big enlargement from your image in order to do this. It is important to perform this test before shooting images that you want to make huge enlargements from to avoid being disappointed with your results. If I had to make an image with a lens I hadn’t tested and wanted to get the sharpest image possible with maximum depth of field, I’d use the setting one stop bellow the smallest (highest numbered) aperture. As a rule of thumb, that is likely to be the best for producing the sharpest possible image, although some of the modern zooms have "sweet spots" at various apertures.  F/8 and above are usually sufficient to achieve enough depth of field for most shots.  If you want a blurred background, try using the largest (smallest f/#) aperture your lens allows.  Try to put as much distance as possible between your subject and the background and get as close to the subject as you can.  If you need more "blur," this can be enhanced in Photoshop with a bit of training and practice.

Shutter Speed

The third and final factor, shutter speed, is also the easiest to understand. The shutter speed is simply how long you allow the light to strike the sensor in your camera. If the shutter speed is too low, the image will exhibit “motion blur.” Motion blur occurs when the shutter is open long enough to record the movement of the subject as opposed to freezing it in place. When shooting flowing water and using a tripod, you can use motion blur to produce a magical, “flowing” look to the water. This should appear when your shutter speed is set to 1/15th of a second or longer.

So how fast is fast enough to avoid motion blur when shooting without a tripod? One of the special advantages of digital imaging is “image stabilization technology.” It is included on just about every camera made today (with the exception, perhaps, of the one on your cell phone – unless you have a really nice cell phone).  Because of this, today that question is more complicated than it was back in the pre-digital era. Shutter speeds that use to be too slow and result in motion blur are fast enough to produce sharp, motion blur free images with today’s image stabilization technology activated.

The old rule was to take the reciprocal of the focal length you were using and that was the longest hand-held shutter speed. That would mean if you zoomed your lens in to 55mm you would need to use a shutter speed of at least 1/55th of a second or faster. These days with the image stabilization technology activated, a shutter speed of just 1/15th of a second might be fast enough if you were very careful to hold the camera as still as possible as you pressed the shutter release button. The image stabilization systems should give you about a two stop improvement on your longest, motion-blur free shutter speed. Using a monopod, which is much easier than setting up a tripod, can give you sharp images at even slower speeds. Once again, the only way to be sure of avoiding motion blur is to test your equipment and determine your minimum shutter speed for sharp hand-held and monopod photography.

Understanding these three factors and how they affect your images will allow you to scoff at the automatic “dummy” mode on your camera and take control of the images you produce. You’ll still have at least one use for the fully automatic mode on your DSLR assuming you occasionally hand your camera to someone who has zero photographic knowledge so they can make the photo and you can be one of the subjects in it. In the future, I’ll be going over how to use the histogram (the best way to judge the quality of an exposure in the field) in conjunction with the aperture priority and shutter priority modes available on most digital cameras to make capturing well exposed images as quick and easy as possible without giving up precise control over our three factors. I hope this has been a good primer or refresher on understanding basic exposure control.

Happy Shooting,

Al Williams

The Elephant’s Eye Photography

1 comment: