Where to start with exposure?

In the last post I discussed the three elements of the exposure triangle: ISO, Shutter Speed, and aperture. This time I want to give my ideas about how to get you images properly exposed in the camera.

Previously I suggested that most advanced photographers do not use the Auto or Programmed modes of exposure because they prefer to retain creative control over the exposure triangle.  The reasons for this desire are apparent when you consider the pros and cons of each element of the triangle. High ISO allows shooting at high shutter speed or low light conditions but can add digital noise to the image. Low ISO reduces digital noise from the sensor but creates the need for a longer shutter speed or larger aperture. High shutter speed can be used to freeze a subject in motion or reduce the possibility of blur in the image caused by camera shake but requires a higher ISO or larger aperture to capture the image.  A slower shutter speed can smooth a waterfall but requires a low ISO, small aperture, and possibly, a filter to limit the light getting to the sensor.  Large aperture (lower f-stop) can be used to lower the depth of field and blur the background but may require a fast shutter speed to limit the light to the sensor.  A small aperture (higher f-stop) will increase the depth of field but require a higher ISO or slower shutter speed.

The pressing question is how do we estimate the proper exposure and how do we adjust the components of the exposure triangle to reach this exposure while using the components to achieve the best creative effects?

In the olden days, photographers followed the sunny f/16 rule to estimate the proper exposure in the outdoors. The sunny f/16 rules says that under bright outside light the proper exposure will be when the shutter speed is equal to the ISO of the sensor. In other words on a bright day, shooting at f/16 and an ISO of 100, the shutter speed should be about 1/100 second. If you want, for creative reasons, to vary the f-stop or shutter speed you do so by using the concept of "stops" of light.  A stop is a change in the exposure triangle that effectively doubles or halves the exposure.  Doubling the exposure time (shutter speed) from 1/100 to 1/50 of a second doubles the light reaching the sensor and is said to increase the exposure by one stop. Doubling the the time again to 1/25 of a second increases the exposure by 2 stops. Exactly the same effect is seen when increasing the ISO.  Changing from ISO 100 to ISO 200 doubles the sensitivity of the sensor and increases the exposure by one stop.  Changing the ISO or shutter speed create simple exposure changes and shooting at a faster shutter speed, say 1/200 second and a lower ISO say 50 create no change in exposure - one is increased by one stop and one decreased by one stop resulting in no net change.

So far it sounds easy -- but the confusion occurs when we want to change aperture. Aperture changes also occur in stops that double or halve the amount of light reaching the sensor. However, aperture changes do not follow the simple arithmetic of doubling or halving the number like ISO and shutter speed do. The standard f-stops follow the numeric sequence: 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32.  Moving up the series of standard, full-stops effectively cuts the light reaching the sensor by half. So, using the sunny f/16 rule, if we needed to double the light reaching the sensor, we could open the aperture to f/11. This would accomplish the same thing as increasing the ISO from 100 to 200 or the shutter speed from 1/100 to 1/50 of a second. The concept of working in exposure stops allows us to change exposure by varying one or more of the elements of the exposure triangle or keep the same exposure by changing any combination of elements in the opposite directions. 

In the next posting we will wrap-up the discussion of exposure by examining the metering modes available on modern digital cameras and the exposure modes that allow creative imaging without using the Automatic or Programmed modes.

The Exposure Triangle

Every photographer knows that proper exposure is essential for a good photo but far too many picture-takers rely on the Auto or Program modes for exposure control and don't realize the creative possibilities of using manual exposure modes. So what do the auto modes actually adjust to obtain a "proper" exposure and why would anyone want to regress to manual modes?

Exposure is a poorly defined term that describes the effect of light on the media used to capture an image. Today the media is the digital camera sensor.  There are three factors that affect the exposure and they are the sensitivity of the sensor, the amount of light coming through the lens, and the duration of time that the lens is open. Each of these elements of the exposure triangle has impact on the image, each has a different effect on the way the final image looks, and each adjustment has pros and cons influencing its use in manual modes of exposure.  Let's look at each element and its pros and cons.

In the days of photographic film, the first decision a photographer had to make was what kind of film to load into the camera and, very importantly, how sensitive or "fast" was the film in recording the image. In the olden days, film speed was measured by the ASA of the film.  Many remember good old Kodachrome-64 and Kodachrome-25 used for 35mm slides, and Panatomic-X and Tri-X used for black and white photos. Over time, the ASA (American Standards Association) speeds of film were converted to ISO (International Standards Organization) terms with higher numbers meaning greater sensitivity of the film and faster capture speeds. Tri-X film was very fast with an ASA or ISO of 400 (and even higher if special darkroom processing was used.) Fast film (high ISO) could record images in less light or faster shutter speeds needed to capture sports images. The problem was that high ISO meant more graininess in the image and less sharpness for large reproductions or prints.  The same ISO is used to describe the sensitivity of a digital sensor, and the same sort of problems occur when a sensor is made faster by increasing the adjustable ISO exposure setting.  The major pro for using a higher ISO is that you can use a faster shutter speed to freeze motion or can shoot in darker conditions without supplemental flash. The con of increasing ISO is that when the sensor is made more sensitive, digital "noise" (similar to film graininess) can be introduced into the image.  For the clearest image with the least digital noise we want to shoot using the lowest ISO setting on our camera - usually 100 but it may be 50 or 200 depending upon sensor and camera quality.

As you have gathered by now, the second determinant of exposure is shutter speed. Shutter speed tells us how long the shutter is open allowing light to reach the sensor.  Shutter speed is generally expressed as fractions of a second but may extend into seconds or even minutes.  The shutter speed of old manual film cameras was usually about 1/60 to 1/100 of a second. This speed is often fast enough to capture a still subject image without blur caused by camera shake.  But what if the subject is moving as a bird in flight? Usually it takes a much faster shutter speed to freeze the bird's wings - maybe 1/500, 1/1000, or 1/2,500 of a second. The faster shutter speed will freeze the motion but if other factors of the exposure triangle do not change, there will not be time for enough light to hit the sensor to produce a proper exposure. This is the concept of exposure balance. If one factor changes one or both of the other factors must change in the opposite direction.

The third element of the exposure triangle is aperture.  The aperture is the size (diameter) of the lens opening that allows light to reach the sensor. Aperture is the most complicated and, therefore, the most useful, of the exposure adjustments. Aperture is measured by the f-stop setting. A larger f-stop number causes a smaller aperture. What? That is right, a larger number causes a smaller aperture and less light reaching the sensor. How can that be? The reason for this has to do with the definition (and calculation) of the f-stop. The f-stop is not a whole number but rather is a fraction relating the smallest diameter of the light pathway in the lens to the focal length of the lens. Starting to sound complicated yet?  Well, it's not complicated. Focal length of a lens basically tells you the length from the optical center of the lens to the sensor. A longer focal length produces a larger image on the sensor (more about focal length in the next posting.) A "standard" 50mm lens with an optical diameter of 25mm has a maximum aperture of f/2 (e.g., 50 / 25 = 2.)

The iris of the lens is a cluster of metal leaflets that can open or close to change the diameter of the light pathway and, therefore, the f-stop.  If the iris of the 50mm lens described above is closed down to 12.5mm, the resulting f-stop is changed to 50 / 12.5 or f/4. If the iris is reduced further to 6.25mm the f-stop is now f/8. So you can see that by knowing the f-stop is actually a fraction, a greater number will cause a smaller aperture and less light to the sensor. We will discuss the impact of f-stop on the size of lenses in the next posting. For now, recognize that a smaller f-stop means a larger aperture and a bigger lens for any given focal length. 

What are the pros and cons of changing aperture? The most important effect of changing the aperture of a given focal length lens at a given distance from the subject is the depth of field (DoF) or the effective range of focus. You will get the greatest depth of field at the smallest aperture (other factors being constant.) So if you are shooting a nice landscape with a perfect foreground object or person close to the camera, you would want a small aperture (larger f-stop) to get the greatest depth of field for the image and keep everything in focus. On the other hand, if you want to isolate a close-up of a flower from its background, you want a large aperture (smaller f-stop) to create a shorter depth of field to softly blur the unwanted background.

So to summarize, if you need to increase exposure in order to capture an image in low light conditions, you can increase the sensitivity of the sensor (ISO), decrease the shutter speed (a longer duration of exposure), or increase the aperture (a smaller f-stop.) It takes experience to decide which adjustment(s) to make to produce the desired effect on the image as well as the exposure. There will be more on all of this in future postings.

The Exposure Triangle

Every photographer knows that proper exposure is essential for a good photo but far too many picture-takers rely on the Auto or Program modes for exposure control and don't realize the creative possibilities of using manual exposure modes. So what do the auto modes actually adjust to obtain a "proper" exposure and why would anyone want to regress to manual modes?

Exposure is a poorly defined term that describes the effect of light on the media used to capture an image. Today the media is the digital camera sensor.  There are three factors that affect the exposure and they are the sensitivity of the sensor, the amount of light coming through the lens, and the duration of time that the lens is open. Each of these elements of the exposure triangle has impact on the image, each has a different effect on the way the final image looks, and each adjustment has pros and cons influencing its use in manual modes of exposure.  Let's look at each element and its pros and cons.

In the days of photographic film, the first decision a photographer had to make was what kind of film to load into the camera and, very importantly, how sensitive or "fast" was the film in recording the image. In the olden days, film speed was measured by the ASA of the film.  Many remember good old Kodachrome-64 and Kodachrome-25 used for 35mm slides, and Panatomic-X and Tri-X used for black and white photos. Over time, the ASA (American Standards Association) speeds of film were converted to ISO (International Standards Organization) terms with higher numbers meaning greater sensitivity of the film and faster capture speeds. Tri-X film was very fast with an ASA or ISO of 400 (and even higher if special darkroom processing was used.) Fast film (high ISO) could record images in less light or faster shutter speeds needed to capture sports images. The problem was that high ISO meant more graininess in the image and less sharpness for large reproductions or prints.  The same ISO is used to describe the sensitivity of a digital sensor, and the same sort of problems occur when a sensor is made faster by increasing the adjustable ISO exposure setting.  The major pro for using a higher ISO is that you can use a faster shutter speed to freeze motion or can shoot in darker conditions without supplemental flash. The con of increasing ISO is that when the sensor is made more sensitive, digital "noise" (similar to film graininess) can be introduced into the image.  For the clearest image with the least digital noise we want to shoot using the lowest ISO setting on our camera - usually 100 but it may be 50 or 200 depending upon sensor and camera quality.

As you have gathered by now, the second determinant of exposure is shutter speed. Shutter speed tells us how long the shutter is open allowing light to reach the sensor.  Shutter speed is generally expressed as fractions of a second but may extend into seconds or even minutes.  The shutter speed of old manual film cameras was usually about 1/60 to 1/100 of a second. This speed is often fast enough to capture a still subject image without blur caused by camera shake.  But what if the subject is moving as a bird in flight? Usually it takes a much faster shutter speed to freeze the bird's wings - maybe 1/500, 1/1000, or 1/2,500 of a second. The faster shutter speed will freeze the motion but if other factors of the exposure triangle do not change, there will not be time for enough light to hit the sensor to produce a proper exposure. This is the concept of exposure balance. If one factor changes one or both of the other factors must change in the opposite direction.

The third element of the exposure triangle is aperture.  The aperture is the size (diameter) of the lens opening that allows light to reach the sensor. Aperture is the most complicated and, therefore, the most useful, of the exposure adjustments. Aperture is measured by the f-stop setting. A larger f-stop number causes a smaller aperture. What? That is right, a larger number causes a smaller aperture and less light reaching the sensor. How can that be? The reason for this has to do with the definition (and calculation) of the f-stop. The f-stop is not a whole number but rather is a fraction relating the smallest diameter of the light pathway in the lens to the focal length of the lens. Starting to sound complicated yet?  Well, it's not complicated. Focal length of a lens basically tells you the length from the optical center of the lens to the sensor. A longer focal length produces a larger image on the sensor (more about focal length in the next posting.) A "standard" 50mm lens with an optical diameter of 25mm has a maximum aperture of f/2 (e.g., 50 / 25 = 2.)

The iris of the lens is a cluster of metal leaflets that can open or close to change the diameter of the light pathway and, therefore, the f-stop.  If the iris of the 50mm lens described above is closed down to 12.5mm, the resulting f-stop is changed to 50 / 12.5 or f/4. If the iris is reduced further to 6.25mm the f-stop is now f/8. So you can see that by knowing the f-stop is actually a fraction, a greater number will cause a smaller aperture and less light to the sensor. We will discuss the impact of f-stop on the size of lenses in the next posting. For now, recognize that a smaller f-stop means a larger aperture and a bigger lens for any given focal length. 

What are the pros and cons of changing aperture? The most important effect of changing the aperture of a given focal length lens at a given distance from the subject is the depth of field (DoF) or the effective range of focus. You will get the greatest depth of field at the smallest aperture (other factors being constant.) So if you are shooting a nice landscape with a perfect foreground object or person close to the camera, you would want a small aperture (larger f-stop) to get the greatest depth of field for the image and keep everything in focus. On the other hand, if you want to isolate a close-up of a flower from its background, you want a large aperture (smaller f-stop) to create a shorter depth of field to softly blur the unwanted background.

So to summarize, if you need to increase exposure in order to capture an image in low light conditions, you can increase the sensitivity of the sensor (ISO), decrease the shutter speed (a longer duration of exposure), or increase the aperture (a smaller f-stop.) It takes experience to decide which adjustment(s) to make to produce the desired effect on the image as well as the exposure. There will be more on all of this in future postings.

Camera features helpful for nature photography

The first few posts on this blog were what I would call introductory and general postings about the site and its photographs. In this and the next several postings I hope to address specifics about the equipment, techniques, and processing that are fundamental to nature photography. Since there are many experienced photographers, professionals and advanced amateurs, viewing this site, I hope the posts will generate questions, comments, and discussion - so please, join in. 

Selection of a camera is about as fundamental as it gets when you want to take pictures of our natural world. While the brand of camera is not so important, the features available and the quality of the camera are crucial for good image capture. In other words, a good camera can be used poorly and result in mediocre photos but a bad camera, generally, cannot produce good photos. Having said that, there are many good point-and-shoot cameras and even mobile phones that can take very good photos, but at some point all serious nature photographers will want to take the plunge to a digital single lens reflex (dSLRcamera. Why do I think this is true?

The dSLR gives the photographer tremendous flexibility and choices when photographing in a variety of conditions and terrain. So what are the characteristics of a dSLR camera? The major difference between a dSLR and a point-and-shoot or super-zoom camera is the availability of multiple, interchangeable lenses (and the internal mechanics that allow this.) Interchangeable lenses give the photographer the choice to determine perspective, range of focus (depth of field, DoF), and composition of a photo. Changing lenses changes the "feel" of a photo and not just the sense of closeness to the subject.  Modern dSLR cameras can use standard lenses, wide-angle lenses, telephoto lenses, and a variety of specialty lenses used in creative photography. Manufacturers of the lenses offer a variety of quality and, therefore, price of the lenses to allow lower cost investments initially and higher cost investments for higher quality as skills progress. Lenses are the most important and most expensive part of a nature photographers equipment and I will use a later posting to discuss lens selection in greater detail.

Other features in a dSLR camera that offer flexibility and creative control include the ability to move from automatic control of the camera settings to manual control of virtually all of the settings. You might think that automatic control is a good thing, and sometimes it is, but being able to adjust the settings manually can dramatically alter the quality of your photos. The three components of proper exposure of a photo can each be controlled manually on a dSLR camera. The aperture size determines how much light passes through the lens to the image sensor. Aperture, as we will discuss in more detail in the next posting, also is a key component of the range of focus or depth of field (DoF) in an image.  Shutter speed determines how long the sensor is exposed to light coming through the lens. Shutter speed is another key component of exposure but also determines the effects of movement (either the subject or the camera) on the image. The third determinant of exposure that can be controlled on a dSLR camera is the ISO or the sensitivity of the sensor to light. These three components of exposure (aperture, shutter speed, and ISO) will be discussed in depth next time.

Another setting that can be adjusted automatically or manually on a dSLR camera is the focus. The ability to focus manually is crucial to the nature photographer who often has to shoot through a cluttered scene. Most auto focus systems adjust the lens to the point of greatest contrast of the nearest object in the area of focus. Sometimes this is the subject of the photo and sometimes it is a tree branch in front of the subject. With a dSLR camera you can focus manually or you can choose the point in the viewfinder that the camera will choose as the primary subject. Most dSLRs allow multiple ways to select the focus automatically as a single point, from multiple points, and even from points tracking a moving subject. Since focus is one of the five ways to ruin a photo, this flexibility is needed in nature photography.

Auto-focus speed is another key factor in selecting a dSLR camera. Shutter lag was a technical problem introduced by digital photography. There is nothing worse than pressing the shutter button and having the camera take the photo a third of a second after the bird took off! Shutter lag is still common in point-and-shoot and super-zoom cameras (although it is improving) while quality dSLRs capture the image when you press the shutter release.

Most dSLRs offer the ability to shoot in high speed bursts of images. This is very helpful when shooting birds in flight or a rapidly moving animal. The burst rates have increased from about 3-4 frames per second to well over 12 fps. This feature is useful in sports photography as well as nature photography.

While image sensors are improving rapidly in all cameras, dSLRs tend to have the largest sensors and the highest resolution in each manufacturer's product line. Sensor size and resolution are key components to high quality images and the ability to print these images at a large size. While the pixel count is highly touted in camera ads, it is really the sensor size and pixel size that determine resolution of the image. Larges sensors that approximate the size of 35mm film are referred to as full-frame sensors while smaller sensors are called crop sensors. There are advantages and disadvantages of each but dSLR cameras seem to be moving towards larger, full-frame sensors, even though the cost is typically higher.

The last advantage of a dSLR camera over others tends to be the quality of the construction. The dSLRs tend to be larger, heavier, and have a ruggedness with more metal rather than plastic construction and better seals to prevent dust and water from entering the camera and damaging the sensor or internal mechanics of the camera. These are important to the nature photographer but also create problems of greater bulk and weight when photographing in remote locations.

So like everything in life there are trade-offs when selecting your "perfect" camera. Features are critical, quality is essential, but there are many good quality cameras available that, when coupled with a quality lens, will produce amazing photographs.