Sense and Sensitivity

Sensitivity (ISO) in digital imaging seems to be the subject of quite a lot of confusion - it’s becoming common to hear talk of manufacturers 'cheating with ISO.' So we thought it made sense look at why sensitivity appears hard to pin down, why we use the definition we do and how it’s actually not as complicated as it can sometimes seem.
ISO in Photography

Before we get too carried away with the intricacies of ISO standards, it makes sense to step back and consider how we use sensitivity in photography. Sensitivity is the connection between the physical exposure (how much light you let in) and the brightness of the final image. As such it joins shutter speed and aperture as one of the three factors that define exposure.
Sensors and sensitivity

For ISO to relate exposure to final image brightness, we have to think about the inherent sensitivity of the digital sensor and this is where it risks becoming rather removed from photographic concerns.

Much of the complication arises from the fact that there is no 'correct' way of exposing a sensor. Sensors have a capacity for converting light into electrical charge - limited at the upper end by the point at which the sensor becomes saturated (and cannot convert any more photons of light into electrical charge) and extending down until the signal is drowned-out by electrical noise. The upper, saturation limit of the sensor’s response defines the brightest light intensity that can be turned into meaningful data in the final image. However, this doesn’t tell us much about how to expose the sensor - simply exposing to retain the brightest highlights won’t necessarily ensure a correctly exposed image – we need to work out how to

And there is an added complication before we can get to that point. Sensors respond to light in a very different way to the human visual system - they respond in a linear fashion: twice the amount of light gives twice the signal, whereas the brain doesn’t interpret things that way. In order to make this linear data into a convincing image, a tone curve that attempts to map the data back to the way the eyes respond has to be applied.

This tone curve converts the sensor’s output to the final image brightness, which means it also defines how the sensor needs to be exposed. (In fact there is a subtle interplay between the sensor’s inherent sensitivity, its dynamic range, the tone curve and the camera’s metering.)
A standard with shades of grey

So this is what ISO is defining when you use it: it’s combining considerations of the sensor’s sensitivity with the effects of the tone curve and metering so that you can get the correct final image brightness with your chosen exposure.

However, the standard set down by the International Organization for Standardization (ISO12232:2006, as it happens), contains five separate definitions, each of which can produce a different answer for the same camera. Thankfully, only three of these definitions are widely used and only two, closely-related definitions are used by camera makers.
ISO, courtesy of CIPA

The two definitions of ISO that are actually used by camera manufacturers (and are reported by their cameras) are based on the brightness of cameras’ JPEG output. Both definitions come from standards developed by the Japanese camera trade body CIPA, which were adopted by ISO in 2006. The first definition is probably the simplest and most intuitive, and it’s called Standard Output Specification. Essentially, it defines ISO as the camera behaviour that renders middle grey at the correct brightness (as we’ve just described and pretty much the same way as it did for film).correctly expose a middle grey.

The other definition (Recommended Exposure Index) is fairly similar but is designed to accommodate multi-zone/pattern metering systems. These metering systems aren’t based on trying to represent middle grey and instead aim to achieve whatever the manufacturer considers to be ‘correct’ exposure. As such they can’t be measured because the definition is pretty much circular: whatever the camera chooses is right, by definition.
So what about the others?

The only other definition of ISO you’re ever likely to encounter is one that can be used for RAW data. The problem is that it’s based on a combination of the sensor’s saturation point and a generic tone curve – which isn’t necessarily the tone curve your camera’s JPEGs or metering are based on. So, discrepancies between this figure and your camera’s reported ISOs aren’t the result of under or over-reporting of ISO, they’re a measure of how different your camera’s tone curve is from this generic tone curve.
Why do I need to worry?


If you use the camera’s JPEGs, or a RAW converter that acknowledges the manufacturer’s rendering intent (and that includes many popular RAW converters), then chances are you’re going to get the ISO that your camera tells you. So rather than measuring a slightly obscure aspect of sensor performance, our tests are based on the Standard Output Specification that the camera manufacturers use, that your camera is based on and that, chances are, you use.

Lighting on location

On location
Studio lights need not be limited to the studio. They can be used on location in conjunction with ambient and existing daylight. With color correction of the light source or camera most light sources can produce acceptable and interesting results. Raw file images can be color corrected in post production. See ‘Light’ and
Exterior location
Common examples of studio lighting used on location are the images seen in film and television. The same approach can be taken to still images. Artificial light, whether flash or tungsten, is normally used to supplement the existing light present, usually daylight. In this situation correct color is achieved by balancing to daylight (5500K) and filtering the tungsten light source/sources (3200K) with an 80A lighting gel. When using studio flash on location no filtration is required as the color temperature of the flash is equal to average daylight (5500K to 5800K).
Mixed light
Mixing the color temperature of the light sources can give a more ‘natural’ look. Although the human eye corrects all light sources to what appears to be white light, it is visually accepted, and in most cases to great effect; when we view images created using mixed light sources there should be a difference in the color temperature of the various sources of light within the frame.

Interior location
There can be many different light sources with varying color temperatures confronting the photographer on location. This can range from industrial lighting to the glow from a TV. The possibilities and variations are many but the problems they pose can be either corrected with filtration or white balance adjustment to render ‘correct’ color, or ignored and the differences in color temperature exploited and used to effect.
Daylight balance
In an exterior location all light sources are balanced to the predominance of daylight (5500K). With an interior location (for example, a furnished room with large windows) there can be a mixture of various light sources. Balancing to daylight (5500K) without filtration of the tungsten light sources (3200K) would make the image appear quite different. Daylight in the image would appear ‘correct’ but any tungsten light source whether artificially introduced (studio) or practical (normal domestic lighting, desk lamps, candles, etc.) would create a warm glow at its source and on subject matter predominantly lit by it. The overall effect would be of white light through the windows, and depending upon the lighting ratio created between the tungsten light and the ambient daylight an overall warm cast to the image.
Tungsten balance
If the camera is balanced to tungsten (3200K) without filtration of either the daylight (5500K) or the tungsten light sources (3200K) the result would appear different again. Daylight in the image would appear to have a blue cast and any tungsten light source would appear ‘correct’. The overall effect would be of blue light through the windows, and depending upon the lighting ratio created between the tungsten light and the ambient daylight, a balance of ‘correct’ color within the room. It should be remembered filtration of the camera to match the dominant light source would also produce similar results. However, filtration of the camera removes the possibility of selectively filtering the various light sources and color temperatures.

Balance

In nature there is a natural balance or harmony of texture, shape, form and color. Manyobjects upset this balance and impair the visual relationship between one object and another.It is this control of balance by the photographer, whether to achieve harmony or discord, thatdetermines the level of acceptance of an image by the viewer. As humans we naturally gravitate towards a balanced image (symmetrical).

When there is symmetry between the elements within the frame the image is said to have a sense of balance. A balanced image although pleasing to the eye can sometimes appear bland and conservative. Knowing this a photographer can change the balance of an image to achieve a different result. A dominant element of balance is visual weight created by the distribution of light and dark tones within the frame. To frame a large dark tone on one side of the image and place tones of equal visual weight on the other side will create an imbalance. An unbalanced image (asymmetrical) will often create visual tension,
interest and a sense of things not being as they should be. The communication of harmony or tension is the deciding factor when composing an image intended to convey a specific message.

lighting still life

The assignments are written with the assumption that the aspiring photographer has a limited knowledge of the practical application of studio lighting. It is important the assignments are completed in the order in which they are written. Each lesson learnt from one applies to the assignment that follows. To attempt to produce the assignments out of sequence will mean certain information relating to lighting development and metering techniques will be missed.

The criteria set out in each assignment should be followed as closely as possible as it forms a major part of the practical learning process. Where possible it is advisable that each assignment be workshopped at a group level with guidance and direction. Individuals should then attempt to complete their own interpretation of the brief within a specified period.

The assignment brief is itemised into specific requirements. These requirements must be successfully completed as a basis for any assessment of the completed image. Interpretation of the brief beyond these requirements is encouraged but not at the expense of the basic criteria. All information relevant to the completion of each assignment should be compiled in the Record Book.

Health and safety

Power supply

It cannot be stressed strongly enough that the lighting equipment and studio power supply be either installed, or checked in the case of existing supply, by a qualified and licensed electrician. Without question working with powered light sources is dangerous.
As a photographer it is inevitable that light sources are taken for granted and unfortunately familiarity leads to complacency and poor safety practices. Always observe a few simple rules.

• Electricity is dangerous. It can kill you.
• Never attempt to repair lights or wiring unless you are absolutely confident you know
• what you are doing.
• Always be cautious when moving or connecting lights.
• Always turn off the power to the flash pack when changing flash head outlets.
• Use heat resistant gloves when handling tungsten lights.
• Always turn off the power and disconnect the cable before changing a globe.
• Never touch any part of a light or cable with wet hands.
• Exercise extreme care when photographing liquids.
• Never use liquids near electricity.
• Wear shoes with rubber soles.
• Ensure you know where and how to use the first aid kit.
• Ensure you know where and how to use the fire extinguisher.
• Ensure you are aware of emergency procedures related to work area.
• Ensure adequate ventilation of the studio area.

the studio

Essential skills
• An understanding of the use of artifi cial light sources, camera and associated
equipment in a studio environment.
• An awareness of the equipment and organisation required for the photographic
control of lighting ratios, contrast and exposure.
• The study and observation of the importance of the studio in the production of
photographic images.
• The compilation of reference and visual information infl uencing the approach taken
to produce the photographs for each activity.
• To produce photographic images and collate information relevant to the technique
and production of each photograph.

Introduction
Studios range in size from small areas surrounded by black curtains to large film stages in Hollywood. The instant photographic booths found in many public areas are miniature studios.They are an area devoid of external light in which there is a controlled light source. This is thebasis of any photographic studio. Size is not as important as efficiency. To set up a studio that will function within the requirements of this book need not be a complex or unachievable task.

Size
A floor area for each photographer working with camera, lights and table-top set-up shouldbe approximately 6m x 6m, with a working height of 4 metres. This is an ideal minimum. The reality is sometimes different. Whatever size can be achieved it is important to ensure the area is uncluttered and free of anything that could cause injury. Bear in mind that other than the lit subject the studio area will be in almost total darkness.

Power
After determining size the most important criterion is the supply of power. Ensure it is safe.Have a qualified electrician install sufficient power (amount of current-amps, and number of outlets) for the equipment to be used. An imperative safety factor is the installation of circuit breakers (breaks power circuit at the instant of any electrical fault). Distribution boards (the supply is divided into multiple outlets) with overload switching facilities (breaks delivery of current to the equipment being used) is also recommended as an extra precaution. Also make sure the normal lights within the studio can only be turned on or off from within the studio and there is adequate ventilation.

Darkness
The only light in a studio should be created by the photographer. To achieve this blacken outthe entire work area. This can be done with dark heavy curtains over windows and painting the walls and ceiling a dark matte gray. Where possible the floor color should also be dark. The result should be an area with no external light entering and surfaces of minimum reflectance.Work areas within a large studio should be separated from each other by non-reflective curtains
so more than one photographer can be working at a time.

Scenic Photography

The history of scenic photography is as old as photography itself. The first known permanent photograph, taken in 1826 by French chemist Nicéphore Niépce, is a view of the courtyard outside his laboratory. Because early photographs required exposures that were extremely long (Niépce's scenic required an eight-hour time exposure in full daylight), stationary subjects like trees and mountains were perfect pictorial fodder. When early photographers weren't strapping our ancestors into chairs for painfully tedious portraits, they were creating scenic photographs.

Of course, we've come a long way since then, both in the way photographs are taken, and the esteem in which they are held. That first photograph was made using a pewter plate coated with asphalt, and when Niépce submitted the "heliograph" to England's King George IV and the Royal Society, it was rejected. Today, the same photo could be exposed onto a solid-state sensor in 1/16,000th of a second (the fastest shutter speed on the latest digital cameras), and photographs are more highly valued. For example, a signed 16 x 20 print of Ansel Adams' "Monolith, Face of Half Dome" will set you back $37,000 at the Ansel Adams Gallery (www.anseladams.com, if you'd rather have the photograph than a new SUV).

Scenics are a great way to practice your digital photography skills. Vacations are a good time to investigate this type of photography. You're relaxed. You're out to have fun. You definitely have your digital camera along with you to document the sights and sites that you've paid thousands of dollars to visit. Even if you're on a whirlwind 14-city tour of Europe, you'll find that between each of those cities are scenes created by Mother Nature that deserve your artistic attention. This chapter focuses on several different varieties of scenics and offers some tips for getting some interesting shots of landscapes, sunsets/sunrises, fireworks, and other types of scenic images.

PRACTICE: PORTRAITS & OTHER STILL PHOTOS

Still photos are best taken using the one-shot autofocus mode, which ensures that a focus lock has been achieved before the exposure begins. The usual focus point requirements of contrast and strong lighting still apply, although one needs to ensure there is very little subject motion.

For portraits, the eye is the best focus point—both because this is a standard and because it has good contrast. Although the central autofocus sensor is usually most sensitive, the most accurate focusing is achieved using the off-center focus points for off-center subjects. If one were to instead use the central AF point to achieve a focus lock (prior to recomposing for an off-center subject), the focus distance will always be behind the actual subject distance—and this error increases for closer subjects. Accurate focus is especially important for portraits because these typically have a shallow depth of field.

Since the most common type of AF sensor is the vertical line sensor, it may also be worth considering whether your focus point contains primarily vertical or horizontal contrast. In low-light conditions, one may be able to achieve a focus lock not otherwise possible by rotating the camera 90° during autofocus.
In the example to the left, the stairs are comprised primarily of horizontal lines. If one were to focus near the back of the foreground stairs (to maximize apparent depth of field using the hyperfocal distance), one could avoid a failed autofocus by first orienting their camera in landscape mode during autofocus. Afterwards one could rotate the camera back to portrait orientation during the exposure, if so desired.

Note that the emphasis in this tutorial has been on *how* to focus — not necessarily *where* to focus. For further reading on this topic please visit the tutorials on depth of field and the hyperfocal distance.

action photos

IN PRACTICE : ACTION PHOTOS

Autofocus will almost always perform best with action photos when using the AI servo or continuous modes. Focusing performance can be improved dramatically by ensuring that the lens does not have to search over a large range of focus distances.

Perhaps the most universally supported way of achieving this is to pre-focus your camera at a distance near where you anticipate the moving subject to pass through. In the biker example to the right, one could pre-focus near the side of the road since one would expect the biker to pass by at near that distance.

Some SLR lenses also have a minimum focus distance switch; setting this to the greatest distance possible (assuming the subject will never be closer) can also improve performance.
Be warned, however, that in continuous autofocus mode shots can still be taken even if the focus lock has not yet been achieved.

Light and Color in Photography

Light is one of a photographer’s most important considerations. In fact, any photograph captures the effect of light on objects, whether taken in natural light or the warm glow of incandescent light. A photographer who understands how the eye and lens perceive the color spectrum can make full use of light in photography.
The Color Spectrum

Both the natural light of the sun and artificial incandescent light appear white to the naked eye. However, if you shine light through a prism, it splits into a rainbow of colors. This effect shows how the “color” white contains every shade in the color spectrum.

When light hits an object, the object absorbs some of the color spectrum and reflects back the rest of the colors. The portion of the color that is not absorbed by the object but reflected back to eye is the color that the object appears to the human eye. For instance, a white wall reflects most of the color spectrum black. So our eyes (and the camera lens) see white. While a red rose reflects the red portion of the color spectrum and absorbs the rest, a black object absorbs the entire color spectrum.

Natural Light in Color Photography
Natural light in photography is not as consistent as you may think. At different times of the day, different shades of the color spectrum dominate natural light. For instance, at midday, the blue portion of the color spectrum is dominant, producing a “cool” light. Color photography taken at midday produces the clearest, sharpest pictures in bright light.

In contrast, natural light at sunrise and sunset emphasizes the red portion of the color spectrum. Known as warm light in photography, sunrise and sunset light produces warmer pictures with a softer contrast.

Artificial Light in Photography
When photographers use artificial light in photography, the effect of the lighting depends on the type of bulb used. The warm dispersed glow of incandescent light produces an effect that is dramatically different from that produced by the sharp, narrow, focused beam of a street light.

Photographs taken with incandescent light often have a yellow tinge. A photographer can combat this tint in two ways. One method is to use a blue filter to offset the excessive yellowing. If you’re using a film camera instead of a digital one, you can use tungsten film that is designed for professional studio lights. Although it wasn’t intentionally designed with incandescent light in mind, tungsten film reduces the yellow tones so often found in indoor color photography.
Florescent lights produce a diffuse light that often translates into greenish tinges in color photography. Using a fluorescent or daytime filter helps remove this unappealing color.

Street lamps, like florescent lights, may cause a greenish tint in the resulting pictures. However, a photographer can use this effect to his advantage: in the darkness, this greenish quality can produce mysterious or ghostly effects.

Diffuse and Direct Light in Photography
The direction of light in photography is important: different angles of light produce different shadows, changing the appearance of your subject.
Light in photography may be diffuse or direct. Direct light, such as light from the noon sun, hits the subject from one direction. If you’re looking for high contrast between light and shadows, direct light is a good choice.

On the other hand, diffuse light hits the subject from several directions. Florescent lighting is one example of diffuse light in photography. The loss of contrast that diffuse light produces mutes colors and softens the image.
Digital Cameras and Light in Photography
Digital cameras come with specialized light settings. Here are some of the common settings:

• auto: when you want the camera to adapt to changing light
• cloudy: for outdoor use in cloudy or overcast skies
• daylight or sunny: for outdoors and bright sunlight
• flash: for taking pictures with the flash
• fluorescent: for scenes lit by fluorescent lights
• incandescent or tungsten: for incandescent lighting
• manual: when you want to manage the lighting of each shot. This requires that the photographer aim the camera at white paper to change the lighting settings manually.

Color Photography or Black and White?
While lighting is important for color photography, it is even more important for black and white or sepia photography. While this fact seems counterintuitive, the lack of color means that the key feature of black and white photography is the contrast between light and shadow.

The basic rules of light in photography apply to black and white photos just as they do to color photography. For example, direct light creates a higher contrast than diffuse light. Because the contrast between light and shadow is much more obvious in black and white than in color photography, the photographer should opt for direct light when composing the shot.