Digital Is Different: Part I

It certainly is true that photographs are still photographs, whether you shoot film or digital. In many ways, digital photography is identical to traditional film photography. Both use cameras that consist of a light-tight box with a lens to focus an image onto a light-sensitive medium. Both have ways to control the light coming in (exposure) and a viewfinder for the photographer to compose the image.

However, the experience of digital is different, as we’ve just discussed, and the capturing of images does change a bit with digital photography. Today, digital cameras are everywhere; they are even built into phones! But when they first caught the attention of photographers, a lot of people wondered what they might mean for photography. Would they be too different? The first digital cameras actually had designs that were much too “unusual,” seeming to spring from the minds of designers who had little idea of what a camera was used for or what it needed to be.

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People wondered if digital cameras meant learning something totally new. Did this technology mean that everything you knew about film was no longer true? Would digital replace film? Would something be lost if it did? Most of these fears are long gone. Digital cameras look and act like traditional cameras in many ways.

There was another barrier to digital camera acceptance that you still hear about today even though it is no longer true. Many technically minded people did the math relating to image resolution and concluded that true photographic quality required a camera with many more mega pixels than anything available. It didn’t help that the earliest digital cameras had terrible resolution, yet the folks marketing them hyped these cameras as having “high resolution.”

As photographers began to work with digital cameras, however, they discovered that this film-biased evaluation of resolution was incorrect. There was supposedly no way a 3-megapixel camera could make an 8 X 10 print that matched the quality of film — but when a print was made, photographers found that a 3-MP camera could produce absolutely beautiful, true photo-quality 8 X 10 inch prints. It’s true that if you put some of these pictures up close to your eye you might find some artifacts or image noise, but photographs are never meant to be viewed that way.

Since of an image, by the way, is always an important issue with digital cameras. Resolution is quite incomplete and rather meaningless without a reference to how an image will used and the size at which it will be presented. Now we are seeing amazing 16 X 20 inch prints from 8-megapixel cameras. The ideas presented in this book can help you do that, too.

Sensor vs. Film

The light-sensitive sensor is a feature of digital cameras that film cameras don’t have. It is like film in that it also reacts to light. However, a big difference is that film both senses the light and stores the image on its light-sensitive surface. With digital, the sensor “reads” the light, but the camera translates that information into digital data for storage elsewhere.

This is certainly interesting, but probably more useful for trivia contests than for picture taking. However, it is important to understand how a sensor responds to light and how it differs from film. Film “sees” detail equally when the light is in the middle of its range (where that middle is for a given scene depends on exposure). Film changes, though, in its response to the lightest and darkest parts of the image, gradually blending the middle tones into black and white, creating a very smooth tonal change.

Digital cameras tend to see light in a more linear way. This response, however, is quite variable, not because of the sensor, but because of proprietary image processing circuits that manufactures put into their cameras. The sensor typically captures all tones throughout the exposure range, mid-tones may be seen the same as blacks and whites. However, there is much less of the tonal gradation seen in film’s bright areas. Because of this, bright highlights in digital images often exhibit a sharp transition of tones in highlight areas. (the tonal gradation exhibited by film is missing.) An abrupt cut-off of detail in very bright areas is often a five-away that a digital camera was used.

This is important for photographers—we can expect overexposed highlights to lose detail quickly with digital cameras. This is similar to the way that slide film records highlights. It is interesting, however, that digital cameras frequently capture more detail in dark areas than slide film, making them react more like print film in the darker areas of the image. You will be surprised, if you are used to slide film, to see more detail in the shadows.

Note: With small digital cameras and their associated small sensors, dark areas can have more noise than the other areas of the photo.

Sensor Size Affects

Focal Length

A point-and-shoot digital camera has a very small sensor area compared to a frame of 35mm film. Typically, sensors are approximately 9 X 6.6 mm (.35 X 26 inches), or 2/3 inch across diagonally. Many of the ultra compact cameras have sensors that are even smaller. This is a fraction of the size of 35mm film, which has a frame size of 36 X 24mm or 1.5 X 1 inches.

The area of coverage of a lens is independent of its focal length and is directly related to the film format or sensor size. With a 35mm SLR camera, a 50mm lens is considered a ‘normal’ lens. It produces an image on the film that approximates a person’s field of vision. When used with a small sensor, a 50mm lens becomes a telephoto lens recording only a narrow angle of view.

This means that a shorter-focal-length lens (wide angle lens) must be used to give a normal angle of view for a digital camera. To get the 35mm film equivalent of a 28-80mm lens, for example, the camera might have a lens with an actual focal length of 7-20mm, That would be an extremely wide-angel lens for a 35mm film. This change in focal length affects three things; lens size, f/stop and depth of field.

f/stop - Because the lenses have such short focal lengths, lens apertures get small. In addition, the reduced glass area affects what lens designers can do with f/stop availability. The result is that lenses on small digital cameras often don’t stop down beyond f/8 or f/11. Lens designers limit that choice for quality reasons related to the size of the lens.

Lens size - The lenses on compact digital zoom cameras can be reduced in size because you don’t need as much glass to give an image area that covers a small sensor. This affects the size of the camera as well as the lens, and has no effect on lens quality (other things being equal, such as f/stops and quality of glass used).

Depth of field - Short focal lengths typically give more depth of field. In addition, the smaller physical size of apertures will have an effect of more depth of field, too.

This is a boon for those photos that need a lot of depth of field, but a real challenge if you seek a selective focus (narrow depth of field) effect.

Continue on to Digital Is Different: Part II