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Saturday, October 26, 2019

Photography Basics for Genealogists: Part Two: Digital vs. Analog

Robert James Wallace - Plate VII from "The Silver 'Grain' in Photography" by Robert James Wallace, The Astrophysical Journal, Vol. XX, No. 2, Sept. 1904, pp. 113–122, Chicago. As reproduced by Google Book Search.
We live in an analog world. The term "analog" is defined as relating to or using signals or information represented by a continuously variable physical quantity such as spatial position, voltage, etc. Historically, photographs were made on light-sensitive film. The images on photographic film are created by the chemical process of light interacting with silver halide crystals. See "Film Processing Chemistry, How Does It Work?" for an example. Even though we may not realize it, film photos and the paper prints made from all have a "grain." The size of the individual silver halide crystals determines the "resolution" of the photograph.

The difference between film images and digital images (i.e. analog and digital) is somewhat artificially maintained. A digital image is made when a light-sensitive "sensor" made up of an array of electronic devices that capture photons (light) and convert the light into electronic signals instead of silver halide crystals. See Wikipedia: Image sensor. The main difference between the two processes involves the fact that once the light is converted to an electronic signal, the resultant information can be transmitted, stored, retrieved, and viewed by reconstituting the electronic signal in a display or with some type of printing mechanism.

The graininess or RMS granularity of photographic film is "a numerical quantification of film-grain noise, equal to the root-mean-square (rms) fluctuations in optical density, measured with a microdensitometer with a 0.048 mm (48-micrometre) diameter circular aperture, on a film area that has been exposed and normally developed to a mean." See Wikipedia: Film grain. Translated into English, that means that photographic films vary in the amount of film density and therefore some films are grainier than others.  Here is an image from the same Wikipedia article that shows a grainy image.

By RX-Guru - Own work, CC BY-SA 3.0,
You can get similar issues with digital images because of the limitation of the number and size of the sensors. With digital images, the problem is usually called "pixelation." That is to say that the grainy digital images are usually overlooked because most everyone knows that there is a pixelation limit that appears during the magnification of a digital image. Since the size of the grains in photographic film varies with the film's sensitivity, the more sensitive the film is the larger the grains. There is a rough correlation between the sensitivity or speed of the film and its graininess. Film sensitivity or speed is measured by a scale referred to as the film's ISO (International Standards Organization) number (previously ASA and DIN numbers). A slow film, i.e. ISO 100, is less grainy than a fast film, i.e. ISO 800. A popular slow film was Kodak's Kodachrome and a popular fast film was Kodak's Tri-X Pan.

On the other hand, digital noise or apparent graininess is always related directly to the size of a pixel, regardless of the ISO setting.

If you would like to know how the detail of photographic and digital images compare, you should read this short article, "Film Resolution (Pixel Count)." This article points out that a 35mm film image is the rough equivalent of 175 MegaPixel (MP) digital image. However, actual practice will also be affected by the quality of the camera lens, lighting conditions, the sharpness of the camera's focus and a long list of other factors. Right now, the highest resolution digital consumer camera is the 100 MP Hasselblad H5D-100C camera going for around $33,000.00 for the camera body only.

So why is it so important to digitize photographic images? The answer lies in the issue of the individual nature of photographic analog images. Simply put, each photo is a unique original and loss of the original (negative or positive) is permanent and irreplaceable. However, digital images can be copied instantly and every copy is exactly like the original including any defects.

There is also an important limiting factor about all image resolution digital or analog: the resolution of the human eye which most sources put at about 300 dpi (dots per inch). To reproduce what your eye can see in your total field of view, you would need about 576 MP resolution. But when viewing a photograph that would drop to between 5 and 15 MPs, well within the range of the average smartphone camera or a less expensive digital prosumer camera. See "How Many Megapixels Is the Human Eye?"

The uniqueness of a single photograph has driven a commercial market for art prints of original photographs. Digital images are sold on websites with millions of other photos.

Next: More about Digital vs. Analog

For the previous post, see

Part One:

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