|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.
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, https://commons.wikimedia.org/w/index.php?curid=4760480
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: https://genealogysstar.blogspot.com/2019/10/photography-basics-for-genealogists.html