14th Blog Post, First Published on October 2, 2016, as a Newspaper Column.
(Photos and possibly an extended text will be added soon).
Hi, and Welcome to my “HighLight & Shadow” Photo Blog
Professional & amateur photographers, and even the general public, have occasions when we need to understand how to refer to a digital photo’s size; times when reading articles, in casual conversation and more serious discussions, and times we want to submit photos to contests or for publication and we need to follow submission guidelines on photos sizes. However, many people, even people in the business, don’t know how to describe the size of digital photos accurately and that lack of knowledge leads to confusion and even causes mistakes that can be a deal buster for many reasons.
This month, I want to address this basically simple issue, and to talk about a couple terms used when describing the size of digital photos with the intention of helping people talk as easily about the actual size of digital images as we do about the size of analog photos, so we can sidestep incorrect use of terms that are confusing and even nonsensical.
A good starting place is to look at a simplified version of the complex and magical innovation of digital photography, by talking about how digital photos are created which will help us better understand how to correctly discuss the size of digital images.
What is a Digital Photograph?
Digital cameras create photographic images when light from a scene is focused on, and then recorded by, the camera’s sensor or chip. The chip is a grid of thousands of columns and rows of microscopic, light-sensitive photo sensors or pixels. Each pixel records the intensity and color of light that strikes it during the exposure. When that pixel’s recorded light is combined with the intensity and color of light recorded by the other millions of pixels in the grid, the result produces an image that accurately represents the scene the lens projected onto the sensor.
A couple weeks ago, I joined a group for a day-hike sponsored by the Wyoming Wilderness Association in the gently striking Honeycomb Buttes in Wyoming’s Red Desert. Of course, while on the hike, my camera’s grid of millions of individual pixels created numerous photographs including the original digital image of photos A, B and C. The copies here are un-cropped, newsprint reproductions of those three original images. An un-cropped photo is usually referred to as a “full frame” photo, meaning that what the viewer sees is a reproduction of the complete or full image the camera’s sensor created when that image’s exposure was made.
The camera that recorded photo A has a 24-megapixel sensor, or a sensor with 24 million photo sensors. The original, digital photo A was created when each of those 24 million, minute, individual photo sensors recorded the color and intensity of the light that struck them during the shutter’s precisely controlled time that exposed the camera’s sensor to the lens’ focused light. That orderly combination of 24 million separate exposures was then transferred to the camera’s memory card for preservation.
The actual size of the sensor is 6,015 columns by 4,016 rows of photo sensors which, when multiplied together, equals an actual total of 24.160 million pixels in my camera’s sensor. Rounded off, it equals 24 million pixels.
Common Mistakes in Describing a Digital Photo’s Size
Often when talking about digital photos, the most common terms of 72, 150 or 300dpi (dots per inch) are used in describing the size of the photo. Erroneously, the number of dots per inch (72, 150 or 300) is often the only information provided when some people talk about a photo’s size.
There are actually two mistakes with this size description.
First, when talking about a photo that is in a digital state, there are no dots, just pixels, so the correct term is 72, 150 or 300 PIXELS per inch, or ppi. However, because most people use the term dpi when meaning ppi for digital photos, dpi (DOTS per inch) has become the standard acronym for that portion of a photos dimensions. So I’ll use dpi with the numbers 72, 150 or 300, but will refer to the individual “dots” as pixels or photo sensors.
The second mistake is that simply using the terms 72, 150 or 300dpi to describe the size of a digital photo is only telling half the story. The usual way of describing a digital photo’s actual size is to talk about how many total pixels make up the long side of the image, and to determine that size, we need to know how many inches the long side is and what the photo’s dpi setting (72, 150 or 300 etc. dpi) is. Describing a photo’s size by only referring to its dpi setting is like saying that a living room is 12 inches per foot (12ipy), which makes no sense.
So, to determine the length of a digital photo in total pixels, we need to know how many pixels there are per inch (it’s dpi setting) and how many inches and partial inches, there are in the photo. These two numbers are then multiplied together as A x B = C, where A is the dpi setting, B is the number of total inches and C is the total length of the photo in pixels.
Another Way to Describe a Photo’s Size
Another important unit of measurement that describes a photo’s size is the amount of digital memory in kilobytes, megabytes or gigabytes (kb, mb or gb) needed for any version of any photograph. Most photos are more than 1000 kilobytes and less than one gigabyte, so photos are most often described by the megabytes of memory that photo uses to exist in it’s digital state. My experience is that computers are capable of sending or receiving photos via email other means that are less than around 15 to 20mb.
Keep in mind that two photos of the same size in pixels are usually different in the amount of memory both photos use, depending on how visually complex the photos are. Images with a lot of color, contrast, shape and other variety take more memory then photos with a lot of uniformity in color, contrast and shape. So keep this fact in mind and remember that, to know the total number of dots or pixels in a photo’s length, we have to know both the total number of inches in the photo and what resolution (dpi) the photo is set at.
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So till next time, keep your eyes open, your camera handy and your imagination flowing.