The terminology and technology surrounding digital cameras can be a little confusing. A foundational knowledge goes a long way in cutting through information presented in manufacturer’s specifications and user reviews. This brief technology overview aims to make the reader more familiar with concepts and terms used when discussing digital cameras.
The Basics
A digital camera shares many similarities with old film cameras – light travels through a lens to a light-reactive material to create a representation of what is being photographed. Rather than a roll of film that responds to light to make a negative of an image, however, digital cameras have a sensor that captures data about the quantity of light coming through the lens.
Sensors are typically referred to as “CCD” or “CMOS” sensors. While there are differences in how each of these sensors captures image data, they both are similar in that the captured data is translated into an array of pixels. These pixels have a red, green, and blue (RGB) value associated with them, which show up on the screen as the final image.
The number of pixels that a sensor contains is defined in megapixels, which literally translates to 1 million pixels. A sensor that can detect 3115 pixels on the x-axis and 2336 on the y-axis would create a 7.2 MP image (3115×2336 = 7,276,640 pixels).
To translate this into more meaningful terms, the referenced image’s printed equivalent (often defined as 300 dots-per-inch or pixels-per-inch) would be crisp and clear at approximately 10” x 8”. On-screen, given a monitor of 1024 x 768 pixels, the same image would need to be scaled down approximately 33% to properly fit the display.
This raises one of the first big questions in digital cameras – how many megapixels does a camera need? While it would seem that more pixels are always better, certain problems with “noise” can arise when more light-sensitive elements are packed into a small sensor. At its most extreme, this noise can make an image look grainy, and can reduce the useful detail in an image.
There are no hard and fast rules as to how many megapixels are required for clinically-useful images. Generally, the pixel count of point-and-shoot digital cameras on the consumer market is sufficient for clinical use. A review of the cameras available on Amazon.com in May 2010 shows that, of the 9,860 cameras listed, over 8,000 digital cameras have 5.6 megapixels or more.
The Types
Several different types of digital cameras exist on the market. The cameras reviewed as a part of this toolkit were all in the “point-and-shoot” variety, which are viewed as being highly portable, easy to use, and affordable, with a non-changeable lens and smaller physical sensor size. There are several different categories within the point-and-shoot family, ranging from the ultra-compact models that fit into a pocket, up through the “bridge cameras” that provide larger zoom ranges, bigger lenses, and pop-up flashes.
Digital Single Lens Reflex cameras, or DSLR, are typically larger than their point-and-shoot siblings. These cameras are frequently used by professional photographers and serious hobbyists, and provide an array of manual controls that are typically unavailable on the point-and-shoot models. The sensors are larger, which can provide clearer images in low-light settings. The lenses can be changed by the user, allowing for the possibility of large zoom ranges.
A new variety of camera has been in the market since 2008. Dubbed the “Micro Four-Thirds” camera, these have the flexibility provided by interchangeable lenses and manual controls of a DSLR, with a smaller and simplified internal imaging system that is more common in point-and-shoot cameras. The sensors are typically larger than those provided in compact cameras, but not as large as is available with DSLRs.
Additional camera types exist – “medium format” versions that regularly cost well over $10,000 (USD) and line-scan cameras for use in specialized industries, for example. These cameras are well beyond the scope of this toolkit.
Choosing the correct camera type requires considering who will be performing the imaging. The ease of use of point-and shoot cameras makes them a great fit for people who will not be extensively trained on imaging techniques and camera use. The DSLR and Micro Four Thirds camera may be more appropriate for users who will be comfortable using the available manual features when taking images.
The Files
Images that are captured typically are stored in three different file formats – JPEG (Joint Photographic Experts Group, also “.jpg”), TIFF (Tagged Image File Format), and RAW. Most point-and-shoot cameras store images as JPEG images, using various proprietary algorithms to compress the size of the image while maintaining visual quality.
Compression algorithms help reduce the size of the images being stored by storing less information about the individual pixels in the photo. As each company uses different algorithms, photo quality and size can vary from camera to camera. Some cameras can produce higher quality images at a lower file size, while other will produce large files with less detail and photo information.
The Functions and Features
Point-and-shoot cameras have a lot of features that are described in their advertising materials. Some features are critical for use in a clinical setting, while others range from being unimportant to being outright obstacles.
Zoom is frequently discussed in both optical and digital terms. Optical zoom refers to moving the lens to make the subject appear larger or nearer to the viewer. This type of zoom can be desirable, though in extreme cases it may make it difficult for the camera to focus on a near subject.
Digital zoom is less useful than optical zoom. Digital zoom essentially crops an image, and then enlarges it to fit the screen. No additional details are included in the image, as it is just a subset of information from the original picture. A way to observe the loss in quality is to view an image on a computer, and then view it magnified greater than 100%. The image will begin to look jagged or more pixilated.
Macro functionality is the ability of the camera to take an image while very close to the subject. Very powerful macro cameras may be able to get as close as 1cm from the subject, while others may need 20cm or more. A macro of 10cm or less is desired for many clinical applications.
Typically, cameras require the user pushing a macro button (symbolized by a flower) to enable taking pictures in the macro range. Occasionally a camera will have a second “super-macro” setting that is required to use the camera in very close ranges. Recent developments in the camera market have included an “auto-macro” mode, which allows the camera to decide whether or not to use macro.
The flash is a crucial feature of the camera, serving as a way to augment the ambient light in a photo. Shortcomings in how a camera uses its flash can eliminate a device for use in clinical settings. Some flashes will “blow out” an image, making everything appear white, while others will have strong shadows that make elements outside of the flash range difficult to view.
The term “scene” or “mode” is used in much of the camera literature as a way to describe presets for taking pictures in various settings. Examples include scenes for landscapes, snow, portraits, or night photos. There are no standard terms or icons used when describing these scenes, and each camera tends to treat the scenes slightly differently. “Auto” mode is often available, which may be the most appropriate setting to use when imaging. When in auto mode, the camera will decide which scene is the most appropriate for the photo being taken.
White balance is a feature used to tell the camera what type of light is being used to illuminate a scene. While the human eye frequently sees light as white, cameras are more sensitive to the color variations in a light source. If the white balance is not properly set, fluorescent lights are often green, incandescent lights are red or yellow, and a wide range of other bulbs will produce a rainbow of colors. Fortunately, many cameras can detect which type of light is being used, and can choose the right white balance when left in auto.
One important thing to note in white balance is that strong background colors may change how the camera identifies the white balance. It is important to look at how well the camera selects the white balance in a variety of settings when assessing the camera.
When taking images, especially in low-light conditions, many digital cameras are susceptible to “camera shake”. The hands holding the camera while imaging introduce small movements while the camera is capturing an image, resulting in a blurry picture. Some cameras utilize image stabilization, which can help produce clearer and crisper images.
Many other features exist on digital cameras, some of which have possible shortcomings when used clinically. Cameras tout “high ISO” capabilities, which allows for shooting in extremely low light, but such a feature often comes at the cost of more noise or graininess in the images. Face and smile recognition have little utility in a clinical setting, and can be problematic if they prevent the camera from taking a photo due to the lack of a face or smile when attempting to capture an image.