Excessive JPEG compression

With most digital cameras, the default level for image quality is fairly low, adequate for a 4 x 6-inch print, perhaps. Many camera owners use this quality level, which produces a fairly small image file due to high compression, for one reason—they can fit a lot of images into their memory cards. That makes sense, but it's a mistake for anyone who plans to make larger prints. Before we talk about how to avoid the mistake of excessive compression, here's a little refresher on the connection between photo quality and file size.

Image quality The better the quality, the higher the resolution, and the more pixels each image will contain. With more pixels, you get superior definition of detail. A low-quality image has a lower resolution and is composed of very few pixels. Many digital cameras offer several image quality options, from low to super fine.

File size

In addition to choosing image quality, you can usually choose the image-file size, from small to large. The larger the file, the lower the compression, and the higher the image quality. A small file is extensively compressed with internal software, which produces a major loss of important image data and results in poor image quality.

These two features, image quality and file size, work together. When selecting a JPEG-capture mode, you can choose a combination that will make high resolution/large files (for the very finest quality), medium resolution/small files, low resolution/large files, and so on.

Each camera manufacturer uses its own terminology for image quality options and for image file-size levels. Some cameras offer only basic quality options such as normal, better, and best. Read your instruction manual closely to determine what options your camera provides and their actual designations.

The mistake It's tempting to use the camera's default setting, which provides medium-level quality and a fairly small image file. Some people, trying to maximize the number of images their 16 MB memory card will hold, even select the lowest image quality option and the smallest file size. Unfortunately, neither combination produces images that will make for excellent prints.

The fix

Buy a high-capacity memory card and you'll be less tempted to use the low-quality setting or a high-compression option. A 128 MB or a 256 MB card can save many large/high-resolution image files. Regardless of the card, you should frequently review your pictures and delete unsuccessful images. This makes space for new, better pictures.

With a 2- to 4-megapixel camera, use the large/super fine combination if you plan to make 5 x 7-inch or larger prints. If you rarely make prints larger than 4 x 6-inches, you can get by with the medium/fine setting. This will still produce an image file with an adequate number of pixels and a medium level of JPEG compression that should maintain decent image quality.

For the best results, always use the highest image quality option, ideally with a large file size setting. But what should you do if your memory cards are almost full? Select the "Small File/Super Fine" combination. The JPEG image will be extensively compressed but the high pixel count should still assure acceptable quality in a 5 x 7-inch print. 

Tips for buying storage cards

Making sure that the storage card is compatible with your digital camera

Storage cards come in different shapes and sizes. The first thing you need to know is which card your digital camera takes. Some common types of storage cards for digital cameras include CompactFlash, Secure Digital (SD), Memory Stick, and xDPicture Cards. Read your camera's manual or visit the manufacturer's Web site to be sure that you know what type of card is right for your camera.

Determining the capacity-to-price ratio of the card

The larger the memory capacity on the card, the more pictures you can store on it. The higher the storage capacity, the higher the price will be. If you take a lot of pictures, or have a digital camera that shoots large files, then a higher capacity card will be a good choice.

Knowing the speed of the storage card

Storage cards can vary in speed. The speed refers to how fast information is written to or read from the card, such as when it is inserted into a storage card reader that is attached to your computer. Therefore, a faster storage card speed the better. For example, 40x is better than 24x, and 80x is better than 40x. Generally, the higher speed cards will cost more.

Excessive contrast

If you've shot on days with harsh sunlight, you've probably noticed that many of your digital images exhibit extremely high contrast. Such pictures include dark shadow areas and ultra bright highlight areas. Excessive brightness is the most serious problem, with "burned-out" or "blown-out" highlights that obscure detail in, for example, a bride's white gown or a snow covered hillside. With sophisticated image-editing software such as Microsoft Digital Image it's easy to solve certain technical problems. However, it's almost impossible to fully correct blown-out highlights. While these can be darkened, you cannot add detail or texture that was not recorded by the image sensor.

The Fix

In order to minimize this problem, remember the following tips:
If your camera offers a contrast level adjustment control, do not select the high option.
Even in the soft light of a cloudy day, the standard setting should produce snappy
contrast. If your camera does not have a contrast control feature, try to take pictures
when a cloud drifts over the sun. The contrast will be lower under those conditions.

• In extremely harsh, contrast light—as on a sunny day—select a slightly lower contrast setting. This will minimize excessively bright highlights and extremely dark shadow areas. After downloading images to a computer, use image-editing software to increase contrast if the pictures seem a bit "flat." (The software is more effective in increasing rather than reducing contrast.) When taking pictures of people, ask them to move to a shady area and use flash to maintain a bright effect.
• In direct sunshine, use the camera's "Flash Always On" option for nearby subjects to even out the lighting. The burst of extra light can moderate contrast by brightening shadows. If your camera's flash unit produces ultra-bright highlight areas, don't use it with white subjects.
• Overexposure compounds the problem of contrast by making highlighted areas excessively bright. After taking the first picture of any subject, check the exposure in the camera's monitor. If the image seems too bright, set a negative exposure compensation factor such as -0.5. Re-shoot the picture and check it again. A slightly dark image can be corrected later with image-editing software, using the fill-flash or lighten tools.

Color Depth

Color depth is a computer graphics term describing the number of bits used to represent the color of a single pixel in a bitmapped image or video frame buffer. This concept is also known as bits per pixel (bpp), particularly when specified along with the number of bits used. Higher color depth gives a broader range of distinct colors

Truecolor can frequently mimic many colors found in the real world, producing 16.7 million distinct colors. This approaches the level at which the human eye can distinguish colors for most photographic images, though image manipulation, some black-and-white images (which are restricted to 256 levels with Truecolor) or "pure" generated images may reveal the limitations.

24-bit Truecolor uses 8 bits to represent red, 8 bits to represent blue and 8 bits to represent green. 28 = 256 levels of each of these three colors can therefore be combined to give a total of 16,777,216 mixed colors (256 × 256 × 256). Twenty-four-bit color is referred to as "millions of colors" on Macintosh systems.

Beyond truecolor 32-bit color

In the late 1990s, some high-end graphics hardware and scanners started to use more than 8 bits per channel, such as 12 or 16. This has never become common, as the gain in color resolution is almost invisible – 10 bits per channel seem to be enough to reach the absolute limits of human vision under almost all circumstances.

However, professional-quality image manipulation software has started to employ 16 bits per color channel for internal storage, providing protection against accumulating rounding errors when multiple consecutive manipulations are performed on a picture.

RGB vs CMYK

The RGB color model is an additive model in which red, green, and blue (often used in additive light models) are combined in various ways to reproduce other colors. The name of the model and the abbreviation ‘RGB’ come from the three primary colors, red, green, and blue and the technological development of cathode ray tubes which could display color instead of a monochrome phosphoresence (including grey scaling) such as black and white film and television imaging.

CMYK (short for cyan, magenta, yellow, and key (Black), and often referred to as process color or four color) is a subtractive color model, used in color printing, also used to describe the printing process itself. The CMYK model works by partially or entirely masking certain colors on the typically white background (that is, absorbing particular wavelengths of light). Such a model is called subtractive because inks “subtract” brightness from white.

Color Management

Color reproduction can be a complex process

ICC profile color management defines color information in standard terms necessary for proper reproduction of images. Monitors, printers, scanners, and cameras should be profiled. Working and output spaces – such as Adobe RGB, sRGB, SWOP CMYK, (etc.) – should be embedded and preserved when opening files.

An ICC profile is a set of data that characterizes a color input or output device, or a color space, according to standards by the International Color Consortium (ICC). Profiles describe the color attributes of a particular device or viewing requirement by defining a mapping between the device source or target color space and a profile connection space.

Profiles are simply look-up tables that describe the properties of a color space. They define the most saturated colors available in a color space; the bluest blue or deepest black your printer can produce. If you don't have a profile, the trio of Red, Green, and Blue values that make up a color have no particular meaning — you can say something is blue, but not exactly which shade of blue. Accurate profiles are the key to a color managed workflow. With accurate monitor and printer profiles, your prints will closely match what you see on your monitor. Without profiles, you need to rely on trial and error combined with good old-fashioned guessing.

Every device that captures or displays color can have its own profile. Some manufacturers provide profiles for their products, and there are several products that allow end users to generate their own color profile, typically through the use of a colorimeter.

The ICC defines the format precisely but does not define algorithms or processing details. This means there is room for variation between different applications and systems that work with ICC profiles.

Color Spaces

Photographers know that the world is difficult to capture using a digital camera. It be wonderful if we could capture the scene as we saw it. A scene might have a huge dynamic range, the tones between dark shadows and bright highlights. Digital cameras do record the world in a form that’s quite different from how we see the world. The initial RAW data captured has to be rendered to an image; an image as we want it to look, likely encoded into an RGB color space. This RAW data can be rendered to attempt to match the scene or the image can be rendered to create a pleasing reproduction of the scene. There’s a big difference between the two and it’s important to understand the differences.

The actual scene we attempt to record may go beyond the scale of color, luminance, saturation, that our devices can record, and beyond the ability of output devices to reproduce. When rendering we attempt to reproduce the scene colorimetry, the measured color of the scene, we often end up with an image that’s not very pleasing when viewed on a display or printed.

In technical jargon, the measured scene color the camera captures is known as Scene-Referred. Since we need to view this image on something like a display or a print, it’s usually necessary to make the image appear more pleasing on the output device and to produce the desired color appearance the image. These image colors are known as Output- Referred. The need to fit the color gamut and dynamic range to output referred data is called rendering. The camera usually performs this rendering when you select a color encoding setting such as sRGB or Adobe RGB (1998). If the camera is set to capture just RAW data, the rendering becomes the job of the photographer. The photographer expresses their idea of how the scene should be reproduced on an output device such as a standard display or print. The desired color appearance of an image you are editing is dependent on the output medium. This is not the measured color of the scene itself (scene-referred). An example of how a user would handle this output-referred processing would be using a RAW converter to produce the appearance they prefer from the RAW data.

When you set your digital camera controls to capture an image into a color space, there are two parts to this process: rendering the data and then encoding the data. In creating an output-referred image, the camera or computer system has to perform the color rendering processing before it can encode the result of the processing into a color space. Because the color space is rendered to output-referred data it cannot be used to accurately represent the scene appearance, or what would be called the colorimetry (measured color) of the scene. Therefore, first the data is rendered and this rendering process is based upon how a camera manufacturer feels they will produce the most pleasing image appearance for their customers.

As such, this rending varies for different camera manufacturers, and even different models from the same manufacturer, the rendering is not standardized. Think of this rendering process as a perceptual rendering of sorts; the rendering is that which the manufacturer feels produces visually pleasing color, not generally the colorimetrically correct color. This isn’t a problem; different film types have incorporated different looks, which are selected by the photographer based on their preferences. Two color space encodings of the same scene from different camera brands should match but that is rarely the case; no more than two shots of the same scene on two different types of film. However the degree of mismatch between cameras can be more pronounced when printing because the range of scenes captured is so much greater.

The second process after rendering the data is the actual data encoding which is standardized; the rendered data is encoded into a color space. Two identical renderings of the same scene will produce identical encodings in a given color space. When you produce an image file in a given color space you aren't producing a colorimetric copy of the scene you took the picture of, you are producing an image as it would look rendered to an sRGB display or correctly previewed in an ICC aware application like Photoshop. The image file describes the picture on an sRGB display (output-referred). That display should behave as described by the specifications that define sRGB. If the display is profiled and the data being previewed has an embedded profile, the file, will preview correctly in an application like Photoshop. What is seen, and output, isn’t a colorimetric representation of the actual scene (scene-referred). This is one reason why producing "accurate" color from a digital camera can be difficult. Every user has a different definition of what they mean or want when they say accurate. This rendering and encoding process isn’t limited to the process of creating images using digital cameras.

It is important to note the difference between a color space and a color encoding. A color space specifies the color coordinates, but not what medium the image in that color space is intended for. Photographers understand that it is necessary to view prints and transparencies under some kind of defined and correct viewing specifications. If a transparency is 2 stops too dark, viewing that dark transparency on a light box that is 2 stops too bright isn’t the right way to evaluate what is an under exposed transparency. If an image we are viewing on a display system is too dark, don’t increase the luminance controls to make the image appear lighter.

We have defined references that describe how both the light box and the display should behave and how various types of imagery should be viewed. The color image encoding specifies the color space which describes the specifics of the medium on which the colors are being rendered. In other words, how images in a color space are ideally being viewed. The reference medium will specify such parameters as the white, black, and dynamic range of a printer or display as well as the environmental conditions under which the viewing of the image will take place. The sRGB color space reference medium is that of a display with a set of well-defined specifications on how this display behaves including the ambient light around this display. Color encodings provide a bit more detail about the color space based on a reference medium; how that data when output should eventually be viewed. Since digital camera systems need to take their original RAW capture data and encode that data into some RGB color space it’s also important to recognize that the color appearance of an image you are editing is dependent on the output medium. If you want to be sure that your images will be reproduced correctly, it is necessary to communicate both the color space and the reference medium and viewing conditions.

Camera settings for color space are critical when capturing TIFF or JPEG files. (Color space settings are largely irrelevant for raw files, since color space will be determined in the raw file processor.) Most professional digital cameras allow selection of the output color space for JPEG and standard TIFF, with usually two options: sRGB and Adobe RGB (1998).