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Some General interest questions

What is a digital image?

A digital image is an image that is at some point converted into digital form for convenient manipulation by a computer or other digital device. The images discussed in this newsgroup are most commonly of the type known as "raster" images, where each pixel is represented individually (although they may be compressed for storage, depending on the file type).
There are a number of different formats, but the most common for photographic purposes is the 24-bit RGB image, where each pixel consists of a single byte of information for each of red, blue, and green. This means that each of the three colors can have 256 individual intensity shades, giving 16.7 million shades in total. Other formats include greyscale, CMYK and LAB. Greyscale is useful for storing black and white images, but the others are useful only in certain circumstances which are beyond the scope of this document.

What do people mean when they talk about resolution?
There is a common error made regarding the term "resolution"–indeed.

Properly speaking, "resolution" should only be used to discuss the amount of detail present or resolvable within a certain distance. Thus, valid measures of resolution would include dots per inch (dpi), pixels per inch (ppi), and lines per inch (lpi). Notice the length term present in each of these. Without this length term, it cannot be denoted as a measure of "resolution".

What is the difference between dpi, ppi, and lpi?
Dots per inch is generally used to quantify the output of printers. In this usage, it means that a given number of dots of a single color can be printed within a certain distance. Thus, the rated resolution of a printer is valid only in line art mode. As soon as you try to print shades of grey or colour, the effective resolution of the output goes down since you need to blend dots of different colors.

Pixels per inch is the proper term to use when specifying the resolution of a scanner. The ppi rating of a scanner is the number of pixels (each composed of red, green, and blue values) that can be sampled within the space of an inch. Thus, a film scanner rated at 4000ppi would be able to sample 6000 pixels along a segment of film measuring 1.5" long.

Lines per inch is the normal way to test the resolution of optics and film in the analog domain. A printed target of black lines is used to determine at which point you can no longer distinguish between the lines. Since there is white space between the black lines, it actually would take a line of white and a line of black to give a single "line" of resolution. Thus, a scanner capable of 1200ppi resolution, would be theoretically capable of a maximum of 600lpi.

How do I get a digital image into my computer?
There are two ways to get an image of a real-world object into the computer–a digital camera or a scanner.
The digital camera uses lenses to project an image of the actual object onto a light-sensitive charge-coupled device (CCD). The CCD then converts the image directly into digital form, at which point it may be stored or sent to the computer. Digital cameras may be used anywhere that film cameras are used, and generally look similar to film cameras.
The scanner takes an analog image and converts it to digital. The original item is placed in the scanner, at which point a digital image is sent to the computer. Flatbed scanners are useful for scanning prints, while film scanners are generally higher resolution and are optimized for negatives and slides. Scanners require at least two steps (nature->film->digital) as opposed to the single step (nature->digital) of the digital camera.

What is better for digital images, a scanner or a camera?
It really depends what you want to do. At present, digital cameras in the price range even remotely affordable by the amateur are quite limited in their pixel size. Currently they are in the range of 1600x1200, with some going somewhat larger. This limits your print size to about 4"x5" at near photographic quality. It is possible to do a pretty good 5"x7" and even a decent 8"x10", but it is noticeably softer than the 4"x5". If this range of output size is satisfactory, or if you are only going to be displaying the images on a computer screen or LCD projector, then a digital camera should suit you just fine.
If, on the other hand, you want a larger print, or want to be able to crop and still print the image at the above sizes, then you'll need to spring for a scanner. In this case, dedicated film scanners give the best results, but flatbed scans of prints can give excellent quality as well.

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How do scanners work?
Briefly, scanners shine a light on (or through) a subject, and then an array of sensors picks up the image and converts it to digital form.

What different kinds of scanners are there?
There are two kinds of scanners which we will be considering, flatbeds and film scanners.
Flatbed scanners are designed for reflective material, which means that the light source (generally a fluorescent bulb) and the sensor array are on the same side of the original. Flatbeds are generally able to accept inputs of up to letter or legal size (8.5"x11" or 8.5"x14"). Larger scanners do exist, at a much greater price. Consumer flatbed scanners can have resolutions up to 1200ppi, although some professional (and extremely expensive) scanners have resolutions that are quite a bit higher.
Flatbed scanners are capable of doing an excellent job of scanning prints, magazines, artwork, and even small objects.
Film scanners are optimized for film (including both negatives and slides). This means that they are designed to handle smaller originals (from APS to medium format in size), and they are designed for non-opaque originals. This means that the light source (either a fluorescent tube or an array of LEDs) is on one side of the original, while the sensor array is on the other side.
Film scanners generally have much higher resolutions than flatbeds, which is possible mainly because the originals are so much smaller. Film scanner resolutions in the range 2400-2700dpi are common, with the newer scanners coming in at 4000dpi. Most film scanners are optimized for 35mm negatives and slides, but it is possible to find ones that support APS and medium format as well.

How many pixels can I get from a scanner?
Well, a high-end consumer film scanner has a resolution of 4000ppi. With a standard 35mm negative, this would give a pixel size of 3780x5670 pixels, or a total of 21 millions pixels.
Scanning from an 8x10 print using a flatbed at 600ppi, you would end up with an image size of 4800x6000 pixels, for a total of 28.8 million pixels. In comparison, a normal digital camera will give up to about 2000x1312 (for the Nikon D1) pixels, or a total of 2,624 million pixels.

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How do digicams work?
In a digital camera, a series of one or more lenses is used to project an image onto a light-sensitive charge-coupled device, or CCD. This CCD detects the image, and converts it to electrical signals, which are then converted into a standard image format and either stored on the camera or transferred to a computer.

How many pixels can I get from a digicam?
This varies with the camera. Low-end ones will give 640x480 pixels, or sometimes even less. Current state-of-the-art consumer digital cameras will give up to about 2000x1312 pixels, or a total of 2,624 million pixels.
In comparison, a high-end consumer film scanner has a resolution of 4000ppi. With a standard 35mm negative, this would give a pixel size of 3780x5670 pixels, or a total of 21 millions pixels.

How are the pictures stored?
The camera can be tethered directly to the computer, in which case the image is sent directly to the printer. More commonly, however, the image is written to some form of temporary storage while in the field.
The two most common forms of temporary storage are the floppy disk or flash memory capable of holding up to 128MB, with capacity still increasing. Flash memory comes in two main forms, CompactFlash and SmartMedia. Sony has recently released a third form, called the MemoryStick.

At the current rate of digital camera development, we should reach the 8.64 MP level of consumer level cameras in a year to a year and a half. The 34.56 MP level should be reached within three years, assuming we've got flash cards big enough to hold these images and the patience to download them to our PCs.

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What is compression and why would I want it?

Compression is when, instead of saving the actual information contained in the image file, you save information about that image information. Imagine if you took a picture of 2000x3000 pixels that was completely white. You could save the uncompressed image (at a file size of 18MB), or you could save a file that just said "there are 6 million pixels that are all white" (at a file size of under 1KB). This is a crude example of what image compression does.
The reason for compression is rather obvious–which would you rather download, the 18MB file or the 1KB file? Now of course in real life the difference isn't quite that striking, but it is still quite large.

Is there anything bad about compression?
When you receive the image, you have to decompress it. This takes CPU time. If you have a slower CPU but a very fast internet connection it can actually be faster to transfer the uncompressed image rather than compress the image, transfer the smaller file, and decompress the image. For storing files on your own hard drive, it is generally faster to read in a larger uncompressed image than it is to read in a smaller image and decompress it. Aside from this, most compression schemes will throw away some information to compress the image to a smaller size. They are pretty smart about what they throw out (most of it wouldn't be visible to a person anyway), but they are still throwing out information. This is known as lossy compression, of which the commonly used JPEG is a form.

What is the difference between lossy and lossless compression?
Lossy compression has already been mentioned. It tries to compress to a smaller size by throwing out information that it feels you probably wouldn't have noticed anyways. An example of this is JPEG. As you compress the file smaller and smaller, the artifacts (the differences between the compressed image and the original) caused by throwing out this information become more and more noticeable.
Lossless compression uses similar methods as the popular .zip file to compress the image without throwing information out. These give better image quality at the cost of a larger file size. Because they don't throw out any information, they can't compress as small as the lossy compression. Examples of lossless compression are the GIF, LZW-compressed TIFF, and PNG image formats.

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What resolution is required for photographic quality?

The general consensus is that 300dpi of effective resolution is sufficient for "photographic quality". Of course, if you peer closely at the output you will be able to determine that it is not a true photograph even at 300dpi, but that close an inspection is very rare. 300dpi gives very good images that the average person will not be able to tell from a chemical print.

How do image pixels correspond to ink dots?
Early printers had a direct correlation between the input and the output ink dots. Current printers use sophisticated algorithms (error diffusion is one) to convert the input image pixels to output ink dots. Because of this, there is no longer a direct relation from one to the other. Rather, it depends on image content.

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Colour Management

Why doesn't the image on the screen match the real object?
There are three reasons; your scanner/camera, your monitor, and your software.
The scanner/camera will not be perfect in turning the image into digital form. The human eye has a lot of adaptability that electronic devices do not have. This leads to bad white balance, incorrect exposure, and other such difficulties. High contrast scenes may simply be impossible to accurately sample with consumer equipment.
If the digital image is correct, your monitor may be adjusted incorrectly. Brightness and contrast controls, as well as the color temperature setting of your monitor, can make a huge difference in what the image on the screen looks like.
Finally, if the digital image is correct and your monitor is set properly, your software may be messing things up. Make sure that if your software has color management settings, you are using them properly. Improper settings will cause problems.

Why doesn't the image from the printer match the image on the screen?
There are again three possibilities; the monitor, the software, and the printer.
If the image doesn't match the image on the screen, but it does match the original, then you have a problem with either your monitor or your software.
If the screen image matches the original, then it is a problem with either the software or the printer. If the software has color management settings for the printer, you need to use them to adjust the output until it matches what you see on the screen and the original item.

Picture CD/Photo CD

What are the differences between Picture CD and Photo CD?
Although both Picture CD and PHOTO CD Discs are ways your service provider can deliver your images to you on a Compact Disc, requesting one CD or the other will give you completely different results.
In general, images stored on a PHOTO CD Disc are intended for professional or commercial use, available in six levels of resolution, ranging from 128x192 to 2048x3072 pixels.
Images stored on a Picture CD are intended for the average picture-taker; they have one resolution, excellent quality at 1024x1536 pixels. You can store approximately 100 images on a PHOTO CD Disc, and you can add to the images on the CD many times. Images are written to Picture CD at the time of the original processing from a single roll of film (number of exposures will vary), and you cannot add more images to it later. PHOTO CD Discs require the use of enabled software to view and use the images. Picture CD comes with software included on the CD.

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