There are very many image file formats, but the most common and the most important for cameras, printing, scanning, and internet use, are JPG, PNG, GIF and TIF. TIF has been the standard for commercial work, but web browsers cannot show TIF file images (there are browser plugins for TIF, but it will be very incompatible with others).
Frankly, JPG is used when small file size (for transfer or storage, web pages, email, memory cards, etc) is more important than maximum image quality. But a High Quality setting to create JPG is good enough in most cases, if we don't overdo the compression. Perhaps good enough for some uses even if we do overdo it (web pages, etc). But if you are concerned with maximum quality for archiving your important images, then you do need to know two things: 1) JPG should always choose higher Quality and a larger file, and 2) do NOT keep editing and saving your JPG images repeatedly, because more quality is lost every time you save it as JPG (in the form of added JPG artifacts ... pixels become colors they ought not to be - lossy). More at the JPG link at page bottom.
JPG2 There is a format called JPG 2000 with .jpg2 files. It uses a different compression without the JPG artifacts, but it typically reduces image sharpness. Little used, and few programs support it, so it is likely incompatible with most programs. Web browsers don't show JPG2 files.
TIF format is very versatile. There are many TIFF formats for all kinds of data and compressions. CCITT data for standard text document storage, which supports multiple pages in one file. Standard fax is another TIFF format. Designers can be assigned special data tags to declare other data and compression types. One case is that some camera Raw files are actually TIF format, but with unique proprietary data tags for their special purpose, which then is no longer compatible with TIF viewers.
We could argue that there really is no concept of RAW files from the scanner (scanners are RGB). Vuescan does offer an output called RAW, which is 16 bit, but RGB, not raw like from cameras. The difference is that it only defers gamma correction until a later pass. And its file can include the scanners fourth Infrared noise correction channel data if any. Vuescan itself is the only post-processor for these Vuescan raw files (except any Photoshop-like Levels can adjust gamma). But scanner color images are already RGB color, instead of Bayer pattern raw data like from cameras.
Camera RAW images are not RGB, and must be converted to RGB for any use (our monitors and printers expect RGB images). The idea and big advantage of camera raw is that all camera and JPG processing options (such as white balance and contrast) are deferred until later, when we can see the image to decide what it precisely needs without having to undo JPG processing. That makes it better, and much easier to get it right. Then the converted RGB image can be saved only one time as high quality JPG (no JPG artifact issues). When and if the image needs additional processing, we discard that JPG copy and resume from the raw archive original.
I strongly recommend always archiving your original unedited image from camera or scanner. Especially for JPG, archive the first pristine copy (which is automatic with Raw files). Your download folder should be your permanent archive location of the camera's unchanged original file, and edited copies go elsewhere. Good practice is when editing that image, always save any change to a different file in a different location, always. Never overwrite or delete your only original file. Always keep your pristine original, because you can't otherwise go back. Or else there could be times when you realize the edited image is damaged, especially important on the really special ones. JPG especially, each JPG compression is lossy. If you did edit that original JPG file a few times, for white balance, brightness, resampling or cropping, JPG quality suffers with each new JPG compression (lossy), and it is irreversible if the original image is lost. You can't go back, so don't risk destroying your pristine original image. Any work should only make a copy. Beginners tend to worry about the disk space used by that archive, but this is just the nature of the game, JPGs are small anyway, and disks are inexpensive (a 4 TB Western Digital USB 3.0 external drive is about $100 USD). Disk space becomes a trivial concern. Retaining your original image is not trivial. Make a frequent backup too, onto another disk. It's a choice of being safe now, or sorry later.
|A few features of common file types|
|Web pages can show it||ALL||ALL||ALL|
|8-bit RGB color (24-bits)||ALL||Yes||Yes|
|16-bit RGB color (48-bits)||Yes||Yes|
|CMYK or LAB color||Yes|
|Indexed color option||Yes||Yes||ALL|
The term ALL means it is the only option. Yes means it is an available option. Blank means there is no option.
Different color modes have different size data values, as shown.
|Image Type||Bytes per pixel||Possible color|
|1/8 byte per pixel||2 colors, 1 bit per pixel.|
One ink on white paper
|TIF, PNG, GIF|
|8-bit Indexed Color||Up to 1 byte per pixel if 256 colors||256 colors maximum.|
For graphics use today
|TIF, PNG, GIF|
|8-bit Grayscale||1 byte per pixel||256 shades of gray||Lossy: JPG|
|16 bit Grayscale||2 bytes per pixel||65636 shades of gray||TIF, PNG|
|24 bit RGB|
|3 bytes per pixel (one byte each for R, G, B)||Computes 16.78 million colors max. 24 bits is the "Norm" for photo images, e.g., JPG||Lossy: JPG|
|32 bit CMYK||4 bytes per pixel, for Prepress||Cyan, Magenta, Yellow and Black ink, typically in halftones||TIF|
|6 bytes per pixel|| 2.81 trillion colors max.|
Except we don't have 16-bit display devices
The camera sensor is dimensioned in mm, but it also has dimensions in pixels. For example, a full frame 36x24 mm sensor might be divided into 6000x4000 pixels. The sensor size in mm is all important for computing Field of View or Depth of Field. And the sensor mm dimensions also affect the necessary enlargement factor to print size, but the pixel dimensions are also important for viewing the image on screen or paper.
Data Size is the image uncompressed size in bytes when file is opened into computer memory (but image size viewed on the monitor screen is still dimensioned in pixels).
File Size is its size in bytes stored in a file (which is Not a meaningful number regarding how the image might be used). Image size is in pixels, not bytes (file size is in bytes). Data compression can affect file size drastically smaller, but it is still the same image size in pixels. So saying “I have an 8 megabyte JPG file” says nothing to describe the image size. Image size is dimensioned in pixels.
Print Size is its size when printed on paper (dimensioned in inches or mm, as is the print paper). The size of film is also inches or mm. Sensor size (mm) or film size (mm) must be enlarged to the print or viewing size. By varying the printing resolution (pixels per inch on paper), we can print the image about any size we wish, but the quality will vary. 250 to 300 dpi are usual high quality goals.
In strong contrast to paper, monitor screen size is dimensioned in pixels, and image size is also dimensioned in pixels. The image pixels fit the screen pixels one for one, so to speak. A 600x400 image will show as 600x400 pixels on the screen. If the image size is larger than the screen size, we normally are shown a temporary resampled copy of more suitable smaller size. However, print paper is dimensioned in inches or mm, so images for printing must be scaled to be spaced out so many pixels per inch or mm (often called dpi, jargon for pixels per inch on paper). See basic differences, and more detail between using images printed or on the video screen.
The most common type of color image (such as any JPG file, but Not Raw files) is the RGB 24-bit choice. Note that uncompressed 24-bit RGB data is three bytes per pixel, regardless of image size. However many/most files are compressed into a smaller file size (JPG is normally compressed to unusually small size, which can involve some quality losses). Compressed files are uncompressed again when opened into computer memory for showing (the count of pixels remains unchanged).
|Photographic Images||Graphics, including|
Logos or Line art
|Properties||Photos are continuous tones, 24-bit color or 8-bit Gray||Graphics are often solid colors, with few colors, limited to 256 colors, with text or lines and sharp edges|
|For Unquestionable Best Image Quality||TIF LZW or PNG24 (lossless compression,
without JPG artifacts)
|PNG8 or TIF LZW or GIF (lossless compression,|
without JPG artifacts)
|Smallest File Size||JPG with a higher Quality factor can be both small and decent quality.||TIF LZW or GIF or PNG8 (graphics/logos without gradients normally permit indexed color of 2 to 16 colors for smallest file size)|
Windows, Mac, Unix
|TIF or JPG or PNG24||TIF or GIF or PNG8|
|Worst Choice||256 color GIF or PNG8 is very limited color, and is a larger file than 24-bit JPG||JPG lossy compression adds artifacts, can smear text and lines and edges|
These are not the only choices, but they are good and reasonable choices — More information:
Major considerations to choose the necessary file type include:
But file size is also very dependent on image size (in pixels). A 0.24 megapixel image file (600x400 pixels) will be vastly smaller than a 24 megabyte image file (6000x4000 pixels), regardless of compression. 24-bit color is always 3 bytes per pixel (before compression).
The only reason for using lossy compression is for smaller file size, usually for internet transmission speed or storage space. Web pages require JPG or GIF or PNG image types, because some browsers do not show TIF files. On the web, JPG is the clear choice for photo images (smallest file, with image quality being less important than file size), and GIF is common for graphic images, but indexed color is not normally used for color photos (PNG can do either on the web).
Other than the web, TIF file format has been the undisputed leader when best quality is desired, largely because TIF is so important in commercial printing environments. High Quality JPG can be pretty good too, but don't ruin them by making the files too small. If the goal is high quality, you don't want small. Only consider making JPG large instead, and plan your work so you can only save them as JPG only one or two times. Adobe RGB color space may be OK for your home printer and profiles, but if you send your pictures out to be printed, the mass market printing labs normally only accept JPG files, and only process sRGB color space.
All photo editor programs will support these next file formats, which will generally support and store images in the following color modes:
Note that if your image size is say 3000x2000 pixels, then this is 3000x2000 = 6 million pixels (6 megapixels). Assuming this 6 megapixel image data is RGB color and 24-bits (or 3 bytes per pixel of RGB color information), then the size of this image data is 6 million x 3 bytes RGB = 18 million bytes. That is simply how large your image data is (see more). Then file compression like JPG or LZW can make the file smaller, but when you open the image in computer memory for use, the JPG may not still have the same image quality, but it is always still 3000x2000 pixels and 18 million bytes. This is simply how large your 6 megapixel RGB image data is (megapixels x 3 bytes per pixel).
Photo images have continuous tones, meaning that adjacent pixels often have very similar colors, for example, a blue sky might have many shades of blue in it. Normally this is 24-bit RGB color, or 8-bit grayscale, and a typical color photo may contain perhaps a hundred thousand RGB colors, out of the possible set of 16 million colors in 24-bit RGB color.
Grayscale 8-bit images can have 256 shades of gray (0..255 from black to white). RGB color images have three component values (of Red, Green, Blue), each of 8 bits, which can combine to be one of a total of 256x256x256 = 16.78 million possible colors (called 24 bit color). For example, one color of Orange might be RGB(255, 165, 0) (which is #ffa500 hexidecimal). See more detail about RGB. Colors can be 16 bits, but our monitors and printers are 8 bit devices.
Graphic images are normally not continuous tone (gradients are possible in graphics, but are seen less often). Graphics are drawings, not photos, and they normally use relatively few colors, maybe only two or three, often less than 16 colors in the entire image. In a color graphic cartoon, the entire sky will be only one shade of blue where a photo might have dozens of shades. A map for example is graphics, maybe 4 or 5 map colors plus 2 or 3 colors of text, plus blue water and white paper, often less than 16 colors overall. These few colors are well suited for Indexed Color, which can re-purify the colors. Don't cut your color count too short though, there will be more colors than you count. Every edge between two solid colors likely has maybe a couple of shades of anti-aliasing smoothing the jaggies (examine it at maybe 500% size). Insufficient colors can rough up the edges.
Scanners have three modes to create the image:
JPG files are very small files for continuous tone photo images, but JPG is poor for graphics, without a high Quality setting. JPG requires 24-bit color or 8-bit grayscale, and the JPG artifacts are most noticeable in the hard edges of graphics or text. GIF files (and other indexed color files) are good for graphics, but are poor for photos (too few colors possible). However, graphics are normally not many colors anyway. Formats like TIF and PNG can be used either way, 24-bit or indexed color — these file types have different internal modes to accommodate either type optimally.
Our digital images are dimensioned in pixels (not bytes, and definitely not inches). And a pixel is simply a color definition, the color that this tiny dot of image sampled area ought to be. Put all those colored dots together, and our brain sees the image. The losses of image data we are speaking about is about the altered color of the pixels.
Call it dpi or ppi as you prefer. The idea is that this resolution is the spacing of the pixels on paper, pixels per inch.
But NOT on monitor video screens. Images are shown on the video screen at their actual size in pixels. Image pixels are shown one for one on the screen pixels, so to speak. There are no inches or mm inside video monitors. You might have bought a 23 inch monitor, but its screen is dimensioned in pixels.
300 dpi is likely what you want for printing a high quality photo copy job (a line art scan of black text or line drawings can better use 600 dpi, but 300 dpi is about all that will help for photo work). That is speaking of printing resolution, NOT scanning resolution. Small film needs high scanning resolution for enlargement.
This dpi number does NOT need to be exact at all, but planning size to have sufficient pixels to be somewhere near this size ballpark (of 250 to 300 pixels per inch) is a very good thing for printing.
Image data consists of pixels, and pixels are "colors", simply the storage of the three RGB data components (see What is a Digital Image Anyway?).
Any 24-bit RGB image will use three bytes per pixel (see Color Bit-Depth - Memory Size).
So for example- any 10 megapixel camera image data will occupy 3x10 = 30 million bytes, by definition of RGB color. This number is the "data size" (when opened into computer memory for use). A TIF file will be near that size (and is lossless), but JPG is normally compressed very heavily (lossy, not lossless) to store in a JPG file of perhaps 1/10 this size (variable with JPG Quality setting), which is "file size" (not image size and not data size). This example image size is still 10 megapixels (dimensioned in pixels, width x height), and the data size is 30 million bytes, but the JPG file size might be 3 MB (lossy compression takes a few liberties). The image will still come out of the JPG file as the same 10 megapixels and the same 30 million bytes when the 3 MB JPG file is opened. We hope its quality also comes out about the same — the JPG losses are altered color values of some of the pixels).
Image size (pixels) determines how we can use the image — everything is about the pixels. See a summary of digital basics.
The most common image file formats, the most important for general purposes today, are JPG, TIF, PNG and GIF. These are not the only choices, but they are good and reasonable choices for general purposes. Newer formats like JPG2000 never acquired popular usage, and are not supported by web browsers, and so are not the most compatible choice.
PNG and TIF LZW are lossless compression, so their file size reduction is not as extreme as the wild heroics JPG can dream up. In general, selecting lower JPG Quality gives a smaller worse file, higher JPG Quality gives a larger better file. Your 12 megapixel RGB image data is three bytes per pixel, or 36 million bytes. That is simply how big your image data is. Your JPG file size might only be only 5-20% of that, literally. TIF LZW might be 65-80%, and PNG might be 50-65% (very rough ballpark for 24-bit color images). We cannot predict sizes precisely because compression always varies with image detail. Blank areas, like sky and walls, compress much smaller than extremely detailed areas like a tree full of leaves. But the JPG file can be much smaller, because JPG is not required to recover the original image intact, losses are acceptable. Whereas, the only goal of PNG and TIF LZW is to be 100% lossless, which means the file is not as heroically small, but there is never any concern about compression quality with PNG or TIF LZW. They still do impressive amounts of file size compression, remember, the RGB image data is actually three bytes per pixel.
Camera RAW files is one way to bypass this JPG issue, at least until the last one final save as JPG when required. And it offers additional processing advantages too. Better easier tools in RAW than JPG has, the RAW data has wider range than JPG has. Much the same controls as in the camera, which you would have needed anyway, but this step is done after you see the camera results, to know exactly what it still needs, and can simply tweak and judge it by eye (as opposed to settings in the camera done in advance, as hopeful wishing).
We hear: But RAW images require an editing step first. Some people do seem terrified of the word "edit", but no matter what, we do always have to stop and look at our images on the computer, every one of them. That is the same extra step. Surely we have to crop them a bit, and resample smaller, and many of mine will need a slight Exposure or White Balance tweak to be their best. It makes a tremendous difference. That is the same editing, a few seconds each, a few clicks, and then the file must be saved again. You might as well do this step in the RAW software, which has better easier tools to do it, and more range to do it., and we can SEE the image now. If your session included 100 images of same lighting situation, just select them all, edit ONE of them (say White Balance and Exposure, even Cropping, etc), and the same edit clicks are applied to all of the selected RAW images in one click. Extremely convenient. And no JPG artifacts, no losses, and any changes can easily be Undone anytime later, with full recovery of our original RAW master copy. RAW is the trivial, easy, and good way, Day and Night good, if you care about these things. Much more about Raw files here.
We all have our own notions, but here is a popular opinion about the ultimate, in quality, in versatility, in convenience. RAW files are popular indeed, from most DSLR cameras. When we take any digital picture, the camera has a RAW sensor, but normally processes and outputs the image as a JPG file. But often we can choose to output the original RAW image instead, to defer that JPG step until later. We cannot view or use that RAW file any way other than to process it in computer software and then output a final TIF or JPG image, however postponing this processing offers a few serious advantages, better editing options, and we can bypass all JPG artifacts entirely, until the one final output Save for whatever purpose. RAW allows us to tweak exposure and color, and defer White Balance decisions until later when we can see the image first, and judge any trial results. The 12-bit RAW file offers greater range for any of our adjustments, often on multiple files simultaneously. And RAW always preserves the intact original version, so we can easily back out any editing changes we made, crop size for example. An argument is made that processing RAW requires this extra step, but same is true of any editing that is required. RAW is the easy way, with the best results.
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