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File Format


File format refers to how an image file is structured and how it's pixels are numerically encoded.  File format and resolution are not the same.  If I have a 72 ppi JPEG file and a 72 ppi TIFF file, they both have the exact same resolution even though they have different file formats.  File resolution is about how pixels are horizontally and vertically spaced, regardless of format.  The Resolution page discusses resolution in detail.

Common File Formats

Table 1 shows a list of the common formats found in digital photography and when they are generally used.


Table 1.  Common Digital File Formats

JPEG or JPG The JPEG standard, originally developed in 1986 by the Joint Photographic Experts Group (hence, the name JPEG), defines various ways for storing digital image files as small as possible.  Since 1986, and continuing today, the JPEG organization continues to revise and improve the standard to take advantage of technological improvements. Web sites, email and general photographic images.  Any situation where small file size is of greater concern than retaining 100% of the pixels.

Supports the RGB, CMYK and Grayscale color spaces.

Does not support layers.  Does not support alpha channels.  Does not support the Lab color space.  Does not support 16 bit.  Does not support text annotations.  Does not support transparency.
TIFF Tagged Image File Format Scan files, master images and other high quality photographic images.  Any situation where versatility and retaining all pixels is more important than file size.

Supports the RGB, CMYK, Lab and Grayscale color spaces.  Supports layers and alpha channels.  Supports 16 bit.  Supports text annotations.

Offers maximum compatibility between Adobe and non-Adobe products.
PSD Photoshop Document.  Native Adobe Photoshop file format Scan files, master images and other high quality photographic images.  Any situation where versatility and retaining all pixels is more important than file size.

Supports the RGB, CMYK, Lab, Grayscale and Multichannel color spaces.  Supports layers and alpha channels.  Supports 16 bit.  Supports text annotations.

Offers maximum compatibility between Photoshop versions and other Adobe products.
Raw The unprocessed data from a digital capture.  Contains the actual, unaltered data recorded by a digital camera's sensor.  (Technically, a Raw file is a data file and not an image file.)  Since Raw is a word and not an acronym, this web site spells it Raw, not RAW.

Should not be confused with Adobe's Digital Negative format, DNG.
High quality digital camera captures.  Because it gives the photographer unaltered data, many consider it the more flexible format.  However, it is not a format the image should remain in.  Typically, once opened in Photoshop, a copy is made and saved in the TIFF or PSD format.

Supports a broader tonal and color range that JPEG.  File size is larger than JPEG, but smaller than uncompressed TIFF.  Supports 16 bit.

Because it is unprocessed, there is no color space associated with it.  Also, Raw is a manufacturer proprietary, closed source format.  Therefore, Photoshop's Camera Raw plug-in will need to be able to read the specific manufacturer's Raw format before it can open the file.
XMP Sidecar file created by Camera Raw XMP, or Extensible Metadata Platform, files are not actually image files nor do they contain image data.  They contain Camera Raw adjustment instructions.

Photoshop will not update a manufacturer proprietary Raw file.  Instead, if adjustments are made to a Raw file in Camera Raw, these adjustments are stored as instructions in a XMP file.  When the file is opened again, Camera Raw reapplies the adjustments by processing the instructions in the XMP file.
DNG Digital Negative.  Adobe's generic Raw format. Since Raw is a manufacturer proprietary format, Adobe created the DNG format to allow a universal, open source, Raw format.
GIF Graphic Interchange Format Web graphics with a very limited color gamut and distinct detail, such as web page buttons.  Supports animations.  Very small file sizes and allows an image to have a transparent background.

Only supports up to 256 colors.  Not a format for photographic images.
8 bit PNG Portable Network Graphics A competing format to GIF.  A license and patent free graphics format.  Is usually smaller than GIF.  Unlike GIF, supports partially transparent pixels.

Does not support animation.

Only supports up to 256 colors.  Not a format for photographic images.
24 bit PNG True Color Portable Network Graphics Web graphics and photographic images used as a web graphic.  Supports full color.  Supports RGB and Grayscale color spaces.

Larger than JPEG because it is non-lossy.  Supports transparency (JPEG does not).

Does not support layers.  Does not support non-transparency alpha channels.  Does not support the CMYK or Lab color spaces.  Does not support text annotations.

Not a format for master photographic images.
BMP Bit mapped A Microsoft Windows graphic file format.  More appropriate for graphic work than photographic images.

Supports the RGB and Grayscale color spaces.  Supports alpha channels.

Does not support color profiles.  Does not support the CMYK or Lab color spaces.  Does not support 16 bit.  Does not support layers.
EPS Encapsulated PostScript A PostScript file.  Not a format used by photographers.  Typically used by graphic artists.


File Compression

File compression is a technique used to store an image file's data in a smaller space than if the file were not compressed.  Not all file compression algorithms are the same.  At a high level, they fall into one of two categories.  Lossy, which discards some of the file's data when shrinking it.  Lossless, which keeps all of the file's data when shrinking it.  JPEG's compression algorithm is lossy.

JPEG Compression

How does JPEG achieve its smaller file sizes?  I do not go into the details of the JPEG compression algorithm.  But, in concept, this is how it works.  Whenever a JPEG file is saved by an application, whether it is a digital camera's software, a scanner's software or Photoshop, the software applies logic to it to determine what data it thinks can be reasonably reconstructed when the file is next opened.  It then deletes this data.

The above process occurs every time a JPEG file is saved.  It occurs when the file is first created, either in a digital camera, a scanner or software.  And it occurs every time the file is saved by an application, such as Photoshop.  The logic is only applied when the file is saved.  If you use your computer's file browser to move a JPEG file from one directory to another, or if you email the file to someone, the compression logic is not applied.

When any software opens a JPEG file, it applies another algorithm that predicts what it thinks the missing data originally looked like.  It then creates the missing data based on the prediction.  This process occurs every time a JPEG file is opened or viewed.


Comparing JPEG to TIFF is like comparing a sports car to a truck.  A sports car is small, goes fast, but doesn't carry much.  A truck is larger, not as fast, but can carry all kinds of things.  JPEG is the sports car.  TIFF is the truck.  You will want both.  Here is why.

Much scientific, and nonscientific, analysis has been done to prove and/or disprove whether or not JPEG is just as good as TIFF when it comes to digital photography.  After all, JPEG was developed by 'photographic experts' and is a very common format used in digital cameras.  JPEG and TIFF were developed for different purposes.  They are not competing formats.  They each have their pros and cons in any given situation.  The good news is this gives us a choice.  To recap the differences between the two formats, JPEG offers small file size because of its lossy compression algorithm and TIFF offers full retention of data, including layers and alpha channels, with the resulting larger file size.

So, the question is, for a given situation, which is more important?  File size or full information?  The answer will guide you to the format you should use.

I use both formats.  The photographic images on this web site are in the JPEG format.  Why?  Because I need small file size while maintaining decent resolution and color.  The images I capture in my scans, my master images and my print images are all in the TIFF (or PSD) format.  Why?  Because I want to keep all the information in my high quality images.  However, if I then want to email that image to someone, I will create a copy of it, resample it to a much smaller size, and then save it as a JPEG file.

I like having choices, and JPEG and TIFF give us that.

What about JPEG and Raw?

Like JPEG and TIFF, JPEG and Raw are not competing formats.  Both formats can give you quality images.  The table below lists the various pros and cons of each.





In-Camera Processing The only in-camera processing Raw data undergoes is the conversion of an analog signal to digital.  The result is a capture than contains 100% of the data recorded by the sensor.  This is the main reason the Raw format is considered the most flexible. In-camera processing applies all color, white balance and tone conversions.  The image is complete, unless corrections or adjustments are needed.
Convenience Raw is not a ready-to-use format.  Raw allows more control over the image but requires more work.  It must be converted from a data file to an image file before it can be used as an image.  Photoshop's Camera Raw performs this conversion. A JPEG capture is ready to print or email (assuming you do not need to perform any image corrections or enhancements).
White Balance White balance is not actually applied to a Raw file.  It is a value recorded in the file's metadata.  This means you can change the white balance inside Camera Raw with absolutely no loss or degradation of quality. White balance has been permanently applied to the image when saved by the digital camera.  To fix any color imbalance, the image must be color corrected.
Bit depth 8 or 16 bit 8 bit
Color and tonal gamut Since Raw is a 16 bit capture, it can capture more gradations of tone and color than an 8 bit capture. Up to 16.7 million different colors.
File size Much larger than JPEG Small due to lossy compression.
Memory card capacity Because of the larger file size, more JPEG files can fit on a memory card than Raw files. More per card than Raw.
Image quality Image quality is more a function of the photographer's skills than file format. More photographers ruin an image because of poor technique than JPEG's lossy compression ever will.