One of my friend said : ”I bought a new digital camera. It has 10 megapixel. Great!!”. He made photos and then came back to me :”Please help me make the picture smaller”. He didn’t need such big file image.
What is megapixel? Megapixel is million pixel. Pixel is the smallest element of an image. Pixel = Picture + Element. An image with 800×600 pixel is said has 0.48 Megapixel (800 x 600 = 480,000). 0.48Mpixel. In digital camera, it is called “image size”, but in “photoshop” it is called “pixel size”.
Resolution is how compact are the pixels compose the image. For screen (e-file, website, etc.) it is enough with 72 pixel/inch. But for common printing, at least 300 pixel/inch. For high quality printing, of course we need much higher resolution.
So, if you have 800×600 pixel’s image and you want to make a good enough print out, maximum you can get 800/300 x 600/300 = 2.3 x 2 inch size.
The quality and also the file-size of an image depend also to the “color depth”, that has the variable “bpp” = “bit per-pixel.
- 1 bpp, 21 = 2 colors (monochrome)
- 2 bpp, 22 = 4 colors
- 3 bpp, 23 = 8 colors
- 8 bpp, 28 = 256 colors
- 16 bpp, 216 = 65,536 colors (“Highcolor” )
- 24 bpp, 224 ≈ 16.7 million colors (“Truecolor“)
Usually the bits are allocated in image components, in Photoshop called “channel”. The most common, we use three channels Red-Green-Blue, with 24bbp – means 8bit/channel. You can see these variables in your Photoshop. Click menu Image -> Mode.
You will find RGB, CMYK. What are they?
If you want to make what is called “full color printing”, usually we use four color pigments (Cyan, Magenta, Yellow and blacK) instead of RGB (Red-Green-Blue). If you have color printers with separated color ink cartridges, you would understand what I mean with this color pigments. Most of pictures we have are in RGB mode. Screen monitor use the concept of RGB. That’s why we often find the print out is different with what we see in the screen. The printer automatically converts the “data” of the RGB image to CMYK before sending a command to the inks controller for printing. Many software can convert it to CMYK. If you want a more precise print out, it is better to convert it to CMYK first before printing and view it, particularly if you ask a printing shop to do the printing.
FILE FORMAT
This is the definition of file format according to Webopedia: A format for encoding information in a file. Each different type of file has a different file format. The file format specifies first whether the file is a binary or ASCII file, and second, how the information is organized.
For not making you being too confused, I give you example : if you type using Microsoft Word 2003 and save it, commonly you will get a file of Microsoft Word 2003 document (except you save as another format). The file has an extension name “doc”, such as sample.doc – in default, your computer is set to hide that extension name. To see/ unhide it, open the folder with Windows Explorer, than click “tools” -> Folder option. In the dialog box, click tab view -> uncheck “Hide extension for known file types”.
That’s why you should not give file name with “.”, since it will confuse the computer to choose what program should be used to open it.
If an image is taken by a digital camera, first the machine get a “digital negative” or “raw format” image. Most cameras automatically process it to “jpeg” format, but many cameras have choices of the formats.
There are a lot of types/ formats for image. But let us only talk about the most common: .bmp; .tif or .tiff, .jpeg, .gif and .png
An image file has many information, not only the “colors” but can also the process has been made to that picture or even the date and the type of the camera. What data are kept by the file depends on the file type/ format. People try to get the “smaller size” data but as much as needed of data to be kept. For being processed, we need more data inside the file but for view it, we don’t need so much data. That is how we choose the best type/ format.
Tag Image File Format (.tif or .tiff) is the format of choice for archiving images particularly available to be processed. It has a lot of details inside. In consequence, it’s size is really big. It can be compressed with “lossless compression technique”, means even it is compressed; it doesn’t loose any data inside. But of course it will never have the same size as other compressed file such as jpeg or gif. Many digital camera have an option of producing this format.
GIF (Graphics Interchange Format) images are “lossless” – means they contain the same detail as the original (but with only maximum 256 colours), that’s why they are better for clipart and drawn graphics with few colours, or large blocks of colour. Moreover, they can be animated and support transparency. The GIF-64 colours has better quality than GIF-8 colours and soon.
JPEG is (Joint Photographic Experts Group) the best for displaying full-colour photographic images with small file size.
The main disadvantage of the JPEG format is that it is lossy. This means that you lose some of the detail of your image when you convert it to JPEG format. Boundaries between blocks of colour may appear more blurry, and areas with lots of detail will lose their sharpness. On the other hand, JPEGs do preserve all of the colour information in the image, which of course is great for high-colour images such as photographs. JPEGs also can’t do transparency or animation.
PNG (Portable Network Graphics)
There are two types of PNG:
· PNG-8 format, which holds 8 bits of colour information, is comparable to GIF, it often compress better so has smaller file sizes than GIF,
· PNG-24 format, which holds 24 bits of colour, is comparable to JPEG, with often result in larger file sizes than the equivalent quality JPEG but it keeps the original details.
PNG supports transparency like GIF, even more it can have varying degrees of transparency for each pixel, whereas GIFs can only have transparency turned on or off for each pixel so that the transparent PNGs will have nice smooth edges.
Note that unlike GIF, PNG-8 does not support animation.
Since PNG is much newer than JPEG and GIF, old browsers (also Internet Explorer 6) don’t support this format.
.BMP (Bitmap) is used widely by microsoft system. Microsoft Paint software use this format. It has big file size and it keeps the detail informations of the image. Some consider it as “raw” image but not as popular as .tiff.
The last format I want to introduce you is PSD, Photoshop Document. This is for processing using Photoshop. But now, it is also can be used by other Adobe Software such as Adobe InDesign. With this format, you can safe all the Photoshop information of the image.
