When changing the resolution while you keep the amount of pixels the same does not make any difference in size, as the size is dependant on the total number of pixels and the color quality. (Jpeg compression obviously too, but that comes later)
For example: 1 pixel in 8 bit grayscale means this pixel has 256 possible values, ranging from 0 (black) to 255 (white) That means the size of an 100 x 100 px image in grayscale would be 100x100x8 = 80000 bits That’s 10000 bytes (9,77 kilobyte). In color you have three channels instead of one (Red, Green & Blue) So that would make that a single pixel could have 256 x 256 x 256 = 16,7 million values. That means every pixel is 24 bit (3 x 8). A 100 x 100 px image in RGB, 24 bit would be 100x100x24 = 240000 bits, 30000 bytes, 29,3 kilobyte. Pixels have a fixed proportions so this is a constant value.
Next is resolution. When you have a beautiful photo of 1000×1000 pixels (1 Megapixel) in 24bit RGB you probably would like to print it sometimes. Photographic quality can be achieved in multiple ways, but a printer usually needs 200, to 300 pixels per inch of paper to make it look real nice. Let’s say you print at 200 ppi/dpi (dots per inch).
The printed image will thus be 5 by 5 inches. (=12,7 cm). Resolution is thus not a property of your image. Resolution is something you can choose. 4 Megapixel just means that you have 4 million pixels of data. At a resolution of 72 you’ll cover 2272/72 = 31,5 by 1704/72 = 23,7 inches your monitor (as that’s in a resolution of 72 dpi)
It will cover just 7,6 by 5,7 inches of paper at photographic quality (300 dpi)
(When you uncheck the Resample Image box in Photoshop when increasing the resolution, photoshop will create extra pixels while the print size remains the same. You should realize these fantasized pixels don’t contain any new data, and thus are a useless waste of space.)
Last there is this cunning technique called JPEG. What JPEG does is that it puts pixels in clusters of 8 x 8 pixels, and averages the values of those 64 pixels in a variable adjustable way. Modern jpeg compression can reduce the file size up to 100x, but that will rarely happen, as that’s depending on the complexity of the image.
Well, combined with Johan’s post this must give you some more insight in the world of pixels.
Regards,
Tiemen Rapati
"Johan W. Elzenga" wrote in message
philipus wrote:
I have read a lot here in the groups about pixels, dpi, and ppi, and how in Photoshop I can change dpi etc.
But you haven’t understood a lot of it, so it seems. Your question is not about dpi, it’s about JPEG-compression. Changing the dpi doesn’t change the file size.
The photo laboratory I use only provides me with 50 MB on their site and I would like to use it as effectively as possible.
The images from my 4 megapixel camera at highest resolution tend to be 2-3 MB each at 72 dpi. This means that I don’t have space for many images on the site.
I am interested in ensuring good print quality when I order images from the laboratory. Can I somehow shrink the file size while at the same time ensuring good print quality?
If you save your images in Photoshop as JPEG, Photoshop will tell you how large they will be. Look at the bottom of the dialog box where you set the compression. Depending on the image (lots of detail compresses less well), a compression with quality "10" (which is still very good quality) will give you a file size around 1 MB, sometimes even less.
The reason I ask is that when my brother ran the marathon he could buy digital images from his run taken by a photo firm. These images were emailed to him but the file size was less than 1 MB. However, these files could be used to print prints up to 30×40 cm without quality loss.
That doesn’t say too much. Quality is not only depending on the file size, but also the number of pixels and the quality of the camera lens. Were these pictures shot with a 4 Mpixel camera as well? I can compress the images from my 6 Mpixel DSLR to give 1 MB files, and they will still be better than your 4 Mpixel digital compact camera produces at 2-3 MB. The fact that 1 MB files from one particular camera can be used up to 30×40 cm with good quality, doesn’t necessarily mean that 1 MB files from your 4 Mpixel camera can be used the same way. Try it first with one of those 2-3 MB files to see if you like the resulta, before ordering 50 prints of 1 MB files!
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Johan W. Elzenga johan<<at>>johanfoto.nl Editor / Photographer http://www.johanfoto.nl/
