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dgreenbee Dec 29, 2002 3:21 AM

Image size
I would like someone to explain image size to me. When an image is taken with a coolpix 5700 Adobe Photoshop reports the image size as 35 inches by 23 inches or something like that. What does that mean? I know that more pixels makes for a finer image; does a pixel have a fixed size? When you resize an image do the pixels grow or shrink or do you get fewer of them? Does anyone know an information source to help understand the relationship between print size and image size?

Klaus DK Dec 29, 2002 1:04 PM

I'll give it a try:
There's two aspects to size. One is the virtual size of the image in pixels, and the other aspect is the size of the file in bytes. The last aspect is a measure of how much space your photo takes up on your PC. But it's possible to have a small photo - that takes up a lot of space on you harddrive.
Lots of people talk about resizing a photo - they really mean resampling - and the common method is to let you PC do the work by calculating an enlargement or a shrinking. Your software in the PC does the job for you.
When you take a shot with your CP5700 the chip (CCD) render an image of say 2560 x1920 pixels, this mean that the image is 8,5 x 6,4 inches at a resolution of 300 pixels per inch.
(the standard resolution of a standard PC screen is 72ppi)


1. if you change the resolution of this pic from 300 to 72 ppi as a resampling, then the size of the shots pixelsdimensions will change to something like 614x461 pixels. This can be seen on your monitor as a much smaller photo. However it wont change the size of your output printsize - but it will change the resolution!

2. To change the output printsize you have to change the resolution (and not the dimensions - get it?) . This means that if you i.e want to sell you photo for some highresolution printing your maximum printsize at 300 ppi will be (2560/300=8,5 AND 1920/300=6,4) .........8,5 x 6,4 inches. To print your photo you must be sure your graphic editor DONT resample the photo (often seen as a box to uncheck in the resizedialog). Just change the print/documentsize in the resizedialog and the resolution in ppi automatically changes.
3. Finally there's the issue resizing the photo to a larger one than the camera made for you. If you do this - the graphiceditor will use an advances calculation for the guessing of how your photo will end up. It simply will guess the missing pixels and interpolate as it is called. Some digicams (i.e fuji) does this when you actually takes the shot. Just remember it - interpolation. Good in some situation - for enlargement I think not. But if you have to do it - use the BIKUBIC method!

Hope that helped you!

Jim Ghiringhelli Dec 29, 2002 8:55 PM

another way to lookat it
To look at it another way, the number of pixels will stay constant, while the image size can change. The pixels are like a streatch fabric that can be changed in size yet using the same amount of material (pixels). When you make it large the pixels will be a great distance from each other and when you make it small the pixels will be close up tight together.

Tim Grey posted a nice article about resolution in nature photographers magazine. I have the site address below

Jim :)

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