Size/Resolution issues

[usuallyconfused]

Questions, questions...

My Cyber-shot digital camera can take images from 640x480 up to 2048x1536. If I take a range of pictures at 640x480, I get a range of jpeg files which vary in size from 37KB to 78KB. Why are the file sizes different, given the same camera and settings?

If I open of my 640x480 jpeg images, which is 78KB, in PSP and look at the image information, it shows it as being 72 pixels per inch and 16 million colours. If I now save the image, but using different compression factors, a compression factor of 1 produces a file of size 7KB and a compression factor of 99 produces a file of size 173KB. If I want to save the image as a file the same size as the original, I have to use a compression factor of 15. Assuming that the original image has no compression, why is it that saving the image with a compression factor of 1 gives a file so much bigger than the original? I would have expected it to be the same.

Selecting Resize then Actual/Print Size, I can change the Resolution and although the Width/Height changes, it doesn't actually make any changes to the file if I save it. However, manually change the Width/Height and it does change the image. What is the relationship between image size and resolution in this case?

Again, looking at the Resize dialog box, it shows the Width as 22.58cm and Resolution as 72, but when I print it out, it is actually 16.93cm wide. If I change the Resolution to 96, then the Width is shown as 16.93cm, which is interesting because using small fonts, my screen display is 96 dpi. Is there any connection?

However, it is not the size on the screen that matters, but the size when printed. How is the size/resolution related to the size when printed?

I am trying to get my head round this whole issue and any help would be appreciated.

 

[Sheri99]

The Jpg file format is a "lossy" format, meaning the image is compressed and cannot be completely restored to the original. From the camera, the jpg is presumedly a high quality compression of the original "raw" image. There may be a setting on your camera for the compression setting (e.g., Super-Fine, Fine, etc.). Some cameras let you save the Raw image, which is huge in comparison to any jpg. But anyway, if you open a jpg in PSP, edit it (or not) and save it as a jpg, image quality will be lost (even at the lowest compression setting). So if you need it in jpg format for printing (e.g., at a photo lab), you should only save it in that format after completing all your editing changes. Interim saves should be in a lossless format, pspimage.

Concerning resolution and print size, for a decent print you will need a minimum of around 240 dpi (about 95 cm). To see how big in inches (or centimeters) an image will print at 240 dpi, Go into image size and uncheck resample image, then change the dpi (called pixels per inch in PSP) to 240. It will not change the image size in pixels, only tells you how big (in inches or cetimeters) it will print using that resolution setting. Obviously if you print it at a different size, you would be printing at a different resolution. To change the actual dimensions of the image you would enable image resampling and change the height and width. Ideally the height and width would match the output size you plan to make. If the intended output is onscreen web viewing, 72 dpi is appropriate.

Sheri

 

[tviman]

This is a tough one to get your head around. Images displayed on a screen or printer are expressed in "dots per inch". So, let's say you have an image that is 640x480 taken at a resolution of 72 DPI (dots per inch). If you change the resolution to 144 dpi, the image size won't change but the printed size will reduce to half of it's original. This is because the image now has twice as many "dots per inch" than the original. It's backward from what you would expect it to be.

As far as file size, it all depends on the number of colors in the picture. Take a picture of a rose garden, then one of a black word written on a white sheet of paper. Same picture size but (640 x 480) but the file size will be drastically different. The B&W phote has only 2 colors while the roses may have ???? many colors. More colors = large file size.

There's no way to relate how big one picture will be when compared to another. It all depends in the content of the picture.

There's always a better way. The fun is trying to find it!

 

[Larena]

A site that I found really helpful when trying to understand resolutions is

http://www.scantips.com/

 

[usuallyconfused]

Thanks guys. You have given me something to work on.