file sizes

[chris_4622]

With the scan once, use many philosophy do you change the file size (for the web) using "image size" or "resolution"?

[Ted Harris]

If I am scanning once to create a master file for all purposes that file is usually rather large for changing to a web useful file. I downsize it and lower the resolution. You can also use Photoshop's "Save for Web" command to do some of this but the command is not happy with large files.

[Frank Petronio]

The only thing that matters is the number of pixels. A file 10 inches wide by 5 inches tall at a resolution of 72 pixels per inch is identical to a file 1 inch wide and 0.5 inches tall at a resolution of 720 pixels per inch. You may choose to standardize on using a consistent resolution to make printing or organizing easier, but for different uses you will be changing the image size and the resolution to suit.

Funny thing though. I know people who have been working in front of computers as their full time job for ten years who still don't grasp this. About 50% of the graphic designers I know look blankly when I explain this to them. These are the folks who ask for "300 dpi" or "72 dpi" files without any other considerations...

[Bruce Watson]

As Frank says, it's just pixels. The "image size" and "resolution" numbers work together to give you the actual pixels.

For example, if I downsize for printing on my Epson 7600, I set the resolution to 360ppi and then set the image size numbers. If instead I'm downsizing for the web, I set my resolution to 72, then set the image size.

In the end, it's just pixels. All the image editor is doing is giving you an easier way of dealing with pixels by doing the math for you.

[Ralph Barker]

Using the "image size" tool, I downsize in steps of about 50%, with a light touch of unsharp masking at each step. For me, that seems to produce the best results. The specifics of the procedure I use can be found here (http://www.rbarkerphoto.com/scanning-tutorial.html).

[Randy Becker]

Frank said it very well.

I tell my students that computer image files are really math problems. That is Height x Width x Resolution = File Size. Change one thing on one side of the equal sign and the others must change accordingly. So when you need to resize an image, you must "resample" the image. Essentially take the equal sign away and apply a very big complicated math algorithm to it.

I have tried many versions of "downsizing" or "downsampling" and have found no visible difference between doing the downsizing in one giant step or in multiple steps. Now, I am doing this through Photoshop CS2 so maybe the current algorithm may be the real reason it works so well. I don't know for sure but i do know it works quite well.

Best regards,
Randy

[robc]

the answer to your question depends on the software you are using.

For the web you should be interested in making your image x pixels wide by y pixels tall. The resolution or dpi of that image is determined by the screen on which it is finally displayed and not on the resolution value you see in your software.

Assuming you are using Photoshop(PS) then if sizing for the web then use "image size" and key in the width and height pixel values you require. If you have the "constrain proportions" on then you can either enter just the height or just the width and the other edge size will be calculated for you.

You should know that you can also do this by changing the "resolution" in PS but for web its better to think in terms of width and height. You should also know that if you uncheck "resample image" before changing the resolution value, then the image pixel dimensions will not be changed so it will have no effect on the size of image for the web. However, it would have an effect if you print the image because the "resolution" value is used for output to printers. i.e. how many dots to print per inch. Monitors ignore the resolution value. The resolution on monitors is set by the graphics card alone. i.e. how many pixels/dots per inch.

If you are not using PS then I have no idea what your software will do with any of the settings you are shown.

[Greg Miller]

Don't forget to change your profile to sRGB when preparing your file for the web. Otherwise colors will likely look washed out when viewed through a web browser.

[Doug Dolde]

I have noticed that downsizing in PS sometimes produces artifacts especially along high contrast edges much like a sharpening halo.

Another way is to size it visually on the screen about how you want it then hit the "print screen" button. Now create a new image and paste it in and it will be near the size you want. From there use the PS Image Size function to nail it.

[robc]

reasons for that are explained here (http://www.xs4all.nl/~bvdwolf/main/foto/down_sample/down_sample.htm)

download the sample target and have a play. note that when the sample is at anything other than 100% what you see is not what you have in your image which means that making judgments about image quality when not viewing at 100% is problematic.

[chris_4622]

Thank you all for your replies. This helps. I am using PS 7 by the way. Ralph I will read your page, thanks for including the link. chris